Birmingham July 2012

Friday 6th July

The first lecture of the day was given by Dr Christina Lazzeroni on cloud chambers.

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Dr Lazzeroni started her lecture by setting up a cloud chamber which used a fish tank and low sodium salt (which is mildly radioactive).

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Putting dry ice into the insulated base of the bubble chamber.

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Putting the fish tank onto the dry ice filled base.

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Adding propan-2 ol to the top of the fish tank. The little heater at the top of the fish tank and the dry ice at the bottom provide a temperature gradient. The diffusion cloud chamber creates a volume of supersaturated alcohol vapour that condenses on ions left by the charged particles emitted from the radioactive decay of the low sodium salt. This is accomplished by establishing a steep vertical temperature gradient with dry ice. Alcohol evaporates from the warm top side and diffuses toward the cold bottom. The gravitationally stable temperature distribution permits a layer of supersaturating near the chamber bottom. Charged particles passing through the supersaturated air at close to the speed of light leave behind numerous ions along each centimetre traversed. Since each ion becomes a nucleation site for droplet condensation, tracks of alcohol droplets form in this region, indicating trajectories of the charged particles. The fine, threadlike tracks fall to the chamber bottom, leaving room for other tracks to appear in the next moment. Even if the salt hadn’t been added tracks would be seen due to the presence of cosmic rays.

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The blackout cover is to keep some of the outside light out.

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The cloud chamber was invented by Wilson in 1911 to study cloud formation and optical phenomena in moist air. He very rapidly discovered that ions could act as centres for water droplet formation in such chambers. Wilson cloud chambers were based on adiabatic expansion (a gas expands without energy being added or taken away from it).

The diffusion cloud chamber was developed in 1939 by Alexander Langsdorf. The chamber differs from the expansion cloud chamber in that it is continuously sensitized to radiation, and that the bottom must be cooled to a rather low temperature (e.g. dry ice).

The bubble chamber was invented by Donald Glaser in 1952. It reveals the tracks of subatomic particles as trails of bubbles in a superheated liquid (usually liquid hydrogen). As they can be much bigger than cloud chambers and since they are filled with a much-denser liquid they reveal the tracks of much more energetic particles.

Whilst the cloud chamber was set up Dr Lazzeroni began by going through what we now know about the atom.

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The standard model is not complete. One of the differences between quarks and leptons is that quarks clump together and leptons don’t. Muons and low level electrons can be seen due to their tracks in the cloud chamber. The muon tracks are straight,

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Three quarks together produce a baryon such as the proton or neutron. A quark and antiquark produce a meson. A lepton such as the electron is a fundamental particle (as far as we know).

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Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle: 2 protons and 2 neutrons. The atom is transformed into a new element with a nucleus of mass number 4 less and atomic number 2 less.

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An alpha particle has a typical energy of 5MeV (speed of 0.05 times the speed of light. It has a relatively large mass, a +2 charge and low velocity. It is very likely to interact with other atoms and lose energy and is stopped within a few centimetres of air (low range). It has low penetrating power and high ionization ability. It produces short intense tracks in the cloud chamber.

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Cloud chamber with visible tracks from ionizing radiation (short, thick: α-particles; long, thin: β-particles).

During beta decay a nucleus is converted into its next-higher neighbour on the periodic table whilst emitting an electron an anti electron neutrino.

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A beta particle can be an electron (-1 charge) or positron (+1 charge). It has a small mass and generally a low velocity. Having only a single charge it has a small ionising ability, a long range and a medium penetrating power. In a cloud chamber you would expect to see a thin, wiggly track as the particle is easily scattered.

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Cosmic rays are energetic charged subatomic particles originating in outer space. They may be broadly divided into two categories: primary and secondary. The cosmic rays that originate from astrophysical sources are primary cosmic rays; these primary cosmic rays interact with stellar matter (in the upper atmosphere) creating secondary cosmic rays. The Sun also emits low energy cosmic rays associated with solar flares. Almost 90% of cosmic rays are protons, about 9% are helium nuclei (alpha particles) and nearly 1% is electrons. When the primary cosmic rays hit the nuclei of oxygen or nitrogen a cascade of lighter particles is produced (they have less energy than the primary shower but they produce a larger shower). The charged particles reaching the Earth are mainly muons. At sea level this is 150 muons per second per 1 square metre. Muons travel close to the speed of light and with a lifetime of about 2.2 microseconds they would only be expected to travel a distance of about 660m. The fact that the upper atmosphere is about 15-20km up then very few muons would expect to be detected on the Earth’s surface. The high count rate can be explained using relativistic time dilation. Einstein showed that time ticks slowly for particles moving at speeds close to that of light. Whilst the mean lifetime of the muon at rest is of the order microseconds, when it moves at near the speed of light the lifetime is increased by a factor of ten or more (relativistic time dilation). This gives the muons time to reach ground level and accounts for their measured intensity.

clip_image037 A cosmic ray shower made visible.

Muons are not very ionising particles. In a cloud chamber they produce long, straight tracks that cross the whole chamber.

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Cosmic rays could aid the cloud formation in the atmosphere. Varying cosmic radiation with time could change the Earth cloud cover. Cloud formation is investigated at CERN (CLOUD) and at Boulby Mine (SKY – ZERO).

Cosmic rays could be coming from black holes.

The field of particle physics originated in cosmic ray research. The muon, pion, positron, kaon and lambda were discovered in cosmic rays.

When cosmic rays pass through an optically transparent liquid they travel faster than the speed of light. The charged particles cause the electrons in the atoms to move to a higher energy level. When the electrons fall back to the ground state the characteristic blue Cherenkov light is emitted.

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Particle detection

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We were very lucky that this lecture took place within a couple of days of the announcement about the discovery of the boson that is probably the Higgs. Dr Lazzeroni works on ATLAS, which along with CMS, identified the Higgs.

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The Higgs boson is a (proposed) elementary particle in the standard model. It should prove the existence of the Higgs field, the simplest of possible explanations for the origin of the symmetry-breaking mechanism by which elementary particles acquire mass. This process is called symmetry “breaking”, because such transitions usually bring the system from a disorderly state into one of two definite states. To an outside observer the choice will appear arbitrary.

The Higgs field is present all the time but when two protons collide with each other, having been given the required amount of energy, the quarks inside the protons collides. This releases the 125–127 GeV/c^2 of energy where for a very short time interval it is converted into the Higgs particle. As the Higgs particle is so short lived it is the products of its decay that identifies it such as the formation of two photons.

On 4 July 2012, the CMS and the ATLAS experimental teams at the LHC independently announced that they each confirmed the formal discovery of a previously unknown boson of mass between 125–127 GeV/c^2 (this is an alternative mass unit to the kg because in kg the value would be very tiny. It is connected to Einstein’s famous equation E = Δmc^2), whose behaviour so far has been “consistent with” a Higgs boson, while adding a cautious note that further data and analysis were needed before positively identifying the new particle as being a Higgs boson of some type.

The next big thing is to look for gravitational waves. This is expected to take five years.

References:                                                                  http://en.wikipedia.org/wiki/Cloud_chamber http://www.cloudchambers.com/cosmicrays.htm http://imagine.gsfc.nasa.gov/docs/science/know_l1/cosmic_rays.html http://en.wikipedia.org/wiki/Cosmic_ray http://en.wikipedia.org/wiki/Cherenkov_radiation http://www.britannica.com/EBchecked/topic/109373/Cherenkov-radiation http://www.egglescliffe.org.uk/physics/relativity/muons1_.html http://teachers.web.cern.ch/teachers/archiv/hst2000/teaching/expt/muoncalc/lifecalc.htm http://en.wikipedia.org/wiki/Higgs_boson                                      http://www.youtube.com/watch?v=D1bC6LXrVxI

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