Boulby Underground Laboratory is the UK’s deep underground science facility located 1.1km below ground in Boulby mine, a working potash, polyhalite and salt mine in the North East of England. Boulby is a special place for science, enabling a wide range of studies requiring access to the geologically interesting and ultra-low background deep underground environment.
It is a 200-hectare site located just south-east of the village of Boulby, on the north-east coast of the North York Moors in Loftus, North Yorkshire England. It is run by Cleveland Potash Limited, which is now a subsidiary of Israel Chemicals Ltd. (ICL)
Boulby mine is 1.1 kilometres underground
If you are lucky enough to be able to go down the mine you need to dress appropriately. Hi-vis orange outfits and hard hats
The images above show the lift. It can take 60 people at a time but this is not allowed at the moment because of Covid-19
It takes seven minutes to descend the 1.1 kilometres underground, which is very fast and at the bottom, it opens up into what is called the pit bottom and it’s nice and light at this point.
From the bottom of the lift, it takes around 10 minutes to walk to the labs. Some areas are well lit and others not.
Whoever visits the lab, whether they are a scientist, a very important person or Boulby staff. They all have to do this walk
The above image shows the front door to the lab.
The image above shows Emma, the guide for this section of the “visit”.
The main purpose of the lab being so far down is to get away from the cosmic radiation.
Cosmic rays are high-energy protons and atomic nuclei which move through space at nearly the speed of light. They originate from the sun, from outside of the solar system, and from distant galaxies. They were discovered by Victor Hess in 1912 in balloon experiments. Direct measurement of cosmic rays, especially at lower energies, has become possible since the launch of the first satellites in the late 1950s. Particle detectors similar to those used in nuclear and high-energy physics are used on satellites and space probes for research into cosmic rays. Upon impact with the Earth’s atmosphere, cosmic rays can produce showers of secondary particles that sometimes reach the surface. Data from the Fermi Space Telescope (2013) have been interpreted as evidence that a significant fraction of primary cosmic rays originate from the supernova explosions of stars. Active galactic nuclei also appear to produce cosmic rays, based on observations of neutrinos and gamma rays from blazar TXS 0506+056 in 2018.
Victor Franz Hess (24 June 1883 – 17 December 1964) was an Austrian-American physicist, and Nobel laureate in physics, who discovered cosmic rays.
Boulby mine, opened in 1976, was initially set up to mine potash and polyhalite. The underground laboratory was officially opened on 18 April 2003.
The lab provides plenty of space to do lots of science in.
This particular space is called the large experimental cavern. It is about 7m wide and 6m high and was particularly built North-South facing so that it would be a very useful directional dark matter detector.
This is a lovely clean room setup with equipment for a project being developed called AIT. This project will require a second laboratory to be built. This laboratory will be 25 metres cubed and will hold a large water Cherenkov detector.
Cherenkov radiation is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium at a speed greater than the phase velocity (speed of propagation of a wave in a medium) of light in that medium. Special relativity is not violated since light travels slower in materials with refractive index greater than one, and it is the speed of light in a vacuum which cannot be reached (or exceeded) by particles with mass. A classic example of Cherenkov radiation is the characteristic blue glow of an underwater nuclear reactor. Its cause is similar to the cause of a sonic boom, the sharp sound heard when faster-than-sound movement occurs. The phenomenon is named for Soviet physicist Pavel Cherenkov, who shared the 1958 Nobel Prize in Physics for its discovery.
Cherenkov radiation in a TRIGA reactor pool.
When a high-energy (TeV) gamma photon or cosmic ray interacts with the Earth’s atmosphere, it may produce an electron-positron pair with enormous velocities. The Cherenkov radiation emitted in the atmosphere by these charged particles is used to determine the direction and energy of the cosmic ray or gamma ray, which is used for example in the Imaging Atmospheric Cherenkov Technique (IACT), by experiments such as VERITAS, H.E.S.S., MAGIC. Cherenkov radiation emitted in tanks filled with water by those charged particles reaching earth is used for the same goal by the Extensive Air Shower experiment HAWC, the Pierre Auger Observatory and other projects. Similar methods are used in very large neutrino detectors, such as the Super-Kamiokande, the Sudbury Neutrino Observatory (SNO) and IceCube. Other projects operated in the past applying related techniques, such as STACEE, a former solar tower refurbished to work as a non-imaging Cherenkov observatory, which was located in New Mexico.
Astrophysics observatories using the Cherenkov technique to measure air showers are key to determine the properties of astronomical objects that emit Very High Energy gamma rays, such as supernova remnants and blazars.
Cherenkov radiation is commonly used in experimental particle physics for particle identification. One could measure (or put limits on) the velocity of an electrically charged elementary particle by the properties of the Cherenkov light it emits in a certain medium. If the momentum of the particle is measured independently, one could compute the mass of the particle by its momentum and velocity (see four-momentum), and hence identify the particle.
Pavel Alekseyevich Cherenkov, July 28, 1904 – January 6, 1990) was a Soviet physicist who shared the Nobel Prize in physics in 1958 with Ilya Frank and Igor Tamm for the discovery of Cherenkov radiation, made in 1934.
Emma is indicating the apparatus for testing the photo multiplier tubes which are going to be used in the detector.
Photomultiplier tubes (photomultipliers or PMTs for short), members of the class of vacuum tubes, and more specifically vacuum phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. These detectors multiply the current produced by incident light by as much as 100 million times or 108 (i.e., 160 dB), in multiple dynode stages, enabling (for example) individual photons to be detected when the incident flux of light is low.
