1. 1 Anil Gupta, 2 Sonali Bhade, 2 P. J. Reddy, 1 P. K. Kale, 1 K. Narayanan Kutty, 2 D.A.R. Babu, 1 R. G. Purohit and 1 Dr. P. K. Sarkar 1 Health Physics Division, 2 Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai – 400 085 Comparison study of