Particle Tracks

Particles moving through a detector, such as a cloud or bubble chamber, leave tracks that provide evidence about particles. Much of that evidence requires careful measurements and complex calculations to gather and make use of; but, simple observation can provide some clues. Pre-university physics courses only look at these patterns and provide additional information that would distinguish between similar patterns.

Alpha particle, proton, low energy electron, and muon or high energy electron

Neutron

Neutrino

Gamma ray

Nucleus following two alpha decays - first one on the right towards the right, second one on the left going upwards.

For explanations on why the tracks look like this, keep reading.

Detecting particles

Sub-atomic particles are far too small, and usually moving far too fast for any sort of direct observation - the solution, use a particle detector.

Most particle detectors work by amplifying the effect of ionising radiation to a point where that effect becomes visible. The details of how each detector works are not the focus of this article; if you would like to know more about them, we would recommend these sites:

Particle detectors

Cloud and Bubble Chambers

Cloud chambers

Particle detectos at CERN

Two of the earliest particle detectors, cloud and bubble chambers, produce tracks that can be seen with the naked eye.

What do the tracks mean?

The tracks are produced when the medium inside the detector reacts to charged particles passing through it. The width, length and directions of a track can give clues as to what particle has caused the track.

No tracks

Since the tracks are caused by the medium's reaction to a charge, if you see a track start in the middle of the detector then an uncharged particle must have moved through the detector up to that point, these include neutrons, neutrinos, gamma rays, and slow moving electrons and ions produced by radioactive decay. The observant among you will have noticed that the last two are very much charged - yes, but you still won't see them. Here is what you will see:

Neutrons

These massive particles are not fundamental particles, being made up of three quarks (udd) each of which carries a charge. This means that neutrons can interact with matter through the nuclear and electromagnetic forces, but only at very short range - i.e. direct collisions with electrons or nuclei. Therefore, nuclear interaction is much more common than direct electron ionisation.
Possible visible results are:
  The production of a fast electron by ionisation.
  The production of a fast proton by kicking one out of the nucleus.
  The production of radiation (alpha, beta) by being captured by the nucleus, producing an unstable nucleus that then decays.
  The production of atomic fragments by breaking the nucleus apart.*

Neutrinos/Anti-neutrinos

These almost massless particles only interact via the weak nuclear force and therefore cannot produce ionisation electrons. They can, however, trigger nuclear reactions - releasing an electron or positron. The changed nucleus may then undergo radioactive decay or fission. Visually it would be similar to how the neutron interacts.

Gamma Rays

These massless particles display two different patterns depending on their energy. Lower energy gamma rays will be absorbed by an atom, causing an electron to be ionised, and re-emitted at a lower energy in a different direction. This is called Compton scattering and is seen as a series of high energy electrons being produced. However, since the direction of the gamma ray and the distance between interactions is random, it is difficult to associate them with a single ray. Higher energy gamma rays can result in pair production, e.g. electron and positron, when they interact with matter. A third track will be visible if sufficient energy is transferred to an electron during the collision that produces the particle pair (a magnetic field would curve the particles' paths), this is called triplet production.*

There are two other possibilities for very high energy photons, photodisintegration and photofission.* Photodisintegration is where the gamma ray collides with the nucleus and knocks out a proton, neutron or possibly an alpha particle. Photofission is where the gamma ray is absorbed by the nucleus which then breaks apart into fragments. These are several orders of magnitude less likely than pair production even at very high energies (they are not included in the diagram - they would appear similar to neutron tracks).
  Ionisation - about 10eV
  Pair production (electron+positron) - greater than 1.02MeV (2.044MeV for triplet production)
  Photodisintegration - about 10MeV
  Photofission - about 100MeV

Slow moving electrons

While they are charged, they don't have enough energy to cause ionisation. They also do not act as condensation centres since their mass is too small, meaning that they move instead of the medium. A low energy electron leaves no trace at all.