This laboratory has a long history of investigating dark matter. In fact, this is the reason why the laboratory was set up. Detectors for dark matter were developed here. For example, The ZEPLIN project
One of the detectors was the ZEPLIN I detector
ZEPLIN-I, a 3 kg liquid xenon target, operated at Boulby from the late 1990s. It used pulse shape discrimination for background rejection, exploiting a small but helpful difference between the timing properties of the scintillation light caused by WIMPs and background interactions. This was followed by two-phase systems ZEPLIN-II and ZEPLIN-III, which were designed and built in parallel at RAL/UCLA and Imperial College, respectively.
Weakly interacting massive particles (WIMPs) are hypothetical particles that are one of the proposed candidates for dark matter.
The image above is the outer cryostat for ZEPLIN II detector. A cryostat is used to keep the apparatus cold
ZEPLIN-II was the first two-phase system deployed to search for dark matter in the world; it consisted of a 30 kg liquid xenon target topped by a 3 mm layer of gas in a so-called three-electrode configuration: separate electric fields were applied to the bulk of the liquid (WIMP target) and to the gas region above it by using an extra electrode underneath the liquid surface (in addition to an anode grid, located above the gas, and a cathode, at the bottom of the chamber). In ZEPLIN-II an array of 7 photomultipliers viewed the chamber from above in the gas phase.
It was a two-phase xenon detector and revolutionised the search for dark matter.
When dark matter hits the nucleus of a xenon atom a flash of light, called a scintillation, occurs
Scintillation is a flash of light produced in a transparent material by the passage of a particle (an electron, an alpha particle, an ion, or a high-energy photon).
The detector has a special electric field which drives the ionisation into the gas area which causes a second flash of light and the difference between the magnitude or size of those two flashes of light and the time it takes provides all kinds of interesting information about these particles. It also indicates the difference between dark matter and other types of radiation.
The above image shows just one example of the photo multiplier tubes used on the outside of the ZEPLIN II detector.
The laboratory is very long and it is a class 10,000 clean room.
A cleanroom or clean room is a facility ordinarily utilized as a part of specialized industrial production or scientific research, including the manufacture of pharmaceutical items, integrated circuits, CRT, LCD, OLED and microLED displays. Cleanrooms are designed to maintain extremely low levels of particulates, such as dust, airborne organisms, or vaporized particles. Cleanrooms typically have a cleanliness level quantified by the number of particles per cubic meter at a predetermined molecule measure. The ambient outdoor air in a typical urban area contains 35,000,000 particles for each cubic meter in the size range 0.5 μm and bigger in measurement, equivalent to an ISO 9 cleanroom, while by comparison an ISO 1 cleanroom permits no particles in that size range and just 12 particles for each cubic meter of 0.3 μm and smaller.
To keep the cleanroom at the appropriate level, air filtration systems are used. The air particle rate is measured, as is the radon level, temperature and humidity to make sure that the conditions in the lab are absolutely optimum for the kind of science that needs to be done.
There are many other little experiments going on behind the speaker.
A lot of interesting multidisciplinary science happens in the laboratories. It’s not just about dark matter
The above image shows part of the ZEPLIN III detector behind the speaker
ZEPLIN-III was proposed in the late 1990s, based partly on a similar concept developed at ITEP, and built by Prof. Tim Sumner and his team at Imperial College. It was deployed underground at Boulby in late 2006, where it operated until 2011. It was a two-electrode chamber, where electron emission into the gas was achieved by a strong (4 kV/cm) field in the liquid bulk rather than by an additional electrode. The photomultiplier array contained 31 photon detectors viewing the WIMP target from below, immersed in the cold liquid xenon.
ZEPLIN–II and –III were purposely designed in different ways, so that the technologies employed in each sub-system could be appraised and selected for the final experiment proposed by the UKDMC: a tonne-scale xenon target (ZEPLIN-MAX) capable of probing most of the parameter space favoured by theory at that point (1 × 10−10 pb), although this latter system was never built in the UK for lack of funding.
ZEPLIN III, when fully built, is twice the size shown in the photograph and has a mass of 70 tonnes
ZEPLIN-III experiment: the WIMP detector, built mainly out of copper, included two chambers within a cryostat vessel: the upper one contained 12 kg of active liquid xenon; an array of 31 photomultipliers operated immersed in the liquid to detect prompt scintillation as well as delayed electroluminescence from a thin gas layer above the liquid. The lower chamber contained liquid nitrogen to provide cooling. The detector was surrounded by Gd-loaded polypropylene to moderate and capture neutrons, a potential source of background. The gamma-rays from neutron capture were detected by 52 modules of plastic scintillator placed around the moderator. The shielding was completed by a 20-cm thick lead castle.
As mentioned above the outer part of ZEPLIN III was lead. Inside there was copper, some plastic and the detector itself.
When the dark matter detectors are fully functioning, they don’t look very exciting. They just sit inside a box quietly looking for dark matter
Currently ZEPLIN III detector is on display in a museum that is local to Boulby.
There is a new dark matter detector at Boulby that is part of the Cygnus collaboration, which involves various countries around the world. This detector is very special because it’s a directional dark matter detector.
Scientists think they know that dark matter surrounds us in a static cloud and as we move through that cloud there are certain times we are moving into and out of a sort of dark matter cosmic wind. This means that scientists can look in one direction in the night sky, where they think the dark matter is coming from, and this direction seems to lead back to the Cygnus constellation. This is why the project and detector are called Cygnus.
Cygnus is a northern constellation lying on the plane of the Milky Way, deriving its name from the Latinized Greek word for swan. Cygnus is one of the most recognizable constellations of the northern summer and autumn, and it features a prominent asterism known as the Northern Cross (in contrast to the Southern Cross).
