Discovering Density

Dr Preeti Kaur


Dr Kaur works at the ISIS Neutron and Muon Source and part of her talk involved a short tour of the facility


ISIS ISIS 360 Tour (

The ISIS Neutron and Muon Source is a pulsed neutron and muon source, established 1984 at the Rutherford Appleton Laboratory of the Science and Technology Facilities Council, on the Harwell Science and Innovation Campus in Oxfordshire, United Kingdom. It uses the techniques of muon spectroscopy and neutron scattering to probe the structure and dynamics of condensed matter on a microscopic scale ranging from the subatomic to the macromolecular.

Hundreds of experiments are performed every year at the facility by researchers from around the world, in diverse science areas such as physics, chemistry, materials engineering, earth sciences, biology and archaeology

It’s called ISIS for two reasons: it’s named after the local river. The River Thames is called ISIS when it runs through Oxfordshire. It is also named after the Egyptian goddess of recreation and rebirth.


ISIS is kind of super microscope which scientists use to look at lots of different types of materials.

It can look at the microscopic structure of the materials, even looking at the atoms that make up the materials.

An atom is the smallest unit of ordinary matter that forms a chemical element.

Atoms are everywhere, everything is made up of atoms. So, what’s inside an atom.


The above image is a simple diagram of an atom, In the middle are protons and neutrons, which make up the nucleus. Orbiting the nucleus are the electrons.

The atomic nucleus is the small, dense region consisting of protons and neutrons at the centre of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment.


Ernest Rutherford, 1st Baron Rutherford of Nelson, OM, FRS, HonFRSE (30 August 1871 – 19 October 1937) was a New Zealand-born British physicist who came to be known as the father of nuclear physics.

The Geiger–Marsden experiments (also called the Rutherford gold foil experiment) were a landmark series of experiments by which scientists learned that every atom has a nucleus where all of its positive charge and most of its mass is concentrated.

Protons and neutrons are very useful at ISIS.

A proton is a subatomic particle, symbol p or p+, with a positive electric charge of +1e elementary charge and a mass slightly less than that of a neutron.

The neutron is a subatomic particle, symbol n or n0, which has a neutral (not positive or negative) charge and a mass slightly greater than that of a proton.

The electron is a subatomic particle, symbol e− or β−, whose electric charge is negative one elementary charge.

The scientists at ISIS accelerate protons in the particle accelerator.

A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams.



The particle accelerator is under a mound, which can be seen in the above image


The accelerated protons are fired into targets to make neutrons within the experimental halls. As seen above.

Neutrons are used for lots of different experiments and lots of different types of research.

ISIS looks at things in nature such as spider silk, which is a very strong material. It’s five times as strong as steel.

How is it that a spider can make this very strong material? So, the scientists at ISIS study it to find out.


ISIS also studies polar bear hair. Polar Bear skin is black, but its hair looks white even though it is actually clear and colourless. This is because of a phenomenon in nature, called structural colour. The scientists want to try and understand how it is that polar bear hair looks white and, perhaps, apply what they find to things like more environmentally friendly paints

They use neutrons to look at structural medicines. They’ve looked at treatments for diseases and infections and even treatments to help premature babies who can’t breathe properly.

Medicinal chemistry and pharmaceutical chemistry are disciplines at the intersection of chemistry, especially synthetic organic chemistry, and pharmacology and various other biological specialties, where they are involved with design, chemical synthesis and development for market of pharmaceutical agents, or bio-active molecules (drugs).

They use neutrons to look at engineering components such as airplane wings, turbine blades, train wheels and pipelines.

They use neutrons to look at medical implants, archaeological and historical artifacts such as ancient violins and bronze statues.

They use neutrons to look at clean energy materials in order to make better solar cells and batteries.

In fact, neutrons are used to investigate lots and lots of different things that we use in our everyday lives. They are being used to study their material properties in order to understand their behaviour and make them better.

The tour


The parts of the accelerator are 2, 4, 5 and 6.

A linear particle accelerator (often shortened to linac) is a type of particle accelerator that accelerates charged subatomic particles or ions to a high speed by subjecting them to a series of oscillating electric potentials along a linear beamline. The principles for such machines were proposed by Gustav Ising in 1924, while the first machine that worked was constructed by Rolf Widerøe in 1928 at the RWTH Aachen University. Linacs have many applications: they generate X-rays and high energy electrons for medicinal purposes in radiation therapy, serve as particle injectors for higher-energy accelerators, and are used directly to achieve the highest kinetic energy for light particles (electrons and positrons) for particle physics.

