Veto detector

Figure 13.21 Anticoincidence arrangement for a cosmic ray veto detector...

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 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 detector geometry used is a 3 layer design, with a semiconductor array top layer, and two lower layers which are scintillator/photodiode arrays. The semiconductor layer is made of CdTe 2mm thick and the scintillator arrays are 3cm of CsI(Tl). All the pixels are hexagonal with an across flats dimension of 11mm and a centre to centre spacing of 11.75mm. The detector is surrounded on all sides, and on the bottom by a 2cm thick BGO veto crystal. 

The sample, which is considerably smaller than the bore of the beam collimator ( 10mm dia.), is mounted on a thin mylar foil on top of a cup-shaped scintillation detector (veto counter). The mylar is essentially transparent to the incoming muons. Thus,... 

The KamLAND detector, shown in Figure 5, consists of a sphere of liquid scintillator, surrounded by neutron and gamma ray absorbing buffers made of liquid paraffin and water. The liquid scintillator is enclosed in a transparent nylon composite balloon that has low permeability to radon, a background-generating contaminant. The total mass of liquid scintillator is 1000 tons, filling a sphere with a radius of about 6.3 m. A 500 ton fiducial spherical volume is defined within the main volume to help reduce backgrounds from photomultiplier tubes and from external radioactive decay sources. The liquid parafrin buffer is enclosed in a spherical stainless steel vessel, which is in turn surrounded by a water Cerenkov veto counter. The entire detector is shielded by a rock overburden of 2700 meters of water equivalent (m.w.e.). 

Scaling the rate to water and assuming that all neutrons produced by muons within the fiducial volume of the detector thermalize, capture on gadolinium, and escape a muon veto, we obtain an upper bound on the uncorrelated rate of 147,000 neutrons per day in the fiducial volume. Of course, this rate is greatly reduced by the requirement of spatial and time coincidence with the positron signal. 

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. 

Figure IV.C.l Top view of the E-788 experiment illustrating the He detector configuration. Only the active areas of the counters are shown. All support structures, photomultiplier tubes and light guides are hidden. Also not shown is the target veto system.

Fig. IV.D.2 Monte Carlo events required to pass through the KEK detectors and stop in the neutron detector VETO. TTie energy deposited in the KEK from dE/dx is plotted as a function of the measured velocity as measured by time of flight from SL to KEK.

With the increasing number of channels of electronic readout required for these detectors, Fastbus modules have proven to be an economical and reliable solution. The beam-defining microstrips, veto scintillators and the calorimeter blocks are all read out in Fastbus ADC s. In addition, the readout of the drift chambers is now done with Fastbus TDC s which have operated reliably during this year s runs. 

Aerogel Cherenkovs Neutron Det. Arrays Neutron Det. Vetoes Silicon Pad Detectors (E813 only)... 

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