Studying the time response of a vacuum phototriode and measurement of gamma radiation damage to high voltage capacitors and resistors

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.A vacuum phototriodes (VPT) are the photodetectors used in the endcaps of the Electromagnetic Calorimeter of the Compact Muon Solenoid experiment at the Large Hadron Collider at CERN. Software, interfacing with the commercial program “SIMION 3D” was written to allow the simulation of the temporal response of the VPT. Applying Ramo’s Theorem enabled the time development of the VPT signal to be calculated. In order to validate the simulations, experiments were performed using a 60 ps laser pulses (λ = 435 nm) incident on a number of VPT samples. The simulation reproduced the basic features of the operation of the VPT such as gain vs. voltage, and gain vs. magnetic field strength. The simulation also confirmed the need for a fine mesh anode to achieve a useful gain when operating at high magnetic fields. The experimental work represents the first measurements of the time response of a VPT when excited with a very fast light pulse. Both the simulated and experimental response from the VPT were observed to be fast (few ns) and quite complex. Discrepancies between the simulated and experimental signals were partially explained by a SPICE model which includes the VPT interelectrode capacitances and lead inductances. We conclude that the VPT are fast photodetectors with an intrinsic response time of order 1ns for this geometry. The VPT high-voltage filter cards, which operate at 1 kV and in an intense radiation environment, are critical components. A number of commercial off-the-shelf high voltage resistors and capacitors were evaluated at gamma doses up to 345 kGy. No significant change in value or leakage current was observed. As a result of these studies we were able to demonstrate that these inexpensive components were suitable for use in the 3100 filter cards subsequently installed in the CMS apparatus

Simulation of a vacuum phototriode with SIMION 3D

An electron-optic model of a 26 mm diameter vacuum phototriode (VPT) photodetector was developed using SIMION 3D software extended by additional code to simulate secondary emission at the dynode. The predictions of the variation of gain with magnetic field for mesh anodes with 100, 40 and 7 lines per mm and fields from 0 and 4 T are presented. The predicted time development of the signal at 0 T is presented and compared with experimental data obtained by illuminating a production VPT for the electromagnetic endcap calorimeter of CMS with 60 ps laser pulses at a wavelength of 435 nm

Studying the time response of a vacuum phototriode and measurement of gamma radiation damage to high voltage capacitors and resistors

A vacuum phototriodes (VPT) are the photodetectors used in the endcaps of the Electromagnetic Calorimeter of the Compact Muon Solenoid experiment at the Large Hadron Collider at CERN. Software, interfacing with the commercial program “SIMION 3D” was written to allow the simulation of the temporal response of the VPT. Applying Ramo’s Theorem enabled the time development of the VPT signal to be calculated. In order to validate the simulations, experiments were performed using a 60 ps laser pulses (λ = 435 nm) incident on a number of VPT samples. The simulation reproduced the basic features of the operation of the VPT such as gain vs. voltage, and gain vs. magnetic field strength. The simulation also confirmed the need for a fine mesh anode to achieve a useful gain when operating at high magnetic fields. The experimental work represents the first measurements of the time response of a VPT when excited with a very fast light pulse. Both the simulated and experimental response from the VPT were observed to be fast (few ns) and quite complex. Discrepancies between the simulated and experimental signals were partially explained by a SPICE model which includes the VPT interelectrode capacitances and lead inductances. We conclude that the VPT are fast photodetectors with an intrinsic response time of order 1ns for this geometry. The VPT high-voltage filter cards, which operate at 1 kV and in an intense radiation environment, are critical components. A number of commercial off-the-shelf high voltage resistors and capacitors were evaluated at gamma doses up to 345 kGy. No significant change in value or leakage current was observed. As a result of these studies we were able to demonstrate that these inexpensive components were suitable for use in the 3100 filter cards subsequently installed in the CMS apparatus.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Energy Resolution Performance of the CMS Electromagnetic Calorimeter

The energy resolution performance of the CMS lead tungstate crystal electromagnetic calorimeter is presented. Measurements were made with an electron beam using a fully equipped supermodule of the calorimeter barrel. Results are given both for electrons incident on the centre of crystals and for electrons distributed uniformly over the calorimeter surface. The electron energy is reconstructed in matrices of 3 times 3 or 5 times 5 crystals centred on the crystal containing the maximum energy. Corrections for variations in the shower containment are applied in the case of uniform incidence. The resolution measured is consistent with the design goals

Energy Resolution of the Barrel of the CMS Electromagnetic Calorimeter

The energy resolution of the barrel part of the CMS Electromagnetic Calorimeter has been studied using electrons of 20 to 250 GeV in a test beam. The incident electron's energy was reconstructed by summing the energy measured in arrays of 3x3 or 5x5 channels. There was no significant amount of correlated noise observed within these arrays. For electrons incident at the centre of the studied 3x3 arrays of crystals, the mean stochastic term was measured to be 2.8% and the mean constant term to be 0.3%. The amount of the incident electron's energy which is contained within the array depends on its position of incidence. The variation of the containment with position is corrected for using the distribution of the measured energy within the array. For uniform illumination of a crystal with 120 GeV electrons a resolution of 0.5% was achieved. The energy resolution meets the design goal for the detector

Results of the first performance tests of the CMS electromagnetic calorimeter

CMS ECALPerformance tests of some aspects of the CMS ECAL were carried out on modules of the "barrel" sub-system in 2002 and 2003. A brief test with high energy electron beams was made in late 2003 to validate prototypes of the new Very Front End electronics. The final versions of the monitoring and cooling systems, and of the high and low voltage regulation were used in these tests. The results are consistent with the performance targets including those for noise and overall energy resolution, required to fulfil the physics programme of CMS at the LHC

The CMS Barrel Calorimeter Response to Particle Beams from 2 to 350 GeV/c

The response of the CMS barrel calorimeter (electromagnetic plus hadronic) to hadrons, electrons and muons over a wide momentum range from 2 to 350 GeV/c has been measured. To our knowledge, this is the widest range of momenta in which any calorimeter system has been studied. These tests, carried out at the H2 beam-line at CERN, provide a wealth of information, especially at low energies. The analysis of the differences in calorimeter response to charged pions, kaons, protons and antiprotons and a detailed discussion of the underlying phenomena are presented. We also show techniques that apply corrections to the signals from the considerably different electromagnetic (EB) and hadronic (HB) barrel calorimeters in reconstructing the energies of hadrons. Above 5 GeV/c, these corrections improve the energy resolution of the combined system where the stochastic term equals 84.7$\pm$1.6$\%$ and the constant term is 7.4$\pm$0.8$\%$. The corrected mean response remains constant within 1.3$\%$ rms