Alpha spectroscopy is the technique of measuring the energy spectrum
of the alpha radiation
emitted from a sample.
When a radioisotope undergoes alpha decay, it emits an alpha particle (Z=2, A=4) with a specific characteristic
energy, always the same for a given parent isotope and characteristic thereof, at a known rate (related to the
isotope's specific activity). Thus, by measuring the energy of the
emitted alpha radiation, you can identify and measure the alpha emitters in your sample. Certain interesting elements
such as uranium and plutonium are alpha emitters, and so alpha spectroscopy is a useful tool for analyzing these. It
is difficult to characterize mixtures of plutonium and uranium chemically, and impossible to tell their isotopes apart
chemically, but it is frequently extremely important to determine their isotopic comoposition. For example, one may
wish to know if a given sample represents weapons grade or special nuclear material.
Alpha particles have very low penetrating power, due to their large mass. In fact even air absorbs them significantly.
To avoid this, alpha spectroscopy is performed in a vacuum chamber. However, the sample itself also absorbs its own
radiation. Thus it proper sample preparation is critically important. Specifically the sample must be deposited in
a very thin film. There are two principle means of achieving this.
In electrodeposition, the analytes (which are usually heavy metals) are placed in an electrochemical cell with
a platinum anode and a stainless steel disk cathode. The analyte metals are transferred from solution onto the cathode
disk using an electric current. Electrodeposited samples are very thin and provide excellent energy resolution, and
are very durable. However, the process takes up to a few hours per sample.
In fluoride precipitation, the sample is dissolved in hydrofluoric acid along with a rare earth element,
usually cerium, and the mixture coprecipitated onto a stainless steel disc. This technique is fast but
involves using large quantities of hydrofluoric acid, which is extremely toxic (hydroflouric acid poisoning is no fun).
Since it is difficult to perform either technique quantitatively, an obscure isotope such as uranium-232 is added
in known quantity as a spike or internal standard before preparation.
Once the sample is prepared it is loaded into a counting chamber and a vacuum drawn. The chamber has a detector,
usually a silicon surface barrier detector. These are basically reverse-biased diodes; when a particle whose
energy exceeds the band gap impacts the detector, an electron-hole pair is created allowing current to flow. Thus
current in the detector is proportional to the energy of incoming particles. Of course, it is essential to have
a very low level of background and so the counting chambers are very heavily sheilded (ideally, using
low background lead bricks, but these are expensive).
The signal from the detector is analyzed by a computer and plotted as a graph of event rate versus energy, and
individual energy peaks labelled with the isotope which produces them based on a data library.
Alpha spectroscopy is widely used to analyze environmental samples for contamination
with uranium, plutonium, and isotopes found only in spent nuclear fuel. For example, many thousands of environmental
samples a year are analyzed from the Hanford Nuclear Reservation in Washington State.