Cosmic Ray Detector

I mentioned previously that I’ll be trying to use a scintillation counter/cosmic ray detector to measure the high-energy charged particles that are supposedly emitted by ultra-dense deuterium. In this post, I want to describe how a cosmic ray detector works, and how I plan to build one.

The first thing to note is that cosmic rays (which you can read about in more detail on Wikipedia) are a type of ionizing radiation produced when high-energy particles collide with the Earth’s atmosphere. Cosmic rays can be composed of a number of different types of particles, ranging from electrons to heavy nuclei cores, but are predominantly “muons”, which are heavier analogues of electrons.

These muons can interact with a detector in multiple ways. In a bubble chamber, the muon ionizes atoms in a tank of super-heated liquid hydrogen, causing it to boil along the trail traversed by the particle, and leaving a visible trace. This is similar to the function of cloud and spark chambers.

These chambers are quite useful but have significant drawbacks in that they are often difficult to set up and maintain, and are difficult to use when dealing with large quantities of particles (the traces must be counted by hand or with sophisticated computer vision, and disappear after a couple seconds). The commonly used alternative is a scintillation counter. When muons pass through a “scintillator” (which may be an inorganic salt crystal, a plastic, or even a liquid such as toluene) they excite electrons in the material. If the material is doped with a dye, it will release this energy as photons of a known wavelength. Importantly, the number of photons released as the particle passes through the material will be roughly proportional to the energy of the muon.

Because a handful of photons isn’t visible, some form of amplification is required to obtain a signal. Historically, this has been done with a “photomultiplier tube“, which use a series of highly charged plates. While photomultiplier tubes are highly sensitive (able to pick up individual photons) and have a low “dark-count” (rate of false signals), they have drawbacks of being moderately expensive, very fragile, and somewhat bulky (the charged plates need a minimum separation to prevent sparking).

As such, a more recent alternative, known as an avalanche photodiode, has gained some popularity. While avalanche photodiodes have no size constraint and are cheaper than photomultiplier tubes, they have a significant dark-count, and so more work is required to filter out false signals.

For my project, I’m going to be building two different scintillation counters, one using a photomultiplier tube (PMT), and one using an avalanche photodiode. The PMT detector will use two different scintillation paddles spaced about 2 meters apart and will allow me to determine the particle flux in a particular direction, but will only detect particles that make it through the thick steel walls of the vacuum chamber I’ll be using.

The photodiode-based detector will be palm-sized, based on a design by MIT graduate student Spencer Axani and it will be possible to fit it into the vacuum chamber, immediately adjacent to the emitter from which the ultra-dense deuterium should be produced.