Shielding detector

A more specific problem could be experienced when a detector is installed on a nuclear reactor site, particularly in a reactor hall. There, it may also detect Ar (half-life 1.827h, Ey = 1293.6keV) produced by thermal neutron activation of argon in the air within the voids in the 

Notwithstanding these reservations, it is routine practice in ultra-low background counting rooms to purge, not only the detector space, but also the whole laboratory of radon with filtered clean air. 

The earth s atmosphere receives a cosmic ray flux of about 70 % protons, 20 % alpha particles and 10 % other heavier ionized particles. Their energies are extremely high lO GeV to at least 10 GeV. In the upper atmosphere, at about 25 km above the surface, these particles interact to produce secondary radiations of many sorts, largely pions (ir-mesons). At surface levels, the pions interact further and about 70 % of the total flux generates 

The total cosmic ray flux giving events between 2 and 24MeV is about 0.015 particles per second per square centimetre. The peak at 13MeV corresponds to about 0.08 h keV , which diminishes to about 0.01 h keV at24MeV. 

The steep decline up to 2000 keV, the normal spectrometry range, is due to Compton events, backscatter and bremsstrahlung resulting from the decay of muons into high-energy electrons and positrons. On this are superimposed the 511 keV annihilation radiation and all the gamma-ray peaks from the background nuclides and the peaks from activations described below. 

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In both neutron and y-ray detection, the shielding of the detector is extremely important. Especially in the neutron detection measurements, the long target-to-detector distance (2-4 in) which is required to obtain velocity resolution via the neutron TOF technique means many more neutrons are produced than are actually scattered from the sample and then detected. These extraneous neutrons create a disastrous background unless the detector is adequately shielded. We have accomplished this with a large cylindrical shield which contains a lead cavity surrounded by Li2C03 loaded paraffin. The entrance collimator has steel and lead liners the main detector shield weighs about 2000 kg (see B in Fig. 1). 

If samples of very low activity are to be measured, the contribution of the background to the counting rate and hence the error of the measurement are relatively high. The influence of the background can be reduced by intensiflcation of the detector shielding and by coincidence or anticoincidence circuits. 

A detector shield will be needed with a cavity which is able to accommodate large (up to 4 1) samples, constructed of either lead or steel with some type of graded line to degrade X-rays. Lead shields have a much lower back-scatter effect than steel shields. Typically, lead shields have walls 5-10 cm thick, lined inside with graded absorbers made of cadmium ( l-6 mm) and copper ( 0- mm). Other materials, such as plexiglass and aluminium, are also used as absorbers. 

Fig. 3.16 The FDS at LANSCE (a) schematic and (b) cut-away view. Key filter, detectors, shielding, closed cycle refrigerator to cool filter, shielding, sample (usually annular or cylindrical), cryostat for sample, incident beam tube. Reproduced from [20] with permission from Elsevier.

A second strategy is, of course, to avoid the generation of synchronous noise in the system. Typical sources of coherent noise are cavity dumpers, pulse pickers, and picosecond diode lasers. For the shielding of these devices the same rules should be applied as for detector shielding. 

Bremsstrahlung is of interest in radioanalytical chemistry because some of the energy of electrons stopped in detector-shielding material is converted to X rays that can penetrate the shield. Cherenkov radiation permits scintillation counting of radionuclides in plain water samples if the electron energy is sufficiently high and the detection system is sufflciently sensitive. 

A common and effective way to reduce the external background is use of an anticoincidence system (see Figs. 8.3 and 8.4). A separate detector shields the sample detector. When the shield counter detects a pulse, the electronics system momentarily turns off the detector to reduce the background count rate. 

Photoelectric interaction in materials surrounding the detector can result in characteristic X rays in the lower energy region of the gamma-ray spectrum. For example, the Ka (72 keV) and Kp (85 keV) X rays are almost always part of the background in a spectrum of a detector shielded with lead. Commercially available lead shields for gamma-ray-spectrometer detectors are lined with thin cadmium and copper layers to attenuate these lead X rays. 

Excellent beam quality and detector shielding can more than compensate for low neutron flux (Maier-Leibnitz 1969 Kobayashi and Kanda 1983 Matsumoto et al. 1984 Molnar et al. 1997). With careful attention to background, the sample-detector distance can be small and thus the gamma efficiency high. As a result, microgram quantities of boron have been determined in tissue with a neutron flux of only 2 x 10 cm s ... 

Compton scattering within the detector shield - mainly from the sample itself. It also ignores the effect of poorer resolution of larger detectors. Under the conditions implied, bigger does appear to be better. However, the deductions made above would be completely different for detectors with much better, or worse, resolution than is typical. The deductions are also different if the continuum beneath the peaks is due primarily to sources of radiation external to the shielding. 

Active shields in the form of veto detectors are undoubtedly worthwhile above ground and down to a depth of about 100 m w.e. underground. The plastic scintillator veto detector shielding the lAEA-MEL detectors (Povinec et al (2004)) installed at a depth of 35 m w.e. and referred to above, reduces the backgrounds to a level equivalent to 250 m w.e. depth. Reduction factors are between 4 and 11 for the various detectors. However, as depth increases the improvement is less. Above ground improvements of factors of 4 to 10 are achievable. At 500 m w.e., there might be only a 40 % reduction in background and only a few percent at 3000 m w.e. 

Fluorescence in this context, refers to gamma rays from the source, acting on the materials of the detector shielding. 

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