The Cygnus detector is a re-purposed detector, which was part of the drift collaboration
The DRIFT (Directional Recoil Identification from Tracks) programme was a UK-US joint effort at the UK’s Boulby deep underground site that developed and ran detectors designed to determine the direction of the recoil tracks expected from WIMP dark matter particles.
The experiments have lots of support systems, including areas where liquid nitrogen, argon and xenon can be produced.
On to BUGS. This stands for Boulby Underground Germanium Suite. It is in a different kind of clean room. It is a Class 1000 clean room, so it is even cleaner than the clean room outside it
The reason why the room has to be so clean is because it houses in here various germanium detectors.
A semiconductor detector in ionizing radiation detection physics is a device that uses a semiconductor (usually silicon or germanium) to measure the effect of incident charged particles or photons.
Semiconductor detectors find broad application for radiation protection, gamma and X-ray spectrometry, and as particle detectors.
Germanium detectors are very good looking for gamma radiation and scientists can tell all kinds of things from the gamma radiation that comes off everyday objects.
When scientists want to build a very sensitive particle detector, like a dark matter detector or neutrino detector, they build it underground to keep it away from the surface radiation.
They also place it inside a castle or shield to protect it from normal radiation that is all around us. The shield can be made of lead or copper and in some cases water.
Once all the excess radiation has been removed, the radiation the detector “sees”, becomes it background, which tends to be from its own components.
What the scientists do, in this ultra-clean room, is look at each of the components that they are going to build the dark matter detector with and choose the ones with as little radiation as possible. They also want to know exactly how much they have got at the end of the day.
The range of Dominion detectors and the XIA alpha particle counter does this.
Over to Ed who shows what the mine looks like and what scientists are using Mars Yard for.
Ed is on the dirty side of the lab. The room that he is in is the environmental control room where various things like the air filtration system are controlled and various gas levels, including flammable gas are monitored to make sure everything is safe.
When Ed moves to the clean side he needs to put on clean overalls because there is a lot of dust in the tunnels. The scientists need to make sure that the area is clean for their experiments.
Ed is wearing more protection including the hard hat and ear defenders. The latter because it gets very loud in certain parts of the mine. His hat also has a lamp on it which allows him to see and be seen in the dark and the reflective strips on his clothes help with this too.
He is also wearing shin guards and heavy-duty steel toe boots to protect his legs and feet in the working environment.
Ed is now outside the lab but he is still 1.1 kilometres below ground on the North Yorkshire coast.
This is a working mine, but the areas where they’re mining is over 10 kilometres of tunnels away from the lab.
It takes the miners about 45 minutes to drive in a van underground to where they’re mining. The scientists are luckier as they only have a 15-minute walk from the bottom of the lift to get to the laboratory area.
Behind Ed you can see the scope of the tunnels. The walls of these tunnels are made of rock salts and that’s what they’re mining for. Its polyhalite, which is used as a type of fertilizer. It was left over when a sea called this Zechstein Sea evaporated approximately 230 million years ago. This was a sea that expanded from the north coast of England out over the North Sea and into the north coast of what is now Germany and Poland.
Zechstein Sea shown on a map of Northern Europe
There are a couple of mines in Poland that are actually mining the same seam as Boulby
So, after all this water evaporated a very dry, very salty environment was left behind. This is actually sort of similar to the environment that you might expect to find on the surface of Mars.
This is a fantastic opportunity for scientists and engineers who work for the European Space Agency and NASA to use the space down there in Boulby to test equipment that’s going to go on rovers that are going to be sent to Mars and other planets.
Some of the equipment is going on one of the next Mars rovers and were actually tested down in the mine because the environment is quite similar.
The mine is quite isolated but obviously nowhere near as isolated as being on the actual surface of Mars. But it’s a good way to sort of pretend that the rover (or astronauts) is and the various techniques needed for a Mars mission can be tested.
One of the main reasons for going to Mars is to look for life.
Scientists want to know if there is life on other planets and how to test for it.
The main piece of evidence is to look for places where water is or has been because life as we understand is always associated with water.
On Mars, scientists think there used to be surface water and it evaporated. What is perfect is this is exactly the same as what happened at Boulby.
At Boulby, the scientists crack open some of the rock and look inside to see if they can find any microbes or any organic material at all.
Boulby have the facilities for the investigation including labs and the Mars yard hut, which Ed is pointing to in the image below.
The scientists can use the hut if they don’t want to be outside in the tunnels all the time because it is actually quite hot in the tunnels due to the layers of acting as an insulator earth. The temperature stays around 30 degrees even in the winter.
At Boulby they haven’t just been testing rovers. The aim is to create a pretend habitat there so astronauts can practise living on the surface of another planet because the environment there is so alien and so unfamiliar. It is useful as an analogue for the surface of another planet.
It’s not just space experiments that go on at Boulby. They have a lot of different types of experiments, some of which were mentioned earlier.
Ed is indicating another range of experimental areas. Here are some small pieces of equipment which are part of a long-term environmental study to look at how the equipment deals with being left alone for a long period of time in an inhospitable environment.
One experiment at Boulby is looking at radio carbon dating. Another experiment is looking at how life reacts to low level radiation environments. And a very large upcoming experiment will be dealing with anti-neutrino detection.
Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon
A neutrino is a fermion (an elementary particle with spin of ½) that interacts only via the weak subatomic force and gravity. The neutrino is so named because it is electrically neutral
Behind Ed is a new experiment which is looking at using rock salt, the type of salt found in the tunnels at Boulby, to see whether it could be used to store compressed air.
Compressed air is useful for large scale energy storage, particularly for renewable energy where you can’t always guarantee that the supply is going to meet the demand. There needs to be a way to store large amounts of energy. Scientists at Boulby think they might be able to use caverns mined out of the rock there.
