Lower Threshold Study:
Variation between detector channels

We investigated the lower threshold behavior. Note that the threshold can be changed through the input parameter of threshold in each different run at the level of sandwich. However, it is still worth investigating the overall behaviour using some specific runs. Here we examine the Co-57 block calibraion runs because there are high statistics in the Co-57 runs and the 14.4 keV line is just around lower threshold.

Data sets examined:

Co-57 block calibration runs:

Array Number	File Name	Input Threshold Value*
0	blk_3_co57_200_20_0_021120_1.fits	?
1	blk_4_co57_200_20_0_021119_1.fits	12 mV
2	blk_8_co57_200_20_0_0_021209_1.fits	12 mV
3	blk_9_co57_200_20_0_0_021204_1.fits	12 mV
4	blk_13_co57_200_20_0_0_021218_1.fits	12 mV
5	blk_14_co57_200_20_0_0_021219_1.fits	12 mV
6	blk_11_co57_200_20_0_0_021227_1.fits	12 mV
7	blk_12_co57_200_20_0_0_021224_1.fits	12 mV
8	blk_a_co57_200_20_0_021111_1.fits	?
9	blk_17_co57_200_20_30_0_030205_3.fits	12 mV
10	blk_5_co57_200_20_0_0_021201_1.fits	12 mV
11	blk_6_co57_200_20_0_0_021126_3.fits	12 mV
12	blk_10_co57_200_20_0_0_021210_1.fits	12 mV
13	blk_7_co57_200_20_0_0_021211_1.fits	12 mV
14	blk_16_co57_200_0_0_021217_1.fits	20 mV ?**
15	blk_15_co57_200_20_0_0_021218_1.fits	12 mV

*: The values are from the "block-level science calibration record sheets" (white/blue log binders stored in the BAT lab.)

**: It was noticed that the value was set to 20 mV after several runs.

I) Measured lower threshold

I-a) Method:

For each of the above data sets:

1. corrected for the gain, offset and non-linearity for the electronics
2. rebinned into 4096 ch for the range of 3000 mV in pulser voltage

(The 11 testpulse peak and DNL run data were used to convert the raw ADU channel to pulser voltage unit.)

3. found the first five continuous non-zero bins for each detector channel
   LD: the left edge of the five bins was assumed as the lower threshold

I-b) Results for Block 13:

Measured lower threshold (LD) in pulser voltage unit:

The LDs were extracted for each detector channel for all the 16 blocks.

Here we show the results for the 2048 detector channels in block 13.

The guidelines are drawn to separate the 16 sandwiches.

In this specific run, the "threshold" value in the operation panel was set to 12 mV.

This value (12 mV) is far from the actual LD. (Jay has already indicated this difference between the setup value and the pulser voltage.)

There is a variation in LD even within the same sandwiches.

Measured lower threshold (LD) in energy

Converted the pulser voltage into energy utilizing the 122 keV line position

I-c) Results for 16 Blocks:

Measured lower threshold (LD) in energy

II) Combined LD distribution for all the 16 blocks:

Although the data sets were acquired in separate operations, it is interesting to combine the LD distributions for the 16 blocks and to see how the LD distribuetes over the entire CZT array

LDs are ranged from less than 5 keV to over 15 keV.

Here, let us integrate the above histogram.

This "integrated" LD distribution enables us to know the number of detectors above threshold at each energy, or the number of "effective detectors" at each energy.

With 12 mV of the setup threshold over the 16 blocks, we can achieve ~97% of the detectors avobe the threshold at 15 keV.

III) Conclusions:

1. I confirmed that there is a difference between the actual lower threshold in puser voltage and the setup value of the "threshold" in the operation panel.
2. I found a large variation in lower threshold between detector channels more than 10 keV.
3. However, if we can keep these threshold levels (12 mV), the nominal threshold of BAT instrument "15 keV" will be mostly achieved, which is good news!
4. Below ~15 keV, the LD variation should be taken into account for the energy response (effective area).