From tigwiki

[edit] Introduction

The Compton suppression shields for the detectors in the TIGRESS array are used for vetoing the Compton scattered background from the detected gamma-rays. In total each clover can be surrounded by a maximum of 20 segments of scintillator material that are classified into three groups; the front shields, the side-catchers and the back-catchers respectively.

The figure below shows a Ge clover with the cross section of its suppression shields.

Each suppressor unit consists of two scintillators. Light is collected from each scintillator using two photo-multiplier tubes and their light-guides. Therefore, each suppressor unit has 8 cables associated with it. There are 4 HV cables for biasing the PMTs and 4 cables for signal outputs. The table below provides some basic information about the suppressors and their respective PMTs.

[edit] Making the connections

HV CABLING VIEWED FROM BACK OF DETECTOR

SIGNAL CABLING VIEWED FROM BACK OF DETECTOR

• First connect the High Voltage cables from the photomulipler tubes (PMTs) to the HV supply using the cabling scheme shown here. The HV cables to the PMTs can easily be distinguished from the signal cables from the LEMO connectors. The LEMO HV connectors from the relevant PMTs should then be connected to the appropriate channel number in the aluminum HV distribution box on the TIGRESS frame. Each TIGRESS detector has one such box associated with it. This box can identified by following the red cables from the 48 channel, 3 kV HV power-supplies in the TIGRESS shack to the detector set up.

• Connect the two signal outputs from each PMT to its associated pre-amplifier unit on the TIGRESS frame. Each TIGRESS detector has four such pre-amplifier units which can be located on the frame at the back of the detector, one each on the top, bottom, left and right respectively. These two signal outputs are then summed to give one output that is sent to channels 11-15 on appropriate TIG10 digitizers in the TIGRESS shack. The connection scheme is shown here.

[edit] Matching the gains

• To turn on the bias to the PMTs type the following command in a terminal window on the computer in the TIGRESS shack (this can be done from any computer, but it is assumed that you want to be close to the detectors while doing this): telnet tighvxx 1527, where xx is 00, 01, 02 etc. You will need a username and password; ask someone (except Greg) for that information.

• Press Enter on main and again on channels

• The bias supply parameters will come up. Check the name of the cable from the HV supply distribution box to the power supply.

• Make sure all the voltage values (V₀Set) are set to a nominal of +850 Volts and the current values (I₀Set) are set to 3.0 mA for all the cable names as you scroll down (Aquarius, Leo, Taurus, Virgo etc). These cable names are shown as BGOsome number Zodiac.

• After the nominal values have been set all the PMTs for a particular cable can be powered on simultaneously by pressing the space bar button after moving the cursor to the on(off) position in the Pw column using the arrow keys. Individual PMTs can be turned off or on using the same method by moving the cursor to the appropriate channel number shown in the cabling scheme.

• Make sure the preamp units are powered on. If not turn on the toggle switch in the related power supplies (colored gold) in the TIGRESS shack. Make sure the LEDs in the power supply are all green.

• Now take a ¹³⁷Cs source and place it close to one of the front scintillators. The source emits a 662 keV gamma-ray that can be used for gain matching.

• Turn off one of the PMTs associated with this shield by setting the voltage value to zero in the correct channel (using the terminal window HV menu on the computer)

• Connect the output from that channel from the TIG10 module to observe the pulses on a scope as explained in Voltage Matching

• Increase/decrease the bias voltage (and hence the gain) on the tube such that the pulse-height of output pulse is approximately 330 mV. Make sure you do not exceed the maximum allowable bias voltage for the PMTs

PULSE HEIGHT OF SIGNALS WITH ONLY ONE PMT ON

• Now turn off the bias to this PMT and turn on the other PMT and repeat the same procedure.

• Turn on both the PMTs with the adequate bias voltages and check that the summed pulse height is at approximately 662 mV.

PULSE HEIGHT OF SIGNALS WITH BOTH PMTs ON

• These two suppressor PMTs are now 'gain matched' at approximately 1keV/ch. Proceed to do the same on all the other suppressor units.

• Check the histograms on the DAQ computer to see if centroids of all the spectra come at approximately the same channel.