Ed in Mars yard
In this next section Ed talked a little bit more about dark matter and explained some of the concepts not mentioned earlier.
Dark matter was the thing that really kicked off the work in the underground lab.
First of all, what is dark matter. The answer is that scientists don’t really know. That’s why they are looking for it. It’s the big outstanding question.
Scientists think that about 95% of the universe is missing, which sounds like a really terrible statistic, because you can look around you and think that all the matter must be in the visible Universe. But as it turns out, the matter that we can actually see, including everything that makes up Earth, everything that makes up the Sun and other stars only accounts for a very, very small fraction of the total mass of the universe.
This seems a little bit counterintuitive but the evidence for this missing mass comes from different observations that scientists can make.
The main piece of evidence comes from our understanding of gravity.
Scientists think, in this day and age, that they have a very good understanding of gravity, obviously based on Newton’s laws from several hundred years ago followed by Einstein’s theory of relativity that improved them.
Sir Isaac Newton PRS (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, theologian, and author (described in his own day as a “natural philosopher”) who is widely recognised as one of the most influential scientists of all time and as a key figure in the scientific revolution.
Newton’s law of universal gravitation is usually stated as that every particle attracts every other particle in the universe with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres. The publication of the theory has become known as the “first great unification”, as it marked the unification of the previously described phenomena of gravity on Earth with known astronomical behaviours.
This is a general physical law derived from empirical observations by what Isaac Newton called inductive reasoning. It is a part of classical mechanics and was formulated in Newton’s work Philosophiæ Naturalis Principia Mathematica (“the Principia”), first published on 5 July 1687.
In today’s language, the law states that every point mass attracts every other point mass by a force acting along the line intersecting the two points. The force is proportional to the product of the two masses, and inversely proportional to the square of the distance between them.
The equation for universal gravitation thus takes the form:
where F is the gravitational force acting between two objects, m1 and m2 are the masses of the objects, r is the distance between the centres of their masses, and G is the gravitational constant.
Albert Einstein (14 March 1879 – 18 April 1955) was a German-born theoretical physicist who developed the theory of relativity, one of the two pillars of modern physics (alongside quantum mechanics).
General relativity (GR) is a theory of gravitation that was developed by Einstein between 1907 and 1915. According to general relativity, the observed gravitational attraction between masses results from the warping of space and time by those masses.
General relativity has developed into an essential tool in modern astrophysics. It provides the foundation for the current understanding of black holes, regions of space where gravitational attraction is so strong that not even light can escape.
Modern scientists think they have a very good understanding of how it works and they can do the calculations. Things like how fast an apple is going to drop to the ground is a very straightforward calculation to do and even GCSE students’ understanding of gravity will allow them to work it out.
On a slightly larger scale, how fast a planet will orbit around a star is also governed by scientists’ understanding of gravity. The closer an object is to a star, and the heavier the star is, the faster the object needs to orbit to stop it being pulled into the star.
A way of visualising this is to think about a sink full of water. When you take the plug out, the water in the middle is going to start doing a whirlpool and the water on the outside is going to be doing a nice laterally loop around the outside.
The above link is the best video I could find. The star produces a gravitational well because it warps the space around it
Planets show a similar behaviour because of a kind of potential well of gravity that is created by a star. The closer the planet is to the star the faster is needs to travel to maintain its orbit. The relationship between the distance and the speed is well understood, or at least it should be well understood.
The youtube link describes how you can make a model showing the behaviour
Below is a link to a 2-dimensional animation of how gravity works. Via NASA’s Space Place.
Things become less clear on a slightly larger scale when looking at clusters of galaxies orbiting around each other. You would think they are going to follow the same relationship as a planet orbiting a star because we understand how the mass and the distance affects the speed. But what is actually seen when looking at galaxies is a little bit different. Moving further away from a galaxy has a higher orbital speed than expected. This is a bit strange, and it boils down to one of two possible explanations. One explanation is that the way we understand gravity is completely wrong and that that is possible. It’s possible that we need a new theory that explains everything.
However, the current theory of gravity explains most things really well including predicting the orbits of planets. In fact, it predicts all sorts of motion really well. So, physicists don’t really want to have to rewrite everything.
A second explanation, which is the one that physicists are accepting, is that there must be some extra mass, because if there was extra mass, it would explain why everything was moving faster than expected. As mentioned earlier speed is related to the amount of mass.
So, there must be mass present that we just can’t see. Which is why it is called dark matter. It’s literally dark because it doesn’t interact with light so can’t be seen.
So dark matter can’t be seen but its presence can be inferred because of the effects that it has on the other masses within the galaxy.
There are a couple of other theories. One predicts a kind of hole that could be filled by dark particles that have mass but don’t interact with lights. The best candidate so far is called WIMPS. WIMP stands for “weekly interacting massive particle”. Weakly interacting is not to do with the strength of the interaction, but because the particle interacts with the weak force, which is one of the four fundamental forces.
In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is the mechanism of interaction between subatomic particles that is responsible for the radioactive decay of atoms and it participates in nuclear fission.
The effective range of the weak force is limited to subatomic distances, and is less than the diameter of a proton. It is one of the four known force-related fundamental interactions of nature, alongside the strong interaction, electromagnetism, and gravitation.
WIMPs would be massive. They would have mass which means they would be affected by gravity. They would be a special type of particle but scientists don’t know what they are yet but they’ve ruled out a lot of what they aren’t.
A large part of the search for dark matter is actually looking at a lot of possible options and saying, okay, it could be this, this, this and this. The scientists at Boulby are going to build a new detector to look for these particles along with all the detectors that have been built so far, including ZEPLIN.