Radioactive decay ions

When an alpha particle is emitted the remaining nucleus has two more electrons than protons. However, the energy released by the decay is sufficient to ionise those electrons, and several others. An ion with such a large mass and charge would be expected to leave a large track, but it never gets a chance. The ion has a small velocity, since its mass is so large, and strongly attracts the electrons that were ionised. The result is that the electrons recombine to form a neutral atom before the ion can move by a noticeable amount. The atom travels in the opposite direction to the emitted particle, due to conservation of momentum. If it emits another particle there would be a tell-tale gap between the tracks.

Beta decay will produce a similar result, a highly charged ion surrounded by free electrons that have just been ionised. While in alpha decay the nucleus will end up releasing two more electrons than it needs, beta decay produces an extra proton and therefore would be one electron short. However, the momentary large positive charge would pull in a stray electron and create a neutral atom before the nucleus moved far.

Momentum and energy measurements, along with experimental set-ups that limit the particles involved, can help distinguish between these 'invisible' particles.

Straight tracks or wiggley tracks

Assuming that there is no external electric or magnetic field (see below for when there is), the path of the particles is due entirely to the interactions they have with atoms along the way. The more massive a particle is compared to what it interacts with, the less it will change direction, and the vast majority of interactions will be with electrons via the Coulomb force. Since the mass of an alpha particle is about 7300 times more massive than an electron it is deflected about as much as a car is when it drives into a football. Similarly, other heavy particles like protons and muons are not noticeably deflected by electrons.

Low mass particle paths depend on the particle's momentum. If the momentum is large, the speed is high, then the force between the particle and the electron it interacts with only acts for a short period of time. Therefore, even if the force is large due to a small distance between the particles, the amount of momentum transferred during the interaction is small and so the deflection of the particle is small. This means that high energy electrons will have straight paths until they lose the majority of their energy. The lower the velocity the longer the forces act for and therefore the larger the angle the particle can be deflected through during each interaction. This leads to wiggley paths with little distance between changes of direction.

A charged particle that passes near to the nucleus of an atom in the medium will be deflected. As with electron interactions, the lighter particles will experience larger deflections and even those will be small unless they pass very close to the nucleus.

Another possibility, though one with a very small probability, is collision with the nucleus of an atom. Since the nucleus has a large mass compared to the particle the deflection angle is likely to be large. Even less likely is that collision leading to a change in the nucleus - e.g. particle emission or fission (see neutrons above)

Wide tracks or narrow tracks

The width of the track is determined by the ionisation power of the particle; the more ions produced per unit volume, the more condensation takes place and therefore the wider and denser the cloud produced.

The biggest effect on ionisation power comes from the charge of the particle, which depends on the square of the charge. The higher the charge, the bigger the force on nearby electrons, leading to more receiving enough energy to escape their atoms (over a wider area). Hence, the alpha track is much wider than those produced by protons, electrons and muons since its charge is twice as large.

The other property governing the width of the track is the speed of the particle.* It is often said that larger mass particles produce wider tracks, which is like saying that more expensive cars are faster - a million pound supercar will be faster than a cheap city car costing a few grand, but it's not the price that makes it fast. For a given energy, the more massive a particle is the slower it will travel, and that is the key. The longer the particle spends near an electron the greater the energy transferred between them by the Coulomb force, so a slower particle can ionise more electrons in the same distance. Given similar starting energies, proton's track will be wider than a muon's which will be wider than an electron's. To further complicate matters, electrons can reach relativistic speeds with comparatively little energy; at these speeds their electric fields are compressed along the direction of travel by relativity, which means they exert a much larger force as they pass than would otherwise be the case. This larger force leads to greater ionisation and hence a wider track - this makes high energy electrons visually indistinguishable from muons.

Long tracks or short tracks

The length of the track depends entirely on how far the particle can get and still cause ionisation. The same arguments that explain track width apply here; an alpha particle with a +2 charge and low velocity will deposit its energy over a short distance, a proton with half the charge and a larger velocity (for the same energy) will have a much longer track.

