NaI detector runs done the week of 3/18

The 1.5g vial of Gd was replaced with a vial contain much more Gd (Gd(NO₃)₃) (x10?, need to measure mass). The 2“ of poly at the end of the collimator (see Figures 5 and 6 here) was removed. Instead, to generate more thermal neutrons while not scattering them, a 1” thick piece of poly was placed directly in front of the ²⁵²Cf source in the collimator channel, as close to the source as possible. Data collection was not dead time limited, the same as for all the previous NaI runs using a coincidence trigger.

The amount of data collected over spring break was severely limited as one of the NaI PMTs tripped several times after I was gone. This week, using a different +HV source has stopped the tripping. I still need to run a few diagnostic tests on the old power supply.

First, short runs examining the effect of the location of the Gd vial were compared: one where the vial was nearly at the edge of the NaI detectors closer to the source (“Shield 6, Cf-252, Gd Near”), and one where the vial was exactly between the two detectors (“Shield 6, Cf-252, Gd Center”).

Figure 1:	Figure 2:

The run with the vial closer to the source had a higher trigger rate. The trigger rate difference was small, namely (0.086±0.029) Hz, but there was only a couple of hours of the “Near” run data, so we could run a bit longer with this configuration to push down error bars. How should we proceed?

Figure 3:	Figure 4:

The trigger rate difference is (0.378±0.011) Hz where, the subtraction (Source) - (Source+Gd) was done. The peak locations of the two runs are slightly different because of the switch to the new HV source. A run with just the Cf-252 source and no Gd is going to be done such that the subtracted plots will match better.

Figure 5: