Cf-252 Neutron Capture Efficiency Procedure

Experimental Setup

Three CPTA 151 SiPMS were connected using PST-841 grease to WLS optical fibers imbedded in a 61.5 cm x 33.5 cm x 2.5 cm slab of Saint Gobain BC400 undoped plastic scintillator, shown in Figure 1.

Figure 1: Gamma catcher
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Two slabs of BC-400 plastic scintillator were placed on top of one another to construct a gamma catcher. The bottom slab has WLS fibers imbedded in it while the top slab does not. Grease was placed along the fiber channels to couple the two slabs together.

Mounts machined out of plastic were used to hold the SiPMs in place and are shown in Figure 2.

Figure 1: Mount used to hold SiPM in place
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Run Types

For each trigger condition/set of runs, five different types of runs were performed: * No Cf-252 source present, “No Source” * Cf-252 source present but no lead shielding in place, “No Shield” * Cf-252 source present with lead shielding, “Shield” * Cf-252 source present with lead shielding and vial of a gadolinium compound, “Gd Vial” * Cf-252 source present with lead shielding and doped scintillator sample, “Sample Name”

Trigger Conditions

With optimized settings (need to specify what this means!), the TB4 board has a maximum data collection rate of 17.0 +/- 0.1 Hz. Because of the dark count rate of the SiPMs and the probability that two dark counts will occur within the integration window, the minimum threshold that should be used such that collected data is meaningful is 3 photoelectrons. Simulations predict that the mean number of photoelectrons detected by a SiPM for a gamma from neutron capture on Gd is ???? PE.

Objective

The desired signal would be one of a few things. First, an increase in trigger rate from the “Shield” to the “Sample Name” runs could be observed. Second, different peaks in the “Shield” and “Sample Name” runs could be observed when weighting by events.

PS Slab Details

Length = 61.5 cm Width = 33.5 cm Height = 2.5 cm

Fiber locations = 6.6cm

Lead shield: 16 x 16 cm x 1.2 cm