Beam Test Run Plan
- 1 General
- 2 Calibrations/Checks with Cosmics
- 3 Calibrations/Checks with Beam
- 4 Test Beam
- 5 Sample Histograms
- 6 How-to Instructions
- 7 Safety
Beam Request Document (GlueX-doc-620): Media:Bcal_test.pdf
The task of BCAL is to measure the positions and energies of photons emitted from interactions in the GlueX target from 10-117 degrees with respect to the beam direction. The photon position resolution of BCAL is determined by the time resolution. The timing information from BCAL will also be used to determine the time-of-flight information for charged particles entering BCAL after passing through layers of straw drift tubes (central drift chamber - CDC) surrounding the target.
Scope and Objectives
This is the summary of our test-beam goals for one week of beam time (Sep. 22-30, 2006) delivered to Hall-B during the period in 2006 when the CEBAF accelerator is running at 687 MeV. The electron beam will produce a photon beam using the Hall-B tagger system. Photons of various energies will be used to measure the response of a 4-m-long lead, scintillating fiber (Pb/SciFi) prototype module for the GlueX barrel calorimeter (BCAL). The 687 MeV endpoint energy will provide the minimum photon energies needed for these tests. The module will be mounted on a support system in the Hall-B alcove area. The module will be illuminated at several positions along its length and at several angles, from 0-80 degrees with respect to normal incidence. The data will be anchored to results obtained using cosmic rays and will allow us to measure energy resolution, time resolution and linearity and will also allow a validation of our Monte Carlo simulations.
- Measure the energy resolution of the center of the module at normal incidence and also determine the linearity over the range of photon energies from 137 to 653 MeV (see Table 1).
- Measure the timing resolution at the center of the module at normal incidence.
- Repeat the above energy and timing resolution measurements at the center of the module for four angles of incidence at and between 45-80 degrees.
- Repeat the above energy and timing resolution measurements at normal incidence and at four angles of incidence at and between 45-80 degrees for four positions from the center to the end of the module.
A 4~m-long prototype module, termed Module 1 was constructed in 2004 and this will be tested in the 2006 Test Beam at Hall-B. The module, shown schematically in the figure below, has a rectangular cross section of 13 x 23.0 cm^2 active area of Pb/SciFi. Over 70 km of double-clad fiber and 12 kg of epoxy were used in its construction. The scintillating fibers were made by PolHiTech and are blue-emitting with a peak emission wavelength of 420 nm and an attenuation length of ~350 cm. The fibers have a diameter of 1 mm (with 3% and 1% being the thickness of the first/inner and second/outer cladding layers) and the thickness of the lead sheets is 0.5 mm. The module has 186 planes of Pb/SciFi. The composite has a Pb:SciFi:Epoxy ratio of 37:49:14 and an overall density of ~ 5 gm/cm^3 and a radiation length (X_0) of 1.5 cm. Each layer of the module to be tested has 96 SciFi's spaced 1.38 mm apart (center-to-center) and the layers are such that they do not present any gaps in SciFi coverage thus presenting a uniform SciFi density across the shower path. The total weight of the module is 760 kg (including associated aluminum and steel plates) and the support carriage weighs an additional 550 kg. The former rests on top of the latter and both are supported by a cart that has a motorized, computer-driven, remote-controlled turn table.
For purposes of the beam tests, the readout will be based on a segmentation scheme of 3 x 6 light guides, each with a cross section of 4 x 4 cm^2, for a total of 36 channels. The channels are grouped in six (0-5) layers (depth, as the beam sees it) and three sectors (0-2) (width). The PMTs are Philips XP2020 (layers 0-2, front nine on each side - 18 in total) and Burle 8575 (layers 3-5, back nine on each side - 18 in total). This segmentation has been studied via Monte Carlo simulations of the detector and is shown schematically below. The choice of phototube is based on providing the best timing measurement and this is why the XP2020 are mounted on the front half of the Module, where the energy deposition is greatest. The detailed labelling of the PMTs and cables will be indicated in the GlueX E-Log and will be based on Hall-B orientation into North (right - as the particle travels) and South (left) ends.
Module Positioning and Rotation
The figure below schematically shows the 4~m-long BCAL module in several positions in the Hall-B alcove needed to achieve the beam test goals. The carriage and cart position the module so that the beam can enter at any point along the face of the module, but allowing a different range of angles at each point along the length, due to space restrictions of the alcove. The platform in front of the alcove has been modified for this setup and a steel plate has been installed as flooring: it stradles the platform and alcove.
