BGO Calibration
This document covers calibrating the BGO array using a radioactive gamma source.
Note:
On the head or gamma-ray side, each of the 30 BGO signals is split (split anode panel), with one branch going to the ADC (including an appropriate delay to put it inside the gate) and the other fed into a constant-fraction discriminator (CFD) channel. Due to the number of signals, two CFDs are employed, with each able to accommodate up to 16 signals. Each channel of the CFD has an individual output that is fed into a TDC measurement channel. Additionally, each CFD provides an OR output that fires any time one of its channels receives a signal. These two OR outputs are what is fed into the IO32 ECL input to generate the "grand" trigger for the head side. In addition to the CFDs, the accelerator RF pulse is also fed into one of the TDC measurement channels, to provide a time reference that correlates with the generation of a beam particle.
Calibration Instructions
- Get the CmC13 source (Make sure, you have the necessary radiation safety training and source handling training!) from the shielded lead safe on the DRAGON floor (usually somewhere south of the desk). This source produces a 6.13 MeV gamma-ray via the 13C(alpha,n)16O + gamma reaction. The key to the padlock is inside the red DRAGON source folder, located on top of the radiation source safe. The source is placed in a paraffin container inside the shielded safe. Always make sure to put it back into the paraffin before closing the safe after use. Wear gloves when handling the source and lead in the shielded safe.
- Carefully crank the "far" BGO array (N/E) all the way in. Then crank in the "near" array almost all the way. Be careful not to damage the thin copper line that connects to the pressure gauge. The plumbing here is often source of leaks. Make sure any C-clamps have been removed from the target housing, and check that nothing could collide with the BGOs when cranking the array in. You will see a small gap between the top of the gas target box and the detectors. Carefully wedge the 9 x 20mm source capsule in there (z=0) and then close the "near" array fully. Note: You have to calibrate with the array fully closed - the phototubes are sensitive to very small local magnetic field changes and it was determined that the detector position does affect this!
- Go to the DRAGON MIDAS page https://smaug.triumf.ca:8443/, and click "Adjust BGO Thresholds" on the menu bar on the left. Set the variable "Threshold Gamma Energy" to the maximum value of 2500 keV. Note that "Threshold Gamma Energy" corresponds to the threshold in keV. Then go back to the status page and click "Apply thresholds".
- Open up a terminal and ssh dragon@smaug.triumf.ca, issue hvcalib (./hvcalib) to start the calibration program.The start page for the HV calibration program will appear (see Fig. 1).
