Low energy photon detector

Guilmette RA. 1986. A low energy photon detector for the radio assay of Pu and Am in biological samples. Health Phys 51 797-803. 

A similar germanium low-energy photon detector (l.e.p.d.) to that described above has also been used for the measurement of rare-earth elements in Apollo-12 lunar samples. This combination of Ge(Li) spectrometry for overall y-spectrum measurement and l.e.p.d. spectrometry for low-energy measurements extends the application of i.n.a.a. and is now a common procedure. 

A number of special techniques have evolved to increase the detection sensitivity in y-ray counting. One of the most important is the suppression of the Compton scattering events in the y-ray spectrum by the use of anticoincidence annulus around the central y-ray detector. The idea behind a Compton suppression spectrometer is that most events in which the incident photon undergoes one or more Compton scattering events in the central detector will result in partial energy deposition in the detector with a low-energy photon escaping the detector. 

Low energy photons are not able to promote the bound electrons to the free state (low wavenumber cut-off of MCT detector at 600 cm In some MCTs this cut-off is even higher (750 cm ) due to absorption in the detector optical window. 

In this paper we investigate the use of a CsI(Tl)-photodiode detector in a hybrid mode, detecting low energy photons in the photodiode and higher energy photons in the scintillator. The different temporal characteristics of the two types of signal can then be used to distinguish them. 

Figure 3.7 Transmission curves demonstrating the absorption of low-energy photons by detector dead layers and cap materials...

The only detectors currently available which are operable at room temperature are the CdTe, CdZnTe and Hglj detectors. However, these are limited in size and are best suited to low-energy photon measurements. 

Detector type. If measurements at low energy (< 100 keV, say) are not required, then a p-type detector would be fine n-type detectors are more expensive than p-types. If low-energy photons are relevant, then an n-type detector with a thin window is essential. Remember such windows are fragile. For ultra-low energies, special windows are available. If neutron damage to the detector is likely, an n-type detector should be purchased. Neutron damage in n-type detectors is repairable. 

For measurement of low-energy photons, in particular, compare specifications from different supphers, as there are several alternative types of detector available. 

Soft X-ray absorption measurements are done at low-energy synchrotron X-ray facilities such as the UV ring at NSLS or the Advanced Photon Source (APS) at Lawrence Berkeley National Laboratory (LBNL). The beam size is typically 1 mm in diameter. The electron yield data are usually obtained in the total electron yield (EY) mode, measuring the current from a channel electron multiplier (Channeltron). Sometimes a voltage bias is applied to increase surface sensitivity. This is referred to as the partial electron yield (PEY) mode. Huorescence yield (EY) data are recorded using a windowless energy dispersive Si (Li) detector. The experiments are conducted in vacuum at a pressure of 2 X 10 torr. 

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