2024 Summer Downtime changes

• The first half of the calorimeter has been removed (first blocks of lead glass)
• The lead glass is totally black now

PMT circuit & ground management

OLD DOCUMENTATION (before 2024)

General

The positron calorimeter is aimed to measure single positrons that are produced in a laser-electron interaction (pair production) in the picnic basket.

It consists of photomultiplier tubes (PMTs) coupled to lead glass blocks.

Charged particles/energetic radiation and their secondaries that passes through the glass blocks will produce Cherenkov radiation.

The Cherenkov light propagates through the glass and leaves through a small aperture onto the active area of the PMTs.

The PMTs require a high voltage power supply and signal cables with low attenuation at high frequencies.

For the beam time Aug 10-15 2021, 3 calorimeter channels and PMTs were installed.

See also Felipe's paper describing the calorimeter and tracking layer design: https://arxiv.org/abs/2107.03697

Setup in the tunnel and gallery

The lead glass and the PMTs are installed on a frame between the PDC and the EDC chamber.

The high-voltage cables are connected directly to the PMT units and are routed up to the gallery.

Old setup: The HV power supply and the potentiometer are in rack FKG20-23.

Update Nov 2021: We are now using a remotely controlled high-voltage supply with multiple individually controllable channels.

The supplies are CAEN DT5533E.

The signal side is connected to a short (~0.5m) RG58 BNC cable, which in turn is attached to a long (~20-30m) and stiff jumper cable (LMR400).

The long jumper cable is routed from the PDC/EDC area to the IP area and connected there to the Coax Trunk Panel (144-066).

The channels that are used are J1,J2,J3 (for Ch1,2,3).

The patch panel in the tunnel leads up to the patch panel in the gallery in rack FKG20-2X.

RG58 cables connect from the patch panel to the oscilloscope that records PMT traces rack FKG20-26.

J1,J2,J3 are channels 1,2,3 respectively on the oscilloscope.

Channel 4 is the incoming TTL beam trigger.

Scope name: Tektronix TDS 3054C

Images from installation around Aug 2021:

EPICS oscilloscope

The oscilloscope can be accessed on EPICS via:

Home Screen → Experimenter → LI20 → Experimenter Scope B (in column FCKG20-26)

At the moment, this diagnostic is not integrated in any DAQ and it does not have any timestamp/beam ID.
Initially, a simple way to grab this data is using a python/Matlab script, but if this is done for multiple diagnostics, the reading/writing will not allow for 1 Hz operation.

The channels are 

• Ch1: on-axis signal PMT, closest to the pipe
• Ch2: on-axis signal PMT, on top of Ch1
• Ch3: background PMT, on aisle side
• Ch4: beam trigger trace

EPICS HV supply

Since Nov 2021, the new HV supplies are integrated into EPICS.

The supplies are in the bottom shelves of FCKG20-23.

In EPICS they can be found on Main screen → Experimenter → LI20 → FCKG20-23

There are two supplies with 4 channels each as of yet, named HVM:LI20:MC01 and MC02.

Currently, we are using channels (0,1,2) of MC02 for PMT channels (Ch1,2,3), respectively.

The supplies lose connection sometimes. Connection can be re-established by power cycling.

You need to set max voltages and current set points. If you go above the current set point, the channel will disable and the OVC (over current) signal goes on in the gallery.

To change the voltage, enter the voltage set point and press 'enter'. The ramp up rate needs to be >1 V/s for it to respond.

To power cycle the HV supplies remotely, you can use the remote power strip.

Main screen → Experimenter → LI20 → FCKG20-23

There is a new button for the power supplies and on the lower end you will find E320 HV 1 and 2.

Scope trigger

Home Screen → Experimenter → LI20 → Triggers... (in FKG20-24) → Rear Panel Triggers, Ch0, Name: E320 Scope

The current suitable trigger delay is around 1780 ns from the 'Beam' trigger.

Relevant PVs

SCOP:LI20:EX01

Troubleshooting

Oscillations on trace

Channel cross talk

There appears to be some cross-talk between Ch4 and CH3, which leads to a downwards and upwards spike at the beginning and end of the TTL trigger.

Long recovery time

Saturation? Collective effects?

Fluorescence?

Window scintillation?

Radiation/beam dump?

Wrapped channel in aluminium foil to compare with standard reflector foil.

Blocked one to see if any direct hits.

http://physics-elog.slac.stanford.edu/facetelog/show.jsp?dir=/2021/42/22.10&pos=2021-10-22T20:49:59

Update Nov 2021: No clear distinction for long recovery time, still signal on blocked channel http://physics-elog.slac.stanford.edu/facetelog/show.jsp?dir=/2021/47/22.11&pos=2021-11-22T18:38:33

Radiation damage

The lead glass channels experienced significant darkening due to radiation.

We measured the transmission using a red diode laser and a photodiode. We also monitor the dose levels with dosimeters and RCF foils.

January 2022

The transmission in the first block was still 0.6%, the second block degraded to 35% transmission.

Evaluating radiation damage during January downtime:

• inspected lead glass channel for progressing radiation
  damage http://physics-elog.slac.stanford.edu/facetelog/show.jsp?dir=/2022/03/18.01&pos=2022-01-18T17:23:02
  and http://physics-elog.slac.stanford.edu/facetelog/show.jsp?dir=/2022/03/19.01&pos=2022-01-19T22:02:51
• collected dosimeters for analysis and installed a new set http://physics-elog.slac.stanford.edu/facetelog/show.jsp?dir=/2022/03/19.01&pos=2022-01-19T10:56:45
• collected RCF for analysis and installed a new one http://physics-elog.slac.stanford.edu/facetelog/show.jsp?dir=/2022/03/20.01&pos=2022-01-20T00:38:18

October 2021

The transmission was 0.6% through the first (upstream) block and 58% through the second block. http://physics-elog.slac.stanford.edu/facetelog/show.jsp?dir=/2021/40/06.10&pos=2021-10-06T13:59:05

Felipe provided some literature that suggests this requires doses in the multi krad regime.

Reference on f2 lead glass:

Reference on PMT window darkening:

Note that dosimeters in the area are only showing 10s of rad after 160 hours of operation, but they are near the outside of the chamber, so a bit away from the axis.

This indicates that the dose levels are much higher close to the axis, but we need to confirm the numbers.

For that purpose we installed RCF between the tracking layers and the calorimeter (Oct 2021): http://physics-elog.slac.stanford.edu/facetelog/show.jsp?dir=/2021/42/22.10&pos=2021-10-22T20:16:40

We also installed additional dosimeters in the area close to the axis (Oct 2021): http://physics-elog.slac.stanford.edu/facetelog/show.jsp?dir=/2021/45/10.11&pos=2021-11-10T21:49:51