The angle of incidence (with respect to normal) can be set to:
- between 10-25o in the alcove near one end of the BCAL
- between 10-50o in the alcove at 35-50cm from the end of the BCAL
- between 71-90o on the platform near the middle of the BCAL.
These numbers will be checked and further refined by Eric, a week before the run. In the interim, the H-B engineers will move a large bundle of cables from the south railing of the platform to a higher point after installing a support cable. This change will allow the BCAL to overhang the south railing at angles near the normal.
Calibrations/Checks with Cosmics
The initial calibration of the BCAL prototype module (Module 1) will be accomplished using cosmics.
Cosmic Flux and Count Rate
The area of each cosmics counter is 9 x 23 = 207cm2 =0.0207 m2. In this calculation, the solid angle between the two counters when placed above and below the BCAL (and thus separated by about 20 cm) is neglected. The cosmic flux crossing a unit horizontal area is 180 (130 hard/muons, 50 soft/electrons) m-2 s-1. This then leads to an expected rate for the hard muons of 2.7 s-1, or 9,720 hr-1, or when divided equally among 18 PMTs per side, we get 540 hr-1 PMT-1. Thus, each cosmic run needs to be about 4 hrs long to collect adequate statistics for an accurate determination of the ADC centroid and pedestals. (The TDCs, in principle, require less statistics.) If we look at cosmics with the BCAL-OR trigger this number goes up to 24000 hr-1 PMT-1 (over the entire area of of the BCAL), or 400 min-1 PMT-1 or ~7 s-1 PMT-1.
Using cosmics, the PMTs should be tested for functionality. This also tests the integrity of the cabling and electronics, when done on the PIE tower. To resolve cabling issue a scope can be used in the alcove area, connected directly to the PMTs.
PMT Gain Matching
The following procedure should be followed as a guide to gain-matching the PMTs.
- Plateau: Locate the plateau for each PMT and select a high voltage in that region that will allow the best gain match for all PMTs. This can be done using visual scalers, or better yet using the DAQ and integrating the number of counts in the TDC peak, after removal of time-outs. The use of the DAQ allows all 36 PMTs to be scanned for their plateau at the same time. Attention should be given to the threshold used on the discriminators. If it is low the electronic noise contribution should be checked carefully. As the HV increases one should make sure that the noise does not satify the threshold at any appreciable frequency. To expedite this task take the following measurements: 1700 V, 1900 V, 2000 V, 2100 V, 2200 V, 2300 V, 2400 V. Each measurement should have enough statistics in 24 hours. Based on past experience, the guide for the plateauing for different types of PMTs is:
- Burle 8575: Typically the plateau endures between 1900 V and 2400 V. The voltages can be safely set in this range, and its breadth provides flexibility in adjusting the voltages in order to equalize the gains on all PMTs.
- XP2020: The plateau endures between 1900 V and 2500 V for the two Regina XP2020UR. The IU ones should follow a similar behavior, perhaps with the need of an overall shift to account for the different PMT type. Initial checks show that they exhibit lower amplitude for the same HV compared to the Burle PMTs, perhaps a reflection of the bases used, as the XP2020's have higher gain by manufacturers' specifications.
- Gain Match: Select values for each PMT in its plateau region to allow for a global gain match between PMTs. Particular emphasis should be placed in accurately gain-matching the PMTs in each row of three (reading out the same depth of the calorimeter) whereas small adjustments can be made as a function of depth to correct for small differences in energy deposition due to any imperfect mip-bahavior of the cosmic muons. This method aims at a relative gain match. For the overall scale, the energy deposited by photons should be considered, for the entire range of tagged photons, to ensure that the dynamic range based on cell location is adequately covered by the ADCs. As example, look at the energy deposited at each readout cell for 550 MeV photons.
Once the PMT gains have been equalized, cosmic runs with good statistics should be collected to allow the determination of the ADC pedestals and TDC timing offsets. These will be used in the offline analysis (either in an ascii file or in a database). The analysis procedure will be spelled out soon in more detail.
Calibrations/Checks with Beam
Trigger and Commissioning
PMT Gain Matching
===DAQ Monitoring Histograms
Hall-B and Tagger
Hall-B Tagger startup:
- MCC has own procedure to establish beam to tagger dump.