The design of a linac depends on the type of particle that is being accelerated: electrons, protons or ions. Linacs range in size from a cathode ray tube (which is a type of linac) to the 3.2-kilometre-long linac at the SLAC National Accelerator Laboratory in Menlo Park, California.


Animation showing how a linear accelerator works. In this example the particles accelerated (red dots) are assumed to have a positive charge. The graph V(x) shows the electrical potential along the axis of the accelerator at each point in time. The polarity of the RF voltage reverses as the particle passes through each electrode, so when the particle crosses each gap the electric field (E, arrows) has the correct direction to accelerate it. The animation shows a single particle being accelerated each cycle; in actual linacs a large number of particles are injected and accelerated each cycle. The action is shown slowed enormously. (below left)


Gustaf Ising (19 February 1883 in Finja – 5 February 1960 in Danderyd), was a Swedish accelerator physicist and geophysicist. (Above right)

Rolf Widerøe (11 July 1902 – 11 October 1996) was a Norwegian accelerator physicist who was the originator of many particle acceleration concepts, including the resonance accelerator and the betatron accelerator.

A synchrotron is a particular type of cyclic particle accelerator, descended from the cyclotron, in which the accelerating particle beam travels around a fixed closed-loop path. The magnetic field which bends the particle beam into its closed path increases with time during the accelerating process, being synchronized to the increasing kinetic energy of the particles. The synchrotron is one of the first accelerator concepts to enable the construction of large-scale facilities, since bending, beam focusing and acceleration can be separated into different components. The most powerful modern particle accelerators use versions of the synchrotron design.


The synchrotron principle was invented by Vladimir Veksler in 1944. Edwin McMillan constructed the first electron synchrotron in 1945, arriving at the idea independently, having missed Veksler’s publication (which was only available in a Soviet journal, although in English). (below left)


Vladimir Iosifovich Veksler (March 4, 1907 in Zhytomyr, Zhytomyr Oblast, Ukraine – September 22, 1966 in Moscow, USSR) was a prominent Soviet experimental physicist. (Above right)

Edwin Mattison McMillan (September 18, 1907 – September 7, 1991) was an American physicist and Nobel laureate credited with being the first-ever to produce a transuranium element, neptunium. For this, he shared the Nobel Prize in Chemistry with Glenn Seaborg in 1951.

Isis has two experimental halls which contain the instruments that study lots of different materials.



Target Station 2 this is one of the very large experimental halls.


The muon is an elementary particle similar to the electron, with an electric charge of −1 e and a spin of 1/2, 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.


The 360o degree tour will give you an idea of how big the facility with lots going on in here. They’ve got lots of scientific equipment, engineering equipment and science labs.



There are scientific neutron instruments and you can just about make out two in the bottom in the above image, coloured pink one and blue. There are eleven of them instruments in this experimental hall.

There is another experimental hall filled with even more neutron instruments and there is the particle accelerator.

The place is huge. It’s about size of six football pitches and you need a very, very big facility like this to make the very, very small particles.

ISIS accelerates protons to make neutrons and those beams of neutrons are used to examine materials all the way down to their atoms, i.e., 10,000 million times smaller than we are.

ISIS is like a super microscope to study different types of materials, such as clean energy materials, medicines, food (like ice cream, chocolate and artificial meat) engineering components, ecological and archaeological artifacts.


The above left image shows one of the engineers working on the ion source (above right).

The ion source is a very important part of the facility. If this bit doesn’t work, none of ISIS works. Plasma is produced in the ion source.

Plasma is one of the four fundamental states of matter, and was first described by chemist Irving Langmuir in the 1920s. It consists of a gas of ions – atoms which have some of their orbital electrons removed – and free electrons. Plasma can be artificially generated by heating a neutral gas or subjecting it to a strong electromagnetic field to the point where an ionized gaseous substance becomes increasingly electrically conductive. The resulting charged ions and electrons become influenced by long-range electromagnetic fields, making the plasma dynamics more sensitive to these fields than a neutral gas.


Irving Langmuir (31 January 1881 – 16 August 1957) was an American chemist, physicist, and engineer. He was awarded the Nobel Prize in Chemistry in 1932 for his work in surface chemistry.

Plasmas are very abundant in the Universe. Our Sun is made up of plasma as are all stars, it’s created in lightning and plasma balls.


It’s just another state of matter like solids, liquids and gases. Plasma is like a gas. It’s what you get when you superheat a gas.


Above left is an image of the plasma. There is a very bright line in the middle. It’s very small, only about the size of a fingertip, but it’s as hot as the surface of the sun (up to 40,000 degrees Celsius).

The light from the ion source (the plasma) passes through a window (that can be seen in the above left image). The light is separated and rainbow rings are formed.

Some of the particles in the plasma are accelerated.


The LINAC tunnel, which is short for linear accelerator and that’s what can be seen on the right-hand side of the above image. It is the blue structure that extends all the way down. It’s about 40 meters long,


If the linear accelerator were opened it would look like the above right image. It’s made up of lots and lots of structures called copper drift tubes, which are used to accelerate the particles to about 37% of the speed of light (that’s about 250 million miles per hour).

It is very fast, but not fast enough. Which is why the synchrotron is needed.


To get to the synchrotron, you have to pass through the security gates. These are needed because there are lots of hazards associated with the synchrotron. One of these is radiation when the synchrotron is running.

There are high currents, voltages and magnetic fields. All of these need to be powered down before anyone goes inside.

Once all of the power is down you can enter the gates. You need to speak the people in the main control room to gain permission to access the synchrotron.


The above images show the synchrotron

It’s made up of lots and lots of bits of kit and we have some bits that accelerate our protons. Some bits accelerate the protons.




Some bits focus the protons. These bits are magnets (above left).




The synchrotron is fairly big It’s about 163 metres in circumference although not as big as the Large Hadron Collider that some of you may have heard about that one, which is about 27 kilometres.

The Large Hadron Collider (LHC) is the world’s largest and highest-energy particle collider and the largest machine in the world. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and hundreds of universities and laboratories, as well as more than 100 countries. It lies in a tunnel 27 kilometres in circumference and as deep as 175 metres beneath the France–Switzerland border near Geneva.

The ISIS synchrotron is big enough to produce the energies needed.

As well as having things that accelerate protons magnets are needed to focus them.


Magnets are used the bend the paths of the protons. They are the big yellow things in the above left image. These are necessary to keep the charged particles moving in a circular path. They are very heavy and each one has a mass of about 30 tonnes (that’s as much as the mass of six elephants).


In the particle accelerator the protons are accelerated up to 84% of the speed of light (560 million miles per hour). At that speed they could travel six times around the world in one second.

Once they have reached the required speed the protons leave the particle accelerator to make the neutrons.


The inner synchrotron

Like people, electronics don’t like radiation so a shield is needed.



The above images show lots of different types of electronic systems. Some are there to monitor the proton beam in the particle accelerator and it’s all shielded by lots of lots of concrete and steel.


The capacitor room.

A capacitor is a device that stores electrical energy in an electric field. It is a passive electronic component with two terminals.

The effect of a capacitor is known as capacitance. While some capacitance exists between any two electrical conductors in proximity in a circuit, a capacitor is a component designed to add capacitance to a circuit.

Most electrical appliances have a capacitor in them and ISIS has lots and they are very big. They used to power the magnets in the synchrotron.

Those very big magnets are very powerful and use a lot of energy. So, a way of storing and releasing that energy quickly when needed is required. These capacitors are used to store this energy.


The choke room

The big red/brown things are called chokes and are also used to store and release energy.

An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a coil.

A choke is an inductor designed specifically for blocking high-frequency alternating current (AC) in an electrical circuit, while allowing DC or low-frequency signals to pass. Because the inductor restricts or “chokes” the changes in current, this type of inductor is called a choke. It usually consists of a coil of insulated wire wound on a magnetic core, although some consist of a donut-shaped “bead” of ferrite material strung on a wire. Like other inductors, chokes resist changes in current passing through them increasingly with frequency. The difference between chokes and other inductors is that chokes do not require the high Q factor construction techniques that are used to reduce the resistance in inductors used in tuned circuits.

The chokes and capacitors are used to store and release energy to power those very, very big, powerful magnets that 1re used the synchrotron.

ISIS uses a lot of electricity. It consumes about the same amount of electricity as a small town. This sounds like a lot and it is a lot. But it is needed to be able to do all the amazing science and engineering that will eventually make our everyday lives better and save money.

It makes processes better for the environment.


The manipulator room.


Through that window you can just make out some bits of equipment. That is where the neutrons are made.

It is the accelerating protons that make the neutrons.



