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[edit] Building the Setup

[edit] Clover and Source Installation

• The clover is mounted in a normal TIGRESS stand
• Place the clover on the scanning table so the legs are within the bolted-down square in the centre
• The blue and green crystal side should face west
• Secure the detector stand to the bolted-down pieces with bolts
• Connect the Liquid Nitrogen hose and sensor to the detector. It is important to wrap the LN2 sensor in insulating material to electrically isolate it from the detector stand!!
• Connect the pigtail signal cables, HV supply and BSD cables. String all cables through the top of the stand so that they hang with no tension on them
• Ge core and segment signals populate the first 4 tig10s of the scan experiment
• Place four lead bricks on the plate in the centre below the detector in preparation of the source being installed
• Move the table to the x=-200, y=0 location. See #Controlling the motor
• Install the collimated ¹³⁷Cs source under the detector. It is quite a tight fit. To get the source in tilt the top towards the detector and slide it under until the locating pin in the bottom of the source holder fits into the hole at the centre of the plate. Use caution when handing this source, it is a strong source and is well collimated at the top but not at the bottom. Always hold at arms length from your body and dont point it at yourself or anyone else
• Fit the lead shielding tightly around the source
• Move the table back to x=0, y=0. See #Controlling the motor
• Now it is time to calibrate the scan table

[edit] Shielding Installation

Relative positions of the clover can, crystal and lead shielding

• Bolt the two cross support-structs on the east and west sides of the detector stand using the holes ~20cm from the base
• Insert the support plate into the detector stand so that the holes mesh with the alignment bolts of the support structs
• Place four pieces of lead on the support plate. The lead for here is 35mm tall and has most of the volume cut out. One piece for each side of the detector. Take care that the lead is not touching the Ge can
• Attach the side pieces on the north and south sides
• Retract the alignment bolts of the side pieces so that they play no part of the alignment proceedure
• Use the alignment bolts of the support plate to adjust the height so that the top of the lead pieces, on all sides, sits 20.5mm below the rim of the germanium detector can as shown in the diagram to the right
• Adjust the alignment bolts of the side pieces so that they are resting on the bolts
• Check that the alignment of the lead has not changed
• Place a 1mm thick spacer at each corner of the Ge detector to support the next layer of lead
• Place the second layer of shielding on top of the spacers, these are solid pieces of 30mm tall lead. There should be a 1mm gap between these layers
• Place two 1mm thick spacers (easiest to tape them together) at each corner of the Ge detector to support the top layer of lead
• Place the top layer of lead on these spacers. This layer is solid 70mm thick pieces of lead
• Ensure that the lead is not touching the Ge can anywhere

[edit] BGO Installation

Relative positions of the detectors on the coincidence scanning table‎

• Place a layer of insulating material on the support plate and side pieces - This is to reduce RF pick-up in the BGO detectors
• Place the BGO detectors around the lead shielding in number order starting with BGO1 on the south side facing the blue Ge crystal, as indicated in the diagram to the right
• Plug in the corresponding HV cable to each BGO and a lemo-lemo signal cable between each detector and the corresponding input channel of the preamp box
• BGO preamp power is supplied by a quad power supply in a NIM crate with circulat 9-pin connectors in the back
• The outputs from the preamp box must pass through a Phillips 776 amplifier before the signal is put into the spare pigtail cables
• BGO detectors populate the 5th and 6th tig10s of the scan experiment
• The BGO signals must be gainmatched by illuminating the detector with a ¹³⁷Cs source (662keV gamma rays) and adjusting the bias voltage so that the signal (after passing though a Phillips 776 amplifier) observed on a scope is 300mV

[edit] Starting the Acquisition

Here is a table of the clients etc which are required and where they can be found/started.

[edit] Calibrating the Scan Table

The scanning table motor position must be calibrated each time a detector is mounted on the table because each stand sits slightly differently relative to the source.

See Scanning Table Setup

[edit] Setting up the ODB

[edit] Scan - Coincidence Mode

• Set the 'multiplicity' in /equipment/trigger/setting/master/Global multiplicity mask to '0xFFC' for coincidence mode
• Set the 'require mask' in /equipment/trigger/setting/master/require mask to '0x480' for Ge+BGO coincidence. Note that the port mask of the 2nd BGO tig10 has been made equal to the first so that a mask which demands a coincidence between only two tig10s can be used

[edit] Calibrations - Singles Mode

• Set the 'multiplicity' in /equipment/trigger/setting/master/Global multiplicity mask to '0xFFE' for singles and above
• Set the 'require mask' in /equipment/trigger/setting/master/require mask to '0x400' for BGO singles or '0x0' for any singles
• More detailed instructions on single scan setup.

[edit] Starting a Scan

• To start a full coincidence scan from the beginning click the "Start Interpolation" button on the MIDAS status page. The motors frontend then takes control and moves the source to the correct location then starts a new run. The motors frontend then waits for fetigcol to end the run (from either a time or event number limit) before moving to the next point and the next run.

• To start a coincidence scan for a point not the first first change the Start Point variable in the ODB, /equipment/motor/settings/interpolate/, to the point you want the scan to start from. Then follow the same proceedure as stated above.

[edit] Stopping a Scan

When crashes occur the acquisition is usually left running state so must be stopped before the problem can be fixed.

1. First click 'Stop Interpolation' in the MIDAS status window
2. Wait until the words 'Interpolated scan completed' appear in the tab of the 'Motors' frontend
3. Click 'Stop' in the MIDAS status window

If this procedure is not followed (ie the interpolation stopped first) then the motors frontend does not know anything is wrong and proceeds to the next position and starts a new run.

[edit] Controlling the motor

The motors are controlled by the Motors frontend, see #Starting the Acquisition. To move the table you must enter the desired destination in the odb, /equipment/motors/settings/destination(mm), and then tell the motors to move by changing the /equipment/motors/settings/move variable to 'y'.

The present position of the table is displayed in the variables section of the motors odb, /equipment/motors/variables/position(mm)

[edit] BGO Detectors

Electronics diagram for the BGO detectors‎

BGO scintillators are used to create a coincidence with the Ge to identify 90 degree Compton scattered events. The signals out of the LECROY VV100B preamps are too small (~30mV) for direct input into the tig10s so must pass through a Phillips 776 amplifier which gives a x10 gain in signal amplitude (~300mV).

The bias voltages applied to the BGO photomultiplier tubes must be gainmatched to give a similar amplitude signal out. This is done with a ¹³⁷Cs source and the signals gainmatched to 300mV on a 50Ω terminating scope. The latest voltages used are shown in the following table. Last updated 22nd August 2008 (Scan of Clover 10)

[edit] Calibrating the BGO detectors with ¹⁵²Eu coincidence data

Partial decay scheme of 152Eu

Example of energy coincidence gating to identify calibration peaks in the BGO spectrum

Remove the top two layers of lead that surround the Ge detector. Take 4 runs in Ge-BGO coincidence mode with a 500,000 event limit in which a 152Eu source is placed on the lead at each corner of the Ge and the require mask is changed appropriatly. To start a calibration run (ie without moving the motors) just click the 'start' button on the MIDAS status page. The following table shows the configuration:

Calibration points in the BGO spectrum of known gamma rays can be identified by coincidence energy gating of the Ge spectrum. By gating on the 778keV gamma ray and separately on the 1408keV gamma ray, the 122keV and 344keV peaks are observed in the BGO spectrum.