Ionisation isn't the only means of losing energy. Any charged particle that accelerates/decelerates will radiate energy (bremsstrahlung radiation) - the bigger the acceleration, the more energy is lost. Since electrons have a small mass, they have larger changes in direction when they interact with matter and have a larger change in velocity for the same loss of energy compared to protons etc. This means that, despite having a lower ionising power than protons, electrons have a shorter track length. If the electron is travelling at relativistic speeds it may also lose energy through Cherenkov radiation (velocity is greater than the speed of light in the medium).

Changing width

As a charged particle slows down it spends longer in the vicinity of each atom, increasing the possibility of an interaction, and so ionisation is more likely. The trace you see is a 'cloud' expanding outwards from the ionisation trail left behind by the passage of the charged particle. The more ionisation that takes place, the bigger the 'cloud' produced. It would, therefore, be reasonable to assume that the track produced would get wider in the direction of travel of the particle, but it's a bit more complicated than that.

1) The slower the particle gets, the more ionisations will take place in a given distance; each of which slows the particle further. The effect is that the change in track width is concentrated towards the end of the track, so the trace would appear unchanged over much of its length followed by a wider, denser cloud at the end.

2) The region where tracks can form in some types of detector is quite thin. This means that a particle that starts inside the region and heads out of it will start out with a thick trail that gets thinner as the medium thins out.

3) The tracks shown are often from a photograph - an instant in time. It takes time for the tracks to form, spreading out from the particles path, which means that the track will be thinner in the direction of the particles motion in that photograph.

Therefore, don't read too much in to the change in track width in a photograph.

Curved Tracks

Newton's 2nd law tells us that a curved path must be due to an external force; since gravity and the nuclear forces are too weak to meaningfully affect the motion of a particle in the detector, the external force must be electromagnetic. This means that to have a curved path the particle must have a charge. The opposite is not definitely true though, charged particles will take straight paths if there is no electric or magnetic field applied to the detector. So, if we don't know for certain that there is a field then we can't make assumptions about the particles charge; however, a curved path tells us that the particle is charged and that there is a field.

Electric Field - charged particles would take a parabolic path (same as projectile motion). A tighter curve means either a higher charge or lower momentum. Opposite charges would move in opposite directions, but the paths of a particle and anti-particle would not be mirror images unless the photon that they were created from was travelling perpendicular to the field (think same projectile with the same initial velocity, but different starting angles - the projectile paths would look different)

Magnetic Field - charged particles would take a circular path. Since the particle is losing energy every time it causes an ionisation, it would spiral in in smaller and smaller circles. A tighter curve means either a higher charge or lower momentum. Opposite charges would move in opposite directions, and the paths of a particle and anti-particle would be mirror images.

So, knowing the direction of the field tells us the sign of charge but nothing about the magnitude of the charge or mass of the particle.

Split tracks

Split tracks are the result of more than one charged particle leaving the site of an interaction. The most common scenario is an ionised electron gaining enough energy to be able to ionise other electrons - these are called delta rays. If a delta ray causes another electron to also cause ionisation, the tertiary electrons are sometimes called epsilon rays (rarely used these days). Delta rays are usually low energy compared to beta radiation and hence have short tracks.

Alternatively, the two particles leaving the interaction site are decay products, either from an unstable particle decaying or from the decay or fission of a nucleus that is made unstable by the collision. In the case of an unstable nucleus, it is likely to move away from the where the collision happens before the decay happens, due to conservation of energy and the decay not being instantaneous, leaving a gap between the incident track and the split tracks.

Kinked tracks

Sometimes the track will make a sharp change in direction.As discussed above, a large and sharp change in direction could be due to the particle passing very close to a nucleus. The other possibility is a decay that produces only one charged particle e.g. a muon decays into an electron and a neutrino.

Exotic particles

The particles mentioned above are the most common ones to pass through a detector, but there are plenty of other types as well as the anti-particles of each of them. In each case, the effects of charge, mass and velocity are the same. Since exotic particles are unstable more decays will be observed.

Useful links

Cloud chamber lesson plan
Cloud Chambers:Activities for Schools