The scientists have kind of drawn a big line on the work done so far because dark matter particles haven’t been “seen” yet. But the equipment they are building and using are giving more and more precise detections which will hopefully allow them to find WIMPs and work out exactly what they could be.
There are a couple of other fantastic observations that scientists can look at that suggest dark matter possibly exists.
One of the giveaways is something called the Bullet Cluster
Above left: X-ray photo by Chandra X-ray Observatory. Exposure time was 140 hours. The scale is shown in megaparsecs. Redshift (z) = 0.3, meaning its light has wavelengths stretched by a factor of 1.3. Above right: X-ray image (pink) superimposed over a visible light image (galaxies), with matter distribution calculated from gravitational lensing (blue)
In physics, redshift is a phenomenon where electromagnetic radiation (such as light) from an object undergoes an increase in wavelength. Whether or not the radiation is visible, “redshift” means an increase in wavelength, equivalent to a decrease in wave frequency and photon energy, in accordance with, respectively, the wave and quantum theories of light.
The Bullet Cluster (1E 0657-56) consists of two colliding clusters of galaxies. Strictly speaking, the name Bullet Cluster refers to the smaller subcluster, moving away from the larger one. It is at a comoving radial distance of 1.141 Gpc (3.72 billion light-years).
Gravitational lensing studies of the Bullet Cluster are claimed to provide the best evidence to date for the existence of dark matter.
Hot gas detected by Chandra in X-rays is seen as two pink clumps in the images above and contains most of the “normal,” or baryonic, matter in the two clusters. The bullet-shaped clump on the right is the hot gas from one cluster, which passed through the hot gas from the other larger cluster during the collision. An optical image from Magellan and the Hubble Space Telescope shows the galaxies in orange and white. The blue areas in the images show where astronomers find most of the mass in the clusters. The concentration of mass is determined using the effect of so-called gravitational lensing, where light from the distant objects is distorted by intervening matter. Most of the matter in the clusters (blue) is clearly separate from the normal matter (pink), giving direct evidence that nearly all of the matter in the clusters is dark.
The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the Space Shuttle Columbia during STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources 100 times fainter than any previous X-ray telescope, enabled by the high angular resolution of its mirrors. Since the Earth’s atmosphere absorbs the vast majority of X-rays, they are not detectable from Earth-based telescopes; therefore space-based telescopes are required to make these observations. Chandra is an Earth satellite in a 64-hour orbit, and its mission is ongoing as of 2020.
The hot gas in each cluster was slowed by a drag force, similar to air resistance, during the collision. In contrast, the dark matter was not slowed by the impact because it does not interact directly with itself or the gas except through gravity. Therefore, during the collision the dark matter clumps from the two clusters moved ahead of the hot gas, producing the separation of the dark and normal matter seen in the image. If hot gas was the most massive component in the clusters, as proposed by alternative theories of gravity, such an effect would not be seen. Instead, this result shows that dark matter is required.
A gravitational lens is a distribution of matter (such as a cluster of galaxies) between a distant light source and an observer, that is capable of bending the light from the source as the light travels towards the observer. This effect is known as gravitational lensing, and the amount of bending is one of the predictions of Albert Einstein’s general theory of relativity. (Classical physics also predicts the bending of light, but only half of that predicted by general relativity.
Observations of other galaxy cluster collisions, such as MACS J0025.4-1222, similarly support the existence of dark matter.
The major components of the cluster pair—stars, gas and the putative dark matter—behave differently during collision, allowing them to be studied separately.
So, the bullet cluster is two galaxies that collided and the dark matter, because it doesn’t interact, has just passed through, whereas the other material has got caught up in interactions with each other getting stuck in the middle of the cluster
So, physicists haven’t just imagined dark matter because there is a lot of evidence that it, or something like it, exists. They just need to find out what it is.
Secondly, why do the scientists need to work underground to look for dark matter? Dark matter is weakly interacting and interacts with the weak force. It means it doesn’t interact with anything very much at all.
As scientists understand dark matter, they think it should be everywhere in our galaxy. A kind of a diffuse sphere of it all the way around the Milky Way. There are absolutely masses of it flowing about all the time. Always. So why aren’t we being bombarded with it all the time. Well, we might be, but we don’t notice it because it has a very small chance of actually interacting with anything.
The Milky Way is the galaxy that contains our Solar System, with the name describing the galaxy’s appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye.
So, if scientists want to actually detect something that doesn’t interact with much, they need a very, very sensitive detector. However, putting it on the surface of the Earth means it is going to be affected by absolutely everything bombarding the Earth, such as cosmic rays.
Cosmic rays are high-energy protons and atomic nuclei which move through space at nearly the speed of light. They originate from the sun, from outside of the solar system, and from distant galaxies.
We are bombarded by cosmic rays constantly and they are not harmful to life. We’ve evolved to deal with them and millions of them are passing through our bodies all the time. We don’t feel them but a very sensitive detector would because it’s designed in such a way to react to tiny particles, because that’s what they’re looking for.
So, what scientists investigating dark matter want to do is essentially eliminate all the background radiation, like cosmic rays and man-made sources such as the radioactive materials used in hospitals for scans. Even bananas contain a small amount of radiation.
A very easy way to eliminate all the background radiation that bombards the surface of the Earth is to go a kilometre underground where a kilometre of rock does a fantastic job at absorbing the radiation for you.
Now fortunately for the scientists a place like that existed. Boulby mine even had tunnels already dug that could incorporate the labs. All the rock and being underground makes the detections more sensitive and the detectors won’t be reacting to things like cosmic rays.
To reduce background radiation underground as much as possible the detectors are placed in “castles” The term “castle” is just a fancy term for a lead box. Lead is chosen because it is really good at absorbing different types of radiation. The hope is that if the detector does register something it could only be a dark matter particle, because, hopefully, everything else would have been absorbed.
