Conference Paper

Prototype Tests and Construction of the Hadron Blind Detector for the PHENIX Experiment

Dept. of Phys., Brookhaven Nat. Lab., Upton, NY
DOI: 10.1109/NSSMIC.2006.354195 Conference: Nuclear Science Symposium Conference Record, 2006. IEEE, Volume: 3
Source: IEEE Xplore

ABSTRACT A Hadron Blind Detector (HBD) has been constructed as part of the detector upgrade program for the PHENIX experiment at RHIC. The HBD is a proximity focused windowless Cherenkov detector operated with pure CF4 that will be used to detect single and double electrons in relativistic heavy ion collisions and provide additional rejection power against Dalitz pairs and photon conversions. The detector consists of a 50 cm long radiator directly coupled to a set of triple GEM detectors equipped with CsI photocathodes to detect UV photons produced by electrons emitting Cherenkov light. A full scale prototype of the HBD was built and tested in order to study its performance under beam conditions. Tests with the prototype demonstrated good separation between electrons and hadrons using pulse height discrimination and cluster size. The final detector has now been constructed and installed in PHENIX and is presently undergoing commissioning in preparation for its first round of data taking during the next heavy ion run at RHIC. Results of the beam test of the prototype, as well as on the construction and initial testing of the final detector, are presented in this paper.

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    ABSTRACT: A study has been made of the parameters affecting the extraction and collection of photoelectrons from the surface of a CsI photocathode in a triple GEM detector. The purpose of this study was to optimize the photoelectron collection efficiency and GEM operating conditions for the PHENIX Hadron Blind Detector (HBD) at RHIC. The parameters investigated include the electric field at the surface of the photocathode, the voltage across the GEM, the electric field below the GEM, the medium into which the photoelectrons are extracted (gas or vacuum), and the wavelength dependence of the extraction efficiency. A small, calibrated light source, or ldquoscintillation cuberdquo was used to illuminate a GEM CsI photocathode with a known photon flux produced by the scintillation light from 5.48 MeV alpha particles in CF<sub>4</sub>. The photoelectron collection efficiency was calculated by comparing the number of photoelectrons produced to the number collected at the GEM readout pad. Results are presented on the study of the parameters affecting the photoelectron collection efficiency and the construction and calibration of the scintillation cube.
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    ABSTRACT: A study has been made of the parameters affecting the extraction and collection of photoelectrons from the surface of a CsI photocathode in a triple GEM detector. The purpose of this study was to optimize the photoelectron collection efficiency and GEM operating conditions for the PHENIX Hadron Blind Detector (HBD) at RHIC. The parameters investigated include the electric field at the surface of the photocathode, the voltage across the GEM, the electric field below the GEM, the medium into which the photoelectrons are extracted (gas or vacuum), and the wavelength dependence of the extraction efficiency. A small, calibrated light source, or “scintillation cube”, was used to measure the photoelectron yield from the CsI photocathode using <sup>241</sup>Am particles to produce scintillation light in CF 4 and an <sup>55</sup>Fe source to measure the GEM gas gain. The absolute scintillation light yield (photons/MeV) in CF 4 gas was also determined using a CsI photocathode GEM detector. Results are presented on the study of the parameters affecting the photoelectron collection efficiency, the construction and calibration of the scintillation cube, and the measurement of the absolute scintillation light yield in CF 4 .
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    ABSTRACT: The PHENIX Hadron Blind Detector (HBD) is a high performance Cherenkov counter that uses pure CF<sub>4</sub> to detect electrons in relativistic heavy ion collisions at RHIC. It requires extremely high purity gas in order to achieve a high photoelectron yield, preserve the quality of its cesium iodide photocathodes, and maintain stable gas gain in its GEM detectors. In particular, water and oxygen at even the few ppm level can cause significant optical absorption in the wavelength range from 120-180 nm, which is the region of sensitivity of the Csl photocathodes. In order to ensure good optical transparency and low water and oxygen content of the operating gas, a transmission monitor has been constructed to measure the transmittance of the input and output gases of the HBD. The monitor is based on a McPherson 234/302 VUV spectrometer, along with a custom, computer controlled movable mirror system to measure the transmittance of the gas flowing to and from the detector. The monitor is used in conjunction with a recirculating gas system that is designed to supply high purity gas with minimal consumption. Both the transmission monitor and the recirculating gas system are described, along with their performance when used with the Hadron Blind Detector during the 2006-07 heavy ion run at RHIC.
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