Underneath where the circle cursor is hovering is the target, which is made up of a lump of tungsten. Tungsten is very dense.

Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively combined with other elements in chemical compounds. It was identified as a new element in 1781 and first isolated as a metal in 1783. Its important ores include scheelite, and wolframite, lending the element its alternate name.

The target is fairly small. It is only about the size of a packet of biscuits, but it’s very heavy and contains a lot of neutrons.

When you fire protons into tungsten you get lots and lots of neutrons. And that is what is needed for the experiments.

However, when you fire protons into something like tungsten, it becomes very radioactive. So that means any kind of maintenance work that needs to be done on the target or the kit that surrounds it has to be done remotely.

Because high levels of radioactivity aren’t very good for us, we need to protect ourselves from it. That is what the concrete and steel is for.

Any kind of maintenance has to be done remotely. The operator is behind a very thick window of lead glass and uses manipulator arms as shown in the images below.


The above images show a pair of master arms. Inside the room would be a pair of slave arms. So, an engineer would operate the master arms to drive the slave arms to do any kind of maintenance on the target remotely.



Above left shows an engineer using the manipulator arm. He’s standing in a room that has very low radiation levels because there’s lots of shielding due to lots of concrete and steel. He is looking through a thick window of lead glass so he can see the target and see what he is doing.

He is doing all his movements via those manipulator arms.


This is IMAT.


A neutron imaging and diffraction instrument for materials science, materials processing and engineering.

The protons have been accelerated, have hit a target and have produced the neutrons, needed for research.


The neutrons travel down the apparatus shown above.


The neutrons strike a sample, that would be mounted on the apparatus as shown above (where the circular cursor is), and the instrument scientist is be able to look at how those neutrons have passed through or scattered through the sample.

IMAT is a really interesting instrument because it works in the same way as the X-ray machine that would provide an image of a broken arm in a hospital. But it uses neutrons instead of X-rays.

So, you can look inside lots of different things without having to break them open. ISIS have used neutrons to look at things like ancient violins, bronze buddhas, ancient swords and a 3000 year old Egyptian vases.


Neutrons were fired into the vases and IMAT imaged the insides to see what was there without them having to be broken.

Archaeologist, who were studying these vases, got some clues about the seven sacred oils of Egypt.

Ancient Egypt was a civilization of ancient North Africa, concentrated along the lower reaches of the Nile River, situated in the place that is now the country Egypt. Ancient Egyptian civilization followed prehistoric Egypt and coalesced around 3100 BC (according to conventional Egyptian chronology) with the political unification of Upper and Lower Egypt under Menes (often identified with Narmer). The history of ancient Egypt occurred as a series of stable kingdoms, separated by periods of relative instability known as Intermediate Periods: the Old Kingdom of the Early Bronze Age, the Middle Kingdom of the Middle Bronze Age and the New Kingdom of the Late Bronze Age.

Archaeology is the study of human activity through the recovery and analysis of material culture.

Archaeologists study human prehistory and history, from the development of the first stone tools at Lomekwi in East Africa 3.3 million years ago up until recent decades.

As well as archaeological artifacts, plants can be examined on this instrument. Botanists can see how plants take up water through the roots and how that water moves through them.

It can be used to examine things like airplane wings and train wheels. To see if the materials, that these items are made of, have any weaknesses.

So, this instrument is used to study lots of different materials, including a tooth that one of the scientists had removed. It was one of the first samples to be analysed on this instrument.

The next instrument is POLREF

POLREF is a general-purpose polarised neutron reflectometer designed for the study of magnetic and non-magnetic buried interfaces and surfaces.



POLREF is a reflectometer and it is used to look at surfaces and interfaces.



The sample is placed between the magnets, indicated by the circular cursor in the image above.


The circular cursor, above, is indicating the detector, which can swing around to “look” at the neutrons that get scattered off the sample at different angles.


As mentioned before, POLREF is a reflectometer and is used to study surface interfaces and layers. For example, it is used to study the layers in solar cells.

A solar cell, or photovoltaic cell, is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.

POLREF is also used to study the layers in a computer hard drive.

A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage and one or more rigid rapidly rotating platters coated with magnetic material. The platters are paired with magnetic heads, usually arranged on a moving actuator arm, which read and write data to the platter surfaces. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored and retrieved in any order. HDDs are a type of non-volatile storage, retaining stored data even when powered off

The hard drives in a computer have thin magnetic fields arranged in layers in the data storage systems

POLREF can even examine layers in bacteria.


