NaI Detector Shielding

Background, shield 1

Experimental Procedure

The NaI detectors were placed back to back on a trolley and fixed in place with straps. A positive HV source was used to bias the detectors at 1100V and data was read out from the anode. The threshold on the discriminator was set to 30mV and a digitizer was used to read in analog data from the detectors. A coincidence unit was used for Detectors B and C as a trigger. Two runs were performed for the setup- A background run was executed without any shielding, and then another run was conducted with a lead shielding on the floor ( 4 inches of Pb bricks plus 1.5 inches of Pb sheets under cart).

Notes:

Use 0.5V scale with -0.25 offset. Signals become saturated at about 0.8V, which corresponds to about 13 MeV.

Figure 1: Block Diagram of Setup

Figure 2: Shield 1	Figure 3: Shield 1

	Trigger rate (Hz)	1.57 MeV Peak height (counts/hour)	2.94 MeV Peak height (counts/hour)
Table 1: Summary of Data
Background	4.67 +/- 0.01	427.5	86.1
Shield 1	4.34 +/- 0.01	351.9	71.7
Shield 2 (Shield 1 + 1 sheet arched over each detector)	4.20 +/- 0.01	287.2	62.2
Shield 3 (Shield 2 + sheets on top shelf and hanging over sides)	3.96 +/- 0.01	249.8	57.8
Shield 4 (Shield 3 + more hanging over sides + curved around NaI PMTs)	3.78 +/- 0.01	221.4	50.4
Shield 5		175

Figure 4:

Figure 5:

Figure 6:

Figure 7: