Measuring the energy of an alpha particle

Question

I would like to measure the energies of particles emitted from a variety of ionizing radiation sources (alpha and beta), and then convert these energies to velocities. However, I am not too sure about how I might go about measuring these energies. Would this even be feasible? If so, how could it be done? How are these energies measured by professionals?

Answer 1

I believe Germanium is commonly used for alpha particle spectroscopy. It is a semiconductor, measures the amount of electrons that result from the creation of electron-hole pairs as the alpha particle passes through the material. The electronics connected to the anode (+) plate measures the voltage change across the two plates sandwiching the sensitive area of the detector. The number of free electrons produced is proportional to the energy loss of the alpha particle, and (I think) that is also proportional to the energy of the alpha particle.

Answer 2

In Rutherford Backscattering Spectrometry (RBS), the detector of choice for MeV protons and alphas is a surface barrier detector. As @onelineproof mentions, some are made of germanium. For the last 30 years or so, silicon detectors have been much more common. These are basically reverse-biased diodes with a very thick active area to collect charge generated by electronic stopping of the energetic ion. Protons and alphas lose almost all their energy through electronic stopping, so there is very little 'nuclear deficit', or energy lost to nuclear collisions. Right now I've to a 2 millimeter-thick device sitting in front of a neutron source to accurately measure Si recoil energy distributions. As mentioned in my brief comment, Ortec is one supplier of such devices in a variety of areas and thicknesses.

So, an incident ion will (rapidly) generate a pulse of current coming out of the device. This is sent to a pre-amplifier, and then to a spectroscopy amplifier, which outputs a signal with a voltage proportional to the collected current. A multi-channel analyzer is then used to build up a spectrum of the number of particles detected with a given energy. Again, these instruments are quite common in nuclear physics labs, from Ortec, Canberra, ..., often found sitting in NIM bins. Ask around.

Now, if you really want to go old school - the original papers from Geiger, Marsden, and Rutherford (and others) differentiated alphas from different materials through their path length in air. A beam of natural alphas, collimated by apertures, was sent through air, and the number of flashes on a zinc selenide phosphor screen was plotted vs distance from the collimated source. Decay lengths for ~5MeV alphas are on the order of a meter or two. For sources kept in vacuum they used thin mica windows; nowadays mylar would be a good replacement.

There was also mention of magnetic separation. While certainly possible, this has really only been used for medium energy ion scattering (MEIS) where the ~4keV resolution of a surface barrier detector just doesn't cut it. However, you need a magnet and a good position sensitive detector if you want to accurately measure energies. If you just want to separate a few different particle energies you can do just fine.