The above left image is a model of the layers found in the membrane of bacteria and scientists are very keen to study it in order to see how antibiotics interact with it. Antibiotics are very important in killing off the bad bacteria.

This is important because if you get a bacterial infection you want antibiotics to treat it.

Unfortunately, some of these bacteria have become resistant to antibiotics, so scientists want to make new, better and more efficient antibiotics, Studying, how antibiotics interact with the bacterial membranes can help with this.

Some of the ISIS scientists made a model of the bacterial membrane and they’re looking at how they can use this model to test antibiotics and make better antibiotics for us.

The next instrument involves particles called muons. As well as neutrons ISIS makes muons.

Muons exist in nature in cosmic rays.

Cosmic rays are high-energy protons, atomic nuclei and other high energy particles 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.


Victor Franz Hess (24 June 1883 – 17 December 1964) was an Austrian-American physicist, and Nobel laureate in physics, who discovered cosmic rays.


Primary cosmic particle collides with a molecule of atmosphere.

Cosmic rays hitting the Earth’s atmosphere generate showers which have lots of particles, including muons.

ISIS makes its own muons. Protons are fired into a very small piece of graphite and the resultant muons are used to look at lots of different types of materials, just like neutrons.

Graphite archaically referred to as plumbago, is a crystalline form of the element carbon with its atoms arranged in a hexagonal structure. It occurs naturally in this form and is the most stable form of carbon under standard conditions.



Muons are very short lived (they only live for 2 millionths of a second), but in that time they can be used to investigate lots of different things such as magnetic materials, battery materials and even some very special materials used in superconductors.

Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike an ordinary metallic conductor, whose resistance decreases gradually as its temperature is lowered even down to near absolute zero, a superconductor has a characteristic critical temperature below which the resistance drops abruptly to zero. An electric current through a loop of superconducting wire can persist indefinitely with no power source.

So, muons are used in lots of very interesting research.

Neutrons and muons are used to look at lots of different materials in order to study their material properties. This will enable scientists to make better materials.

Questions and answers 1

1) How often does the tungsten target need to be replaced?

The tungsten target lasts about five years, sometimes a little bit less. In order to make it last longer it is coated with tantalum.

Tantalum is a chemical element with the symbol Ta and atomic number 73.

Firing protons into the tungsten target makes it get very, very hot so it is made to cool down. This causes it to corrode a little. Coating it in tantalum reduced the corrosion.


As ISIS is used to investigate material properties the talk continued by focusing on an important material property, known as density.


Rory, who continued the talk, is a student at Manchester University.

What is density

At the simplest level it is how much stuff is packed into a particular volume.

Small objects with a large mass have a high density.


Small objects, like the little balls in the above left image, can be quite heavy when you hold them. So, these are classed as high-density objects. They have a higher density than something like a sponge, which is bigger, but doesn’t feel as heavy when you hold it. This means the sponge has a lower density.

Large objects with very little mass have low density.


Density is a property of a material.

Comparing densities

Below is an example of how densities of different things can be compared.

Do the fireflies in jar 1 or 2 have more room to move around?


Two identical jars with fireflies in them. Which one has the higher density of fireflies?

The answer is jar 2.


The above are pictures of two different forests. The one on the left is pretty lush, there’s lots of trees and plants there. It has a greater density of plants and trees than the one on the right, which is quite bare. There’s only a handful of trees and they’re all very spread out.

Looking at the fireflies and forests it can be quite obvious which one has the greatest density, but sometimes it’s not so obvious when looking at some materials.

If you have two lumps of metals, for example, that have the same size. Just looking at them won’t give you much of an idea of which is densest.

So, we need to use some maths, which will give density a number and that becomes very useful when you’re doing equations.

Calculating density

Giving density a number allows objects to be compared. How is this done? Well, you need the following equation


To work out the density you divide the the mass of an object by its volume. Both these things can be measured.

The mass can be found using the sort of scales you would find at home


There are a couple ways of finding the volume. If the object has a regular shape, like a cube, you can find its volume by measuring its length, width and height, and multiplying these values together.


If the object has an irregular shape such as a grapefruit or a piece of dried lemon slice it would be hard to decide which would have the greatest density.


If you just want to see which is the densest you can put them in water. There is a useful rule in physics that tells you that whether an object floats or sinks is dependent on the value of its density. If the object has a lower density than water it will float in water. If its density is greater than water it will sink.