The detectors used in the Boulby mine would give a particular reading if it detected something. Identification wouldn’t be immediate as the resultant waveform would be carefully examined. The shape of the signal should give the scientists some idea of what particle caused it.
The new range of detectors can be used for other things. Even though the labs are so far below ground some very energetic cosmic rays are able to penetrate 1.1km of rock and the scientists need to be able to detect them. Often these cosmic rays are ones that haven’t been detected before.
For muons able to get through the rock a muon detector is placed on top of the dark matter detector. This means that if both the muon detector and the dark matter detector register a signal at the same time then the particle detected is not dark matter.
The muon is an elementary particle similar to the electron, with an electric charge of −1 e and a spin of ½, but with a much greater mass. It is classified as a lepton. As with other leptons, the muon is not known to have any sub-structure – that is, it is not thought to be composed of any simpler particles. It is an unstable subatomic particle with a mean lifetime of 2.2 μs.
Muons arriving on the Earth’s surface are created indirectly as decay products of collisions of cosmic rays with particles of the Earth’s atmosphere. They decay via the weak interaction.
As mentioned earlier the Boulby laboratories have a couple of dark matter detectors. One of them is Cygnus, which also contains some water, because water can absorb different types of radiation. This provides additional shielding.
One of the brand-new detectors, which is still in the testing phase, is called NEWS-G and it’s a spherical detector.
The NEWS-G collaboration gathers physicists from around the world interested in developing spherical gaseous detectors or SPCs (Spherical Proportional Counters) for various particle physics applications.
Below left: Copper SPC Prototype. Below right: Inner Rod and Sensor
This detector technology provides sensitivity to single electrons from ionisation with energy detection thresholds as low as 10 eVee. This sensitivity arises from a high amplification gain from the avalanche, combined with low electronic noise due to the low intrinsic capacitance of the detector. The latter doesn’t depend on the size of the sphere but only on the size of the sensor, which is a convenient feature for scaling up the size of the detector.
Additionally, the operation of the SPC with light noble gases such as H, He and Ne allows for an optimisation of the momentum transfer for low-mass WIMPs. This results in light dark matter particles depositing larger amounts of energy in SPCs than in any other existing dark matter detector.
Furthermore, SPC technology offers a pulse-shape-based discrimination of surface events down to the lowest energies. This allows for rejecting a large fraction of background events originating from ineluctable radioactive contamination of the inner surface of the copper vessel.
Finally, because background and WIMP event rates evolve differently with the target atomic mass, the operation of the SPC with diverse noble gases allows for the discrimination of a potential WIMP signal from an unidentified background.
This is actually a surprisingly clever technique because it allows scientists to get a very high level of sensitivity on a detector compared to a different shaped one of the approximate same size. There’s a couple of reasons for this. One of which is that a sphere is actually just the shape that has the lowest surface area to volume ratio. Which means for a given shape it’s got the least amount of edge and a detector with lots of edges will give unavoidable odd results, whatever material it is made of. Reducing the number of edges produces a more sensitive detector. Another advantage of the detector being spherical is that it has a very straightforward electric field.
The electric field of a sphere appears to come from the centre of the sphere and spreads out in straight lines like the spokes of a wheel.
The electric field obeys the inverse square law. This means that if you double the distance from the centre of the sphere the electric field strength decreases by a factor of 4.
Scientists researching dark matter are constantly trying new things all the time in order to build the most sensitive detectors and eliminate signals from particles which are obviously not dark matter. They are also trying to narrow down what dark matter actually is.
As mentioned earlier another piece of evidence for dark matter is gravitational lensing. Like a normal lens, which bends light in order to focus it in a particular way, galaxies, because they are so massive and have high levels of gravity, can actually bend light that’s coming from a source behind them.
The image above shows that some parts of the galaxies are spread out
Other pictures show the galaxies as really long thin discs when they should actually be roughly circular. The reason for this is there is a lot of mass in between the object emitting the light and the viewer. Light gets bent around the mass and physicists can actually do calculations to find out how much mass is needed to bend the light in that way.
What they found is that the amount of mass contained in stars, planets and galaxies i.e., normal matter, were just not nearly enough to explain the amount of bending.
There needs to be some sort of extra matter, dark matter, that would explain all the extra bending.
In the false colour pictures of the Bullet Cluster, around the outsides where the blue sections are there is a lot more gravitational lensing than expected. The red section in the middle is where most of the dust and gas, the natural stuff of the galaxies is found, and there isn’t as nearly as much gravitational lensing there. This means that the dark matter, the thing that’s giving all of this extra mass, has just passed through in this collision between the two galaxies. It just seems to have flown past everything because it doesn’t interact with normal matter. Whereas all the normal matter such as the stars, dust and gas got smashed up in the middle.
A way to think about gravitational lensing is to imagine the Universe as a trampoline. Somebody standing on the trampoline would make a big dip in it. If a ball is rolled across the trampoline, it wouldn’t travel in a straight line but go around the person. In the analogy the person represents the galaxy and the ball represents light. The mass of the galaxy is causing a “dip” in the Universe, and because of dark matter the “dip” is bigger than expected and causes light to bend more than expected.
The section below is about an activity that school children were invited to take part.
It contains information about extraterrestrial life and what happens at Boulby, which has a similar landscape to what might be found on Mars.
The activity contains questions about whether there is life on other planets and the children were invited create their own exoplanet. Exoplanets are just planets that orbit around another star.
The children were also asked to think about an extraterrestrial species and draw what they think these aliens would look like.
