Chapter 11 from BASICS OF NUCLEAR PHYSICS AND OF RADIATION DETECTION AND MEASUREMENT - An open-access textbook for nuclear and radiochemistry students by Jukka Lehto

In an ordinary radiochemical laboratory the alpha-emitting radionuclides studied are those listed in Table XI.I. Of these Po, Ra, Th and U isotopes are naturally occurring radionuclides while Pu and Am isotopes are artificial transuranium nuclides. The natural alpha-emitting radionuclides belong to the decay series beginning from ²³⁸U, ²³⁵U and ²³²Th. The sources of the transuranium elements are the the nuclear weapons tests in the 1950' and 1960's and of the Chernobyl accident in 1986 as well as the nuclear waste, especially the spent nuclear fuel. Some of these radionuclides, such as ²³⁵U, ²²⁶Ra and ²⁴¹Am emit gamma radiation, which can in some cases be used for their measurement. The intensities and/or gamma ray energies are, however, typically so low that the gamma spectrometric measurement does not yield accurate results. Moreover, gamma spectrometry does not allow determination of isotopic composition of uranium, which is important information in many studies. Accurate measurements, enabling also determination of isotopic compositions, are obtained either by alpha spectrometry of by mass spectrometry. The former is discussed here in this chapter.

Table XI.I. Most typical alpha-emitting radionuclides studied in radiochemical laboratories.

Semiconductor detectors were discussed already in chapter IX where gamma spectrometry was described. In gamma spectrometry the detector material is germanium whereas in alpha spectrometry the material is silicon. The principal idea in both is the same. They are both diodes composing of an n-type Si/Ge, having an electron donor additive, such as phosphorus, P(V), and a p-type Si/Ge, having an electron acceptor additive, such as boron, B(III). When these are attached to each other a depletion zone is developed around the interface due to combination of electrons and holes on the interface. When a reverse bias voltage is applied across the crystal this depletion zone widens. These are transferred close to electrodes due to the voltage applied. The thickness of the zone is dependent on the voltage applied being typically only 40-60 V in silicon alpha detectors. For the detection of gamma rays the depletion zone needs to be thick, several centimeters, in order to absorb the readily penetrating gamma rays. This is accomplished by using a larger crystal made of very pure germanium and by using a high voltage up to 5000 V. In the case of alpha detection with Si-detectors the depletion zone should be very thin due to the short range on alpha particles in silicon, only 30 μm. Typically the depletion zone in silicon detectors used in alpha spectrometry is 100-200 μm. There are two types of silicon detectors in production (Figure XI.1): surface barrier detectors (SBB) and passivated ion-implanted detectors (PIPS) the latter being a more modern construction mode.

Figure XI.1. Production of silicon detectors for alpha spectrometry. Left: surface barrier detectors. Right: Passivated ion-implanted detector. (http://www.ortec-online.com/Products-Solutions/RadiationDetectors/silicon-charged-particle-detectors.aspx).