So, to find out which is the densest of the grapefruit or dried lemon slice you just put them in water.

And the result is rather surprising. The grapefruit floats and the dried lemon slice sinks.


It’s surprising because the grapefruit is bigger and heavier and the first thing you would think is that it would sink. But if you work out the densities of the grapefruit and the dried lemon slice you would find that the grapefruit has a lower density than water and the dried lemon slice has a higher density than water.

If you want to find out what the actual densities are you would find their masses in the same way as before but to find their volumes you need another method of finding the volume, because the length, width and height of the objects are not constant.

The method of finding the volume uses the behaviour of objects in water and you need to apply something called Archimedes principle.

The Archimedes’ principle states that any object immersed in a fluid is acted upon by an upward, or buoyant, force equal to the weight of the fluid displaced by the object.

When an object is dropped into water, some of the water is displaced. This means the water rises upwards as the object or person pushes water out of the way.

At the same time buoyancy is pushing up the object which changes its weight. If the weight of the object in the water is heavier than the amount of water displaced the object will sink!

If the weight of the object and amount of water displaced are the same or the weight is less, the object will float.

Now if you want to find the volume of the odd shaped object it does need to be dense enough to be completely under the surface of the water.

If the object is totally submerged it will have displaced a quantity of water equal to its own weight


As weight = mass x gravity and gravity, on Earth is constant you can say that the quantity of water displaced by the object is equal to the mass of the object.

Now water has a convenient density of 1g per cm3 (or 1g per ml) meaning 1cm3 of it has a mass of 1g,

Therefore, the mass of the water displaced is numerically equal to the volume of water displaced and that volume equals the volume of the object. The above image shows how you can collect the displaced volume of water

So, the density of an odd shaped object = mass/density as before (although remember that you only get the full volume of the object if it is fully submerged in the water).


Archimedes of Syracuse (c. 287 – c. 212 BC) was a Greek mathematician, physicist, engineer, inventor, and astronomer.

So, the density of the grapefruit is less (it floats in water) that the density of the dried lemon slice (it sinks in water).

Density doesn’t depend on how big or how heavy an object is. It’s just a property of it. So, no matter how big or how small an object is it’s going to have the same density (providing the temperature does not change).

Liquids also have densities and like solids they can have different densities. You can demonstrate these different densities with a little bit of kitchen science.

Making a liquid density tower

Pour equal amounts of each liquid in the cups. All the amounts must be the same. You may want to set the cups in the order you’ll add them into the container: honey, corn syrup, maple syrup, whole milk, dish soap, water, vegetable oil, rubbing alcohol, and lamp oil. Add food colouring to the water and the rubbing alcohol for contrast so they stand out in the finished column.



The above link is a video showing how you can make a density tower using 13 different liquids.


Each liquid needs to be added as slowly as possible because the liquids shouldn’t touch the side of the glass (although don’t worry if it does). To help with pouring, pour the liquids over the back of a tablespoon.

The honey needs to be poured first (about 2 fingers worth)


Then add washing up liquid using the back of the spoon to help with pouring.


The order of the liquids is important as you need to start with the densest one. The honey is denser than the washing up liquid. With careful pouring the washing up liquid should just sit on top of the honey.


The next step is to add water (with a bit of food colouring to show it up).


Then add the oil


There are some bubbles because of the washing up liquid. It should settle down after about 5 minutes. A gap at the top is useful to stop overspill later.

For fun add some solid objects of different densities.


The above image shows a cherry tomato sitting in the washing up liquid layer on top of the water.

Drop a coin on the opposite side to the tomato as you don’t want them clashing. The coin sinks to the bottom. It is denser than all the liquids.


The next object is a mini marshmallow. Unfortunately, it was masked by bubbles.

Next object is two pistachio nuts. One with a shell and one without.


The above image shows the shelled pistachio is in the oil layer. The unshelled pistachio ended up in the water layer. The pistachio without a shell has gone further, which means the shell has affected its density.

This shows that even if you’ve got the same piece of nut (or similar food) the presence of a shell can affect the density. The shell makes the pistachio more buoyant as it has trapped some air.

Don’t forget to clean up.



The above image gives the density values for the different liquids. The density of a liquid is found using the same method as for a solid.

Oil has the smallest density (0.9g/ml) which is why it sits at the top and honey has got the largest density (1.42g/ml), which is why it goes to the bottom.

Things to think about …

What happens if you mix your density tower? Would the layers separate back out or remain stuck together?