Once the children had created their extraterrestrial species there were further questions in order to work out what was necessary for the aliens to have the best chance of life on their planet. If anyone still wants to have ago they would love to see the finished work – do send any pictures through to firstname.lastname@example.org.
Emma on the surface of the site. Behind her are the two mine shafts.
The shaft on the right is called the rock shaft. That’s the shaft that all the material comes out of. The shaft on the left is called the “man shaft”. That’s how people travel down to the mine.
Questions and answers
1) Is dark matter directional?
Essentially dark matter is relatively static actually compared to the galaxy it is in.
Galaxies are believed to have a supermassive black hole at their centres. All matter including stars (with their associated planets) and gas, orbit it.
Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.
Spiral galaxies are named by their spiral structures that extend from the centre into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot stars that inhabit them.
When studying spiral galaxies, it is invariably found that the stellar rotational velocity in the disk remains constant, or “flat”, with increasing distance away from the galactic centre. This result is highly counterintuitive since, based on Newton’s law of gravity, the rotational velocity would steadily decrease for stars further away from the galactic centre. This is seen with the inner planets of our Solar System which travel more quickly about the Sun than do the outer planets (e.g., the Earth travels around the sun at about 100,000 km/hr while Saturn, which is further out, travels at only one third this speed). One way to speed up the outer planets would be to add more mass to the solar system, between the planets. By the same argument the flat galactic rotation curves seem to suggest that each galaxy is surrounded by significant amounts of dark matter. It has been suggested, and generally accepted, that dark matter would have to be located in a massive, roughly spherical halo enshrouding each galaxy.
Rotation curve of spiral galaxy Messier 33 (yellow and blue points with error bars), and a predicted one from distribution of the visible matter (grey line). The discrepancy between the two curves can be accounted for by adding a dark matter halo surrounding the galaxy
Left: A simulated galaxy without dark matter. Right: Galaxy with a flat rotation curve that would be expected under the presence of dark matter.
The dark matter itself is believed to be a little more stationary than normal matter. It does have motion, but it’s not the same circular motion.
If you were moving through dark matter, and you could interact with it, you would feel it coming from a certain direction. If it was like a wind it would either blow you forward or blow you back. Using that sort of understanding should help to determine that what is being detected is actually dark matter. For instance, if scientists see a spike in the observations at a certain time of day when the Earth is in a certain position in its orbit, but this spike disappears when the Earth moves, then this should help confirm that the signals that the scientists are seeing have come from WIMPs i.e., dark matter.
2) Are the general public allowed to visit the lab?
The simple answer is no, not really. They have in the past taken the general public underground on organized visits but you can’t just drop in, it has to be very organized. It’s usually organized for an interested party for some reason.
And at the moment during Covid-19 they’re absolutely not allowed any visitors on site at all.
The mine has very strict rules because it is owned by a company that produce fertilisers. One of the rules is that nobody under the age of 18 is allowed underground. Anybody older than 65 have to have medical tests for safety reasons.
3) As the detectors are in motion with the Earth and the galaxy is dark matter static or does it have its own proper motion.
It does have motion. But the key thing is that it doesn’t have a kind of generalized motion. The motion of the dark matter is going to be much more random. If you look at a random star in the Milky Way it would have motion that kind of correlated with all the rest of the stars because they’re all moving in roughly the same direction, at varying speeds, depending on their distance from the centre, whereas a random single dark matter particle could be moving in pretty much any direction within the dark matter Halo or sphere that surrounds the Milky Way.
Scientists can use the bulk properties of fluids to model dark matter and by looking at the average properties of all dark matter particles they can figure out om what direction the dark matter is mostly coming from. But in general, the particles don’t have a specific direction.
4) How long are the mine tunnels?
It’s a very old mine. They first started digging the first rock shaft in 1968. It is a very big mine at around 10 kilometres from end to end and about five kilometres wide. It has been stated that the company that own Boulby used to mine 40km of material per year
The mine is a bit different now because they have changed the way they mine. About eight kilometres of this mine is actually underneath the North Sea, and it’s not flat. So, when the depth is given as 1.1 kilometres that is simply the distance between the surface and the bottom of the lift. The North of the mine, which is under the sea, is about 800m down and in the South it’s about 1.4 kilometres down. In fact, most of the tunnels are out under the sea.
5) You have some nice lab accommodation. What happens if you have a power cut?
In the lab itself there are some emergency lights so the scientists can still see. They also have lamps which they carry all the time. Some of the most important equipment have battery backups that last about 12 hours. If it’s a small power cut, they just switch off all unnecessary equipment until the power switches back on. If it’s a big power call then they’re in pretty much the same boat as everybody else. They can’t do anything.
The mine has some pretty cool safety equipment. If there’s a huge power cut, for instance, the normal winder stops working and the cage can’t run up and down. But they have big emergency generators and an emergency winder that can be brought in to get people out
6) Are you allowed to sleep in the mine?
Not really. There’s a very old odd bylaw that exists that says it’s actually illegal to sleep underground and there are very good safety reasons for this. The main reason is because it’s very dark. You could easily get run over or lost.
However, in a few years, the mine is hoping to be able to host a bunch of scientists and astronauts, who are going to use the mine and the Mars yard, in particular, as a test habitat, as if they were going to live on another planet. It would only be for about two weeks but in that time the pretend astronauts will be hoping that they can sleep. The team working on this project are doing some work on arranging everything. But in general. No. Sleeping underground is not allowed.
7) Would you like to sleep underground.
Emma: I have no desire to sleep underground When we’re going underground, we have to take all the water, food and everything else with us and I kind of like the idea of being a little bit closer to conveniences, but it would be kind of fun. I, guess, for one sleepover.
