Difference between revisions of "HV and Threshold calibration"

Revision as of 03:21, 14 April 2010

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, and a cut on tdc(L)-tdc(R) 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, except for bars where the gain correction needed is higher then some factor. In these cases we should manually check the bars for any hardware problems (PMT connectivity, base etc..), and perform a HV re-callibration.

• Veto-bars gain calibration with the H(e,e'p) reaction.

Form a veto(L) & veto(R) trigger, to create equal attenuation for both bars.

Find the ADC peak for the protons.

Callibrate the adc gain cofficient so that the threshold will be set below the protons peak.