What would happen if you add the liquids in a different order (In the experiment the liquids were added in order of density)? For instance putting the oil in first.

What other combinations of liquids could you try? (chocolate sauce, milk, different oils, soft drinks).

Questions and answers 2

1) Please mix the layers


The mixture is quite a dark green colour. The honey is quite hard to mix because of how sticky it is compared to the rest of the layer. The mixture is then left for a bit. The far-right image above shows what the mixture looked like after five minutes. The layers are starting to reform because the liquids have different densities

2) Is ISIS doing anything on Covid-19 research?

ISIS isn’t right now, but we are looking at using neutron reflectometry to look at how the virus can find the human cells. You might have heard about the spike proteins on the virus. The spikes are how it binds to receptors on a human cell and neutron reflectometry will look at what happens during the interactions and what affect therapies and treatments have on these interactions.

So that’s something that we’re looking at doing but so other facilities on the Rutherford site have been looking at

Coronavirus disease 2019 (COVID-19) is a contagious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The first case was identified in Wuhan, China, in December 2019. It has since spread worldwide, leading to an ongoing pandemic.

Other facilities on the Rutherford site have been looking at COVID, specifically the diamond light source.

Diamond Light Source (or Diamond) is the UK’s national synchrotron light source science facility located at the Harwell Science and Innovation Campus in Oxfordshire. Its purpose is to produce intense beams of light whose special characteristics are useful in many areas of scientific research. In particular it can be used to investigate the structure and properties of a wide range of materials from proteins (to provide information for designing new and better drugs), and engineering components (such as a fan blade from an aero-engine) to conservation of archaeological artifacts (for example Henry VIII’s flagship the Mary Rose).

The Diamond light source has been using x rays to study the structure of the protein responsible for viral replication. Other facilities have been looking at things like Llama antibodies.

Antibodies derived from llamas have been shown to neutralise the SARS-CoV-2 virus in lab tests, UK researchers announced


The llama is a domesticated South American camelid, widely used as a meat and pack animal by Andean cultures since the Pre-Columbian era.


An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the pathogen, called an antigen. Each tip of the “Y” of an antibody contains a paratope (analogous to a lock) that is specific for one particular epitope (analogous to a key) on an antigen, allowing these two structures to bind together with precision. Using this binding mechanism, an antibody can tag a microbe or an infected cell for attack by other parts of the immune system, or can neutralize it directly (for example, by blocking a part of a virus that is essential for its invasion).

When these Llama antibodies (known as nanobodies) are produced they seem to bind to the Covid-19 spike proteins, which stops the virus binding to the human cell. This is ongoing research.

A single-domain antibody (sdAb), also known as a nanobody, is an antibody fragment consisting of a single monomeric variable antibody domain.


Ribbon diagram of a llama VHH domain.

There are lots of bits and pieces that being done around site and we’re looking at how we can use neutrons as well. But with the neutron science it is still very early days.

3) How do you use neutrons to look at ancient things?

ISIS used neutrons to investigate some bronze age swords.

The Bronze Age is a prehistoric period that was characterized by the use of bronze, in some areas proto-writing, and other early features of urban civilization. The Bronze Age is the second principal period of the three-age Stone-Bronze-Iron system.

An ancient civilization is defined to be in the Bronze Age either by producing bronze by smelting its own copper and alloying with tin, arsenic, or other metals, or by trading for bronze from production areas elsewhere. Bronze is harder and more durable than other metals available at the time, allowing Bronze Age civilizations to gain a technological advantage.

Good swords need to be denser in the areas where they will be used most in a battle.

Neutrons were targeted at a sample of the swords. The scattering pattern through the swords enabled the hardest bits to be identified. This way archaeologists were able to work out how they were used, well, in battle.

The good thing about neutrons is they don’t damage any of the ancient samples. You can use them to examine all types of bits and bobs that are found in the ground.

You can use neutrons in the same way that you can use X rays. They will give you an image of the internal structure of an object without damaging the object. This is really good for very precious and ancient samples.

