HV and Threshold calibration
Procedure for the neutron array HV and threshold calibration
Main considerations -
- calibrate all neutron bars to equal gains.
- set gains as low as posibble, to reduce current on PMT's.
- set threshold energy as low as possible to increase efficiency.
- set threshold energy high enough to remove low-energy EM gackground.
- the ADC full range is channels 0-4000.
optimum was chosen at threshold = 5 MeV = 300/4000 channels - need to re-check.
this is equivalent to 1 MeV = 60/4000 channels
The chosen descriminator threshold voltage is 50 mV, which is choose slightly above the hardware limit of 30 mV.
Calibrations steps -
- Test-area calibration using cosmic muons
Use top-buttom bar trigger, to obtain vertical minimum ionizing muons with known energy deposit.
adjust HV, so that the muon peak will be at-
10.0 cm bar -> E=20 MeV -> channel=1200/4000
12.5 cm bar -> E=25 MeV -> channel=1500/4000
15.0 cm bar -> E=30 MeV -> channel=1800/4000
25.0 cm bar -> E=50 MeV -> channel=3000/4000
- Beam calibration using the H(e,e'p) reaction.
Use the H(e,e'p) reaction to create 1 GeV/c protons which will have energy loss of 32 MeV for a 10cm wide bar.
Calibrate the gain cofficients (software), so these events will be at channel=1920/4000.
This should be a software calibration, exceptfor bars where the gain correction needed is higher then some factor. In this cases a HV re-calibration will be done first.
