Particles and Detectors

 

This section is to help you understand what devices we will use to obtain real and thus unique experimental data and how they will turn into those ones and zeros which are then stored and processed on our computers. It gives very brief introductory notes. Those who want to get deeper knowledge should look for additional information in various sources. If any questions arise, you may ask them at this site.

 

What is a modern elementary particle detector?

Let us define it as a device that indicates a transition of a particle though its sensitive volume by producing an electric signal. The course on experimental methods of nuclear physics for 4th-year and 5th-year students at the Moscow Engineering Physics Institute takes two terms.

Our goal is to give you a general idea of one type of detector on two to three pages. Therefore, we touch upon only the main points.

First, we exclude all kinds of detectors based on photographic detection methods from our consideration. Despite whatever exceptions, their time has gone.

Next, it should be understood that a particle can be detected if it interacted in any way with the detector material. It is obvious but leads to a conclusion that there might be unknown particles which do not interact with ordinary substance and about which we do not know anything yet.

 

How does a particle interact with a substance in general and with the detector material in particular?

Theoretically, it can do this in any of the ways known to us. It means that the particle can undergo interaction of any type possible for it. At present, science knows only four types of interaction, gravitational, electromagnetic, strong, and weak.Let us consider them all - one after another.

  • Gravitational interaction is universal, occurring between any objects that have mass. In addition, it acts at any distance, decreasing as a square of this distance. BUT since the mass of our particles is very small, elementary particles practically do not experience interaction due to gravitation. It is a pure theoretical effect.
  • Electromagnetic (EM) interaction is revealed by all bodies having an electric charge. Like gravitational one, it acts at any distance and also depends on this distance (it decreases as R squared).
  • Strong interaction is strong indeed BUT acts only on particular objects (called hadrons) and at very short (nuclear) distances. It ensures binding of protons and neutrons in the nucleus. It is used to detect some particles (first of all those which do not have a charge, namely, neutrons) or to measure the energy of a particle. Yet, it cannot be employed for solving the most popular experimental problem of multiply measuring the particle passage coordinate/time1.
  • Weak interaction is simply weak. On the one hand, unlike strong interaction, it is universal, i.e., acts on everything (except gamma quanta, or light), and this is good, but on the other hand, in addition to being weak it is also short-range interaction. Thus, it is not good for effective detection.

 

Interaction characteristicsGravityWeekEMStrong
Interaction constant exponent 10-38 10-6 10-3 1
Radius 10-16 10-13
McarrierGev 0 w,z ~ 100 0 ∏ ~ 0.1
Universality All All All Hadrons
Quantization ? Yes Yes Yes

 

It would be naive to think that these brief remarks are enough for you to gain a proper insight into types of interaction, but let it be another impetus for further discussions. Now it is important to understand that the only effective method for detection of a particle is to use the ability of a charge particle to experience electromagnetic interaction.

And it is clear why. The nucleus is very small and sits deep in the atom/molecule. It is hardly probable to hit it (and one must hit it quite accurately because there is neither strong no weak interaction beyond the nucleus for these are short-range interactions). Electron shells occupy much more space. Now imagine that a charged particle travels through a medium (gas, liquid, or solid). It sometimes (very rarely) runs into the nucleus and then strong or weak interaction takes place, but it much more often interacts through EM interaction, which acts at all distances (R squared!). If there is enough force, it removes the electron and excites the atom. This process is referred to as the ionization loss in substance.

It is the process of interaction between the particle and the material of the sensitive volume of the detector that forms the basis for most types of particle detectors.

Let us return to our main task of detecting extended air shower (EAS) particles on the Earth's surface and proving to ourselves and all others that what we detected is exactly an EAS event.

In our simple RUSALKA detecting system, the main detector is a scintillation counter. It is the easiest-to-maintain and most stably operating particle detector. What is the chain of processes that occurs in a scintillation detector from the instant when the particle traverses its sensitive volume till the appearance of an electric signal at its output? The working material of our scintillation detector is a plastic scintillator. It is made from ordinary transparent plastic combined with special additives in the course of manufacture. As a result, when a traversing particle produces the above-mentioned ionization losses in it, this plastic will emit a few light quanta. This is because excitation of atoms of this plastic can be partly "removed" through release of visible light gammas rather than converted to thermal oscillation or other undetectable effects. Light is detected by a photomultiplier tube (PMT), which is also a very reliable device. Again, I will not spend time on describing its operation. The scheme is as usual: you read all you manage to find in the Internet, beginning with the Wikipedia, and ask questions if anything is not clear to you.

Photos of the "inner stuff" of our detectors can be found here.


A whole range of experimental tasks, e.g., determination of angles of particle emission from the interaction point or determination of particle momenta (in the magnetic field), requires multiple measurements of the particle coordinate/time such that the initial characteristics of the particle, its motion direction and energy, are changed as little as possible. It is an example of conflicts often arising in experimental physics: on the one hand, the particle must interact to be detected but, on the other hand, in many cases it is desirable that it would interact without any changes in itself, which is obviously impossible.