Neutrons interact with matter differently than X-rays. X-rays interact primarily with the electron cloud surrounding each atom. The contribution to the diffracted x-ray intensity is therefore larger for atoms with a large atomic number (Z) than it is for atoms with a small Z. On the other hand, neutrons interact directly with the nucleus of the atom, and the contribution to the diffracted intensity is different for each isotope; for example, regular hydrogen and deuterium contribute differently. It is also often the case that light (low Z) atoms contribute strongly to the diffracted intensity even in the presence of large Z atoms. The scattering length varies from isotope to isotope rather than linearly with the atomic number. An element like Vanadium is a strong scatterer of X-rays, but its nuclei hardly scatter neutrons, which is why it often used as a container material. Non-magnetic neutron diffraction is directly sensitive to the positions of the nuclei of the atoms.

A major difference with X-rays is that the scattering is mostly due to the tiny nuclei of the atoms. That means that there is no need for an atomic form factor to describe the shape of the electron cloud of the atom and the scattering power of an atom does not fall off with the scattering angle as it does for X-rays. Diffractograms therefore can show strong well defined diffraction peaks even at high angles, particularly if the experiment is done at low temperatures. Many neutron sources are equipped with liquid helium cooling systems that allow data collection at temperatures down to 4.2K. The superb high angle (i.e. high resolution) information means that the data can give very precise values for the atomic positions in the structure.

X-rays and neutrons are fired into a sample in similar ways. How they are transmitted allows a picture of the internal structure to be found. ISIS has looked at lots of archaeological items such as Roman coins and gold rings from the Ashmolean museum. The composition was analysed.

The Ashmolean Museum of Art and Archaeology on Beaumont Street, Oxford, England, is the world’s second university museum (after the establishment of the Kunstmuseum Basel by the University of Basel in 1661) and Britain’s first public museum. Its first building was erected in 1678–1683 to house the cabinet of curiosities that Elias Ashmole gave to the University of Oxford in 1677.

4) How fast do your fastest particles go?

The protons travel at 87% the speed of light.

The protons are accelerated in our particle accelerator using electric fields and their paths are bent around the particle accelerator using magnetic fields. Their speed is so great that it could travel to Edinburgh and back in the blink of an eye.

5) Why does ice float on water?

This is a good question because people think a solid should sink. But there is something special about how water is structured on the molecular level. When it’s a liquid the molecules sort of come together held in place by weak hydrogen bonds.

The hydrogen bond is a weak electrostatic attraction force that arises between the molecules of the polar compounds as the water molecules, Hydrogen bond is responsible for the abnormal properties of the water, And it is weaker than the covalent bond.


Polar water molecules are linked together by hydrogen bonds

Water molecules can easily flow over each other as the hydrogen bonds are continuously breaking and reforming.

As water cools, its molecular motion slows and the molecules move gradually closer to one another. The density of any liquid increases as its temperature decreases. For most liquids, this continues as the liquid freezes and the solid state is denser than the liquid state. However, water behaves differently. It actually reaches its highest density at about 4°C.

When the temperature drops to the freezing point the hydrogen bonds become permanent, resulting in an interconnected hexagonally-shaped framework of molecules (a lattice). Because these bonds are stable the molecules can move further away from each other. This means that as ice forms it expands and takes up a greater volume than the original water.

As the volume is greater but the mass remains the same it means that ice is less dense than water



6) How long does it take to scan images when looking at the particles.

It depends on the sample and the resolution but it can take hours.

In normal circumstances, we have scientists coming from all around the world to use the instruments and they would normally apply for beam time that lasts for a few days, maybe up to a week and because it usually takes a few hours to get images from neutron scattering.

7) Was it expensive to build to build the ISIS neutron-muon source?

Yes, quite expensive. Target station 2 cost £140 million about 10 years ago.

When ISIS was first built in the 1980s (about 40 years ago) there was just the particle accelerator and one target station and it cost £50 million. So not cheap. But with all the neutrons and muons produced some amazing science has been done and the building costs have been recouped many times over. So, it is worth it.

8) If you have two like items then one is aerodynamically shaped does this affect the density?

If the two items are made of the same material then making one aerodynamic will not affect the density. This is because the density is a property of the material. In fact, aerodynamics won’t affect the density of a material. The reason why aerodynamic objects can “float” better than none aerodynamic objects is due to how fluids flow over their surfaces.

9) Why do the layers separate back out.


The mixture does separate back into layers and it has to do with the density. The layers work in the same way as solids. The coin is denser than the tomato so it will sink further down.

Even when you mix all the layers of the liquids together. They still have different densities. So even though it takes time they will gradually separate out.

10) We’ve just had one thing in the chat about the aerodynamic shaped object and whether it affects the density. It does affect its ability to float. It might end up not floating on water when you would affect it to sink. But the density should be the same.

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