Nuclear Instruments and Methods in Physics Research A 515 (2003) 725–732 A gain monitoring system with a Nd:YAG laser for the photomultipliers of the HERMES experiment H. Tanaka a, *, B. Bains b , M. Bouwhuis b ,R.Holt c , N.C.R. Makins b , Y. Miyachi a , A. Most b , A. Nathan b , H. Ohsuga a , B.R. Owen b , T.-A. Shibata a , S. Williamson b a Department of Physics, Tokyo Institute of Technology, 2-12-1, Ohokayama, Meguro, Tokyo 152-8551, Japan b Department of Physics, University of Illinois, Urbana, IL 61801-3080, USA c Physics Division, Argonne National Laboratory, Argonne, IL 60439-4843, USA Received 6 March 2003; received in revised form 10 July 2003; accepted 21 July 2003 Abstract The Gain Monitoring System (GMS) provides real-time gain monitoring of the photomultipliers of the HERMES experiment,usingaNd:YAGlaserwithawavelengthof532nmasthelightsource.Thesystemsendslightpulsesviaan optical fiber network to the detectors equipped with photomultiplier tubes (PMTs), as well as to an array of reference detectors. Light pulses of different intensities are obtained by rotating a wheel carrying several attenuation plates in frontofthelightsource.ThispermitsthemonitoringofthelargedynamicrangeofADCsignalsfromthedetectors,as well as the linearity of the PMT responses. r 2003 Elsevier B.V. All rights reserved. PACS: 29.40.Vj; 29.40.Mc; 42.60.Lh; 42.70.Hj Keywords: Laser; Gain calibration; Calorimeter; Hodoscope 1. Introduction HERMES is a deep-inelastic scattering (DIS) experimentwhichscattersthepolarizedelectronor positron beam of the HERA accelerator at DESY from polarized and unpolarized internal gas targets. One of the primary goal of the experiment is to determine the polarized structure function of thenucleonandtodecomposethecontributionsto the nucleon spin from each quark flavor, via inclusive and semi-inclusive DIS measurements. For this purpose, particle identification (PID) is extremely important, and the HERMES spectro- meter [1] is equipped with highly efficient PID detectors. Two of the key detectors in performing lepton–hadron separation are the electromagnetic calorimeter and the preshower hodoscope, and bothsystemsarereadoutbyphotomultipliertubes (PMTs). ThestabilityofthesePIDdetectorsismonitored bytheGainMonitoringSystem(GMS).TheGMS alsomonitorsotherdetectorsequippedwithPMTs ARTICLE IN PRESS *Corresponding author. Tel:+81-3-5734-2369; fax:+81-3- 5734-2742. E-mail address: thide@nucl.phys.titech.ac.jp (H. Tanaka). 0168-9002/$-see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.07.057