Ed: I’d have to say round about March this year when COVID was really taking off. I was hopeful that I could just stay down there the whole time. But they wouldn’t let me
8) What is your favourite thing that happens at the mine or what is your favourite research that you’ve been involved in?
Ed: I love the dark matter explorations. It is putting us at the forefront of science and I really enjoy the intersectional multi-disciplinary stuff that we do as well. So, particularly my partner is an archaeologist, and one of the experiments that we do is actually looking at radio carbon dating and we are hoping to improve the accuracy of radio carbon dating. That’s just a really nice little kind of crossover that you wouldn’t necessarily expect. It’s just different fields coming together.
Lauren: It’s not just scientists, there are all sorts of people, not just physicists but biologists, engineers and technicians all working together.
Emma: It’s a tough one for me. I manage the BUGS lab (Boulby Underground Germanium Suite) and it has evolved. Now there’s much more than germanium going on in there. It’s now known as the Boulby Underground screening facility now.
I absolutely love that because we can take absolutely anything. And we can profile it so we can understand what it’s made of by looking at the different radioactive signatures that come from it. So, the gammas, the alphas can really help to understand things. I absolutely love that and unlike the dark matter experiments, we get our data, pretty much straight away. If you want to run a dark matter experiment, you’re going to turn it on, but you’re not going to really see any data for two years. It’s really slow and kind of a weird process. But the other thing I really love about the mine and working here is the mineralogy. The rock salt that we’re in is 230 million years old.
It was originally salt solution and the water evaporated.
The above image shows a bit of salt which has got some inclusions in it. It’s got some liquid trapped in it and one thing I learned recently from Professor Aaron Celestiana is that each of the salt crystals in the mine have very tiny amounts of liquid, but that liquid is 230 million years old. And inside that liquid are bacteria that are still alive. These micro-organisms, the halophiles are 230 million years old. It’s just immensely awesome.
There are all kinds of interesting stuff on the ground. So, you think, oh, it’s just salt. Salt sounds boring. Salt is not boring. It’s super cool. There are so many different things about the salt, and there’s so many different kinds of minerals that get added to it and change it in the potash, and probably phalite.
It is just a really cool place to be. And then, more recently, geologists found a fossilized tree in the polyhalite at the mine, fossilized 250 million years ago. It was really cool.
9) Do the minerals contain fossils?
So, we have found a fossilized tree. There are no animal fossils unfortunately. That time period and this place doesn’t have any animal tracks, which is kind of sad as there were some dinosaurs around.
10) Are all the minerals radioactive?
Yes, they are a little bit. There is some radioactivity that comes off the rock, which why there is lead and water shielding around some of the more sensitive equipment. But one other thing that we’re actually really lucky about is that a lot of mines have to worry about radon.
Radon is a chemical element with the symbol Rn and atomic number 86. It is a radioactive, colourless, odourless, tasteless noble gas. It occurs naturally in minute quantities as an intermediate step in the normal radioactive decay chains through which thorium and uranium slowly decay into lead and various other short-lived radioactive elements. Radon itself is the immediate decay product of radium. Its most stable isotope, 222Rn, has a half-life of only 3.8 days, making it one of the rarest elements.
The problem with radon is it heavier than air so it will sink to the bottom of a mineshaft and give off radiation but because of the natural geology of the area of East Cleveland, where Boulby is, the radon levels are quite low and we don’t have to worry about it too much. Some of the other underground labs around the world have huge massive systems that are trying to extract all of the radon from the air because otherwise it will be giving off more radiation than you really want. We do still do that in the in the BUGS facility in order to try and get absolutely zero radon, but the amounts of radon are not dangerous.
Before we actually build and run these detectors, we do a lot of computer simulations and so in order to have accurate computer simulations we do need to understand the radioactivity of the rocks and the gas in the air.
11) How does one determine the extent of gravitational lensing?
The short version of that answer is general relativity. So, Einstein’s equations allow you to work out how much space-time is warped by mass. So, the heavier, something is the more it’s going to have an effect. It’s sort of like having a weight on a trampoline, which is going to distort all of space around it and the heavier it is the more distortion it creates. Light wants to follow a straight line but if the space itself is curved, then the straight line that light is following is actually not going to be a straight line but is going to be curved, around the mass. That’s how we work it out using lots and lots of maths.
12) Is there anything else that you’ve got a burning desire to tell us about the underground lab that you haven’t told us yet?
Emma: It’s just worth saying that the kind of science that we do here is very widespread. We have lots of visitors here from lots of different universities all over the world and coming to us to do science and there are only around about 10 facilities in the world who are underground like us, some of them in mines and some of them are under mountains. It makes us a very specialized place to be, which means we get to meet a lot of cool people and do lots of really cool things.
And, you know, to work in this kind of industry, you don’t necessarily have to follow the traditional path. You don’t have to go to university and have a PhD. Although lots and lots of people do. And that’s really cool but I don’t have a PhD. I’m the Senior science technician here at I run lots of the experiments here and get to work with lots of awesome people
We have people on site here who care about all kinds of things. So, in fact if anyone was thinking of wanting to work in this really, really awesome place but feel like that they are not brave enough to go and do a PhD because it is pretty scary, but pretty cool. Don’t be put off. There are still opportunities available and still lots of awesome stuff to do. I just want to quickly show you something I found.
This is called Boracite.
Boracite is a magnesium borate mineral with formula: Mg3B7O13Cl. It occurs as blue green, colourless, grey, yellow to white crystals in the orthorhombic – pyramidal crystal system.
This is a really rare blue mineral and it’s just epic.
Ed: Passion is the absolutely most important thing for a job in science or in anything else. Just, just be passionate and interested about whatever it is that you’re into, and you’ll go a long way.