FLC Group Testbeam Studies of the LaserWire Detector

FLC Group Test-beam Studies of the Laser-Wire Detector 13 September 2006 Maximilian Micheler Supervisor: Freddy Poirier

Introduction The goal of the Test-Beam Studies is to measure the performance of the lead-tungstate detector used at the Laser-Wire experiment at PETRA II: → Calibration, for the use of the detector at the Laser-Wire experiment → Resolution, mainly for simulation purposes

Experimental setup • Detector is placed in line with the electron beam as to achieve an EM shower in the centre of the lead-tungstate crystal. (achieved by mounting the detector on an adjustable) • Hamamatsu R 6236 photomultiplier tube (54 mmx 54 mm active photo cathode surface) for detection of the energy dissipation in the crystal • Two scintillators (one horizontal and one vertical scintillator) were placed in front of the detector and connected to a coincidence unit to only record signals for electrons which entered the crystal in the centre • The electron beam energies were selected by changing the currents in the magnets using a computer with a provided software in the Test-beam 24 control room.

Electronics Scintillator signal processed using: • Amplifier • Discriminator • Coincidence unit • TTL signal converter TTL signal used for triggering integration of PMT output signal. Using a Computer (ADC Break-out box in the figure) and an integrator card to integrate the PMT signal and to record the area. The setup of the test-beam studies was chosen to have the same conditions as in the Laser-Wire Hut: • Identical electronics for the read-out system, i. e. ADC box and integrator card • Similar timing for read-out system

Data Acquisition • Signal from PMT is a negative voltage peak. • Integrator card integrates this peak and outputs the area as a positive voltage which is recorded by the computer. • For every single beam energy and PMT supply voltage around 1000 integrator card voltages have been measured. • The 1000 area measurements were plotted on a histogram and fitted with a Gaussian function • The two graphs are taken for a PMT supply voltage of 1115 V at a beam energy of 3. 6 Ge. V • Mean, Standard deviation, and Errors on the Standard deviation from Gaussian function

Detector Performance • Calibration plot: the mean of the integrated signal against the corresponding beam energy for the 4 different PMT supply voltages • This calibration plot was expected to show a linear dependence of the integrated signal on the beam energy • Linear fittings of the individual data sets show a non-linear behaviour Resolution plot Calibration plot • Resolution plot: the PMT normalised resolution (Gaussian width/integrated signal) against the beam energy • Resolution is expected to decrease with increasing beam energy. • At a PMT supply voltage of 1300 V this is not the case

Control Tests Possible reasons why the results differ from expectation: • Saturation of PMT • Cutting the signal → Incorrect delay →Incorrect integration width • Saturation of the read-out system Investigation of the raw PMT output signal recorded from the oscilloscope (as shown). The following quantities were directly calculated from the raw signal: • Signal amplitude (difference between constant base line and minimum voltage) • Signal area (sum of the relative signal w. r. t. the base line within the integration range)

Control Tests – PMT saturation effects • Plot of signal amplitude against signal area shows a linear dependence: Fit: y = 0. 0274 x + 0. 0474 where y represents the signal amplitude and x the signal area. • Plot of signal amplitude against beam energy shows a linear dependence: Fit: y = 0. 404 x + 0. 076 where y represents the signal amplitude and x the beam energy. • Therefore the signal area also shows a linear dependence on the beam energy → No detector saturation while increasing the beam energy

Control Tests – Cut-off effects • 1μs integration delay w. r. t. the TTL trigger, 2. 4μs integration width • The two graphs show a integration with an asymmetric cut-off of the signal from the tail and the front of the PMT raw signal, respectively. tail • Signal area decreases due to cut-off. However, the plots of the signal area against the beam energy for the different cut-off points are still linear. → Cut-off effects do not seem to be responsible for the non-linear characteristics of the integrated signal →No cut-off effects due to integration width as width is sufficiently long enough to detect entire signal (width approx. 110 ns) front

Control Tests – Read-out system saturation At the present time, the read-out system is under study to check how it performs with voltages higher than it is designed for. Conclusions • Resolution plot and Calibration plot achieved for the Laser-Wire detector • Results used for calibration of the signal at the Laser-Wire experiment • After the calibration the data from the Laser-Wire experiment will be compared with previous simulations