Veto guard detectors

The surface system mentioned above in the context of optimization also incorporated a veto detector. Hurtado et al. (2006) examined four different gating scenarios to allow the plastic scintillator veto detector to prevent background counts from being recorded. Two of those, in effect, involved detecting coincidences between veto counts and HPGe counts while the other two used a 

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. 

The account of one particular laboratory s quest for ultimate low background, going to what John Hemingway, in the first edition of this book, termed awesome extremes , is fascinating. It illustrates many of the issues raised in previous sections and that can perhaps justify inclusion of a brief summary. Starting with a commercial low background detector, Brodzinski et al (1985, 1990) followed this extraordinary path  

The detector cryostat was then rebuilt to incorporate low background components. The Nal(Tl) anticoincidence system, which contained Ra, was replaced by an external plastic scintillator veto detector. A layer of cadmium and borated wax was added, to thermahze fast neutrons and absorb the thermal neutrons. This resulted in the second spectrum and count rates down to 1 X 10 to 3 X 10 counts keV min  

The whole assembly was then transferred to a gold mine at 1428 m below ground level to minimize cosmic ray 

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A Compton suppression shield should be chosen only after some thought. It is likely to be satisfactory only for the detection of gammas that are not in cascade. A veto guard detector will reduce cosmic ray events, being especially useful with high energies. 

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