Threshold conversions

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The four thresholds – front slow, front fast, rear slow, and rear fast – are controlled by DACs that can be set from 0x00 to 0xFF (i.e., 0 to 255 in decimal). The slow thresholds were calibrated in detail prior to launch, but it is likely that the calibrations have drifted over time.

Contents

Front slow threshold

Here is a conversion table of selected DAC settings to keV according to the pre-launch calibration. The calibration is a linear function, and the nominal DAC setting is 0x0C, which is ~3 keV.

DAC 1F 2F 3F 4F 5F 6F 7F 8F 9F
0x0C 2.68 2.66 2.58 2.82 2.61 2.51 2.32 2.63 2.52
0x10 3.49 3.44 3.32 3.59 3.40 3.26 3.09 3.42 3.25
0x20 6.68 6.55 6.27 6.66 6.57 6.27 6.15 6.58 6.19
0x40 13.1 12.8 12.2 12.8 12.9 12.3 12.3 12.9 12.1
0x80 25.9 25.3 24.0 25.1 25.6 24.3 24.5 25.6 23.8
0xC0 38.6 37.7 35.7 37.4 38.2 36.3 36.8 38.2 35.5
0xFF 51.2 50.0 47.4 49.5 50.7 48.1 48.8 50.7 47.1

Front fast threshold

The nominal DAC setting is 0x22, which is ~6 keV.

Rear slow threshold

Here is a conversion table of selected DAC settings to keV according to the pre-launch calibration. The calibration is a linear function, and the nominal DAC setting is 0x30, which is ~20 keV.

DAC 1R 2R 3R 4R 5R 6R 7R 8R 9R
0x30 24.1 23.4 24.0 24.3 24.5 23.9 20.8 19.0 21.1
0x40 27.3 26.4 27.2 27.6 27.5 27.1 24.6 22.5 24.1
0x80 40.0 38.5 40.1 40.5 39.7 39.8 39.9 36.6 36.0
0xC0 52.6 50.7 52.9 53.5 51.9 52.4 55.3 50.6 47.9
0xFF 65.1 62.6 65.5 66.3 63.9 64.9 70.4 64.5 59.6

Rear fast threshold

The nominal DAC setting is 0x45, which is ~20 keV. The step size is ~0.28 keV/step.

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