- tagger hodoscope off
- turn on HV for beam-halo monitors (clas_epics > High Voltage > Beam HV)
- position (BPM 2C24 x,y) stable within 0.1mm
- current (I=5nA) stable to within 5%
- harp scan (clas_epics > Motors > Harps > tagger harp scan): beam profile typically: S/N > 10^4, sigma x,y < 150mu
if beam profile acceptable, record x,y positions at 2C21, 2C24
(Note: if you want to check the beam position in the dump: clas_epics > Misc > tagger_dump_lamp)
- move radiator in place (clas_epics > Motors > Radiators > A=5*10^-4)
- turn on tagger HV (clas_epics > High Voltage > Tagger HV > Crates #2,#7,#8)
- check E-counter and T-counter scalers ( ~clasrun/tagger/run_t_scalers, ~clasrun/tagger/ecounter_new/run_tagger_gui)
!adjust HV only if the rate for single counter is far too high/low!
- Check gamma_profiler (photon beam profiler in front of BCAL module: clas_epics > Photon Devices > gamma profile)
- Move 2.6mm collimator in beam (clas_epics > Motors > Collimators > 2.6mm)
- Check gamma_profiler whether collimator positioned correctly (collimator position can be adjusted using 'expert' gui in clas_epics > Motors > Collimators > expert)
- report averaged rate of "Tagger dump" beam-halo PMTs and gamma_profiler
|Photon Energy (MeV)||Fraction of electrons|
|Fraction of photons|
|Table 1: Hall-B Tagger characteristics at an endpoint energy of 687 MeV|
As outlined above, the objectives of this test beam include the understanding of the BCAL's response to photons and the validation of the Monte Carlo Code. The correspondence of the planned measurements to the angular range of the photons on the GlueX experiment are summarized here.
The primary goal of these measurements is to study the energy and timing response of the BCAL Module near its end and at large angles with respect to the normal. (See Module Positioning above for the angular range possible).
The secondary goal is to extract the effective speed of light in the fibers and the attenuation length, from measurements along its length. The latter would be a first-time extraction using photons and will be compared with past cosmics and charged-particle measurements at TRIUMF. (See Module Positioning above for the angular range possible).
Taking into account our goals, the BCAL+cart positioning constraints and general efficient operations, we are converging to the following priorities:
- Start with the BCAL on the platform position, with the beam impinging at its center and at normal incidence. Use the initial beam to check the performance of the BCAL and compare and calibrate it versus simulations. After the initial checks, complete the angular rotation scan at this point (~70-90o with respect to the beam).
- Move the BCAL to the alcove position. This is a ~2hr operation for the move, bolting plus interlock reset. Begin with a carriage setting that has the beam impinge at the maximum distance from one end of the BCAL that allows the full angle range scan (~10-45o). Repeat at closer points to the end (number of points to be specified in the future). Complete this objective by scanning the small angle range close to the end (~10-25o).
- Move the BCAL to the alcove position. This is a ~2hr operation for the move, bolting plus interlock reset. Measure at the two points closest to the ends of the BCAL (complete angular range in both, i.e. ~70-90o) and then continue at points on both sides, between the extremes and the middle, as time permits.
- Proposals from Christine to use a pre-radiator in front of the BCAL and from David to take measurements at different beam currents are being discussed.
From in the PDG:
where is the thickness of the material and the radiation length. For lead mm and this results in the table below.
It should be sufficient to run the 0.5, 1.0 and 3.0 mm pre-radiators. Regina will provide 4x4cm2 lead sheets (large enough to cover the expected beam spot on the BCAL) of thickness 0.5 mm each, to stack up to the desired pre-radiator thicknesses. All these runs should be at normal incidence and preferably at the middle of the Module.
Beam Current Runs
Study possible effects on beam current, from 0.5nA-10nA. (Our nominal mode of operation is 5nA). MCC has been notified that this will most likely take place on Monday, September 25.
Here, the expected number of hours and runs will be placed. Nominally, we take runs that have 10 millon events (0.85 Gb/file). Hall access will be limited to business hours, unless preplanned to ensure that trained personnel are on hand. The Alcove position runs are still being defined...
|Beam Current||Position||Run Number||Z-position||Angle||Height||Converter||Trigger Bit||Threshold||Comments|
Sample histograms of the measured and simulated energy deposition and timing information in the readout cells of the BCAL Module will be continuously updated on the Wiki. Have a look of the first simulated histograms.
How do I change the high voltage on the PMTs? How do I make a log book entry? What do I need to know for the rotation or translation of the BCAL Module? Have a look at our extensive list of HOWTOs.
Temporary Operational Safety Procedure (TOSP): Media:TOSP.pdf
Experimental Safety Assessment Document (ESAD): Media:ESAD.pdf
Radiation Safety Assessment Document (ESAD): Media:RSAD.pdf
Conduct of Operations (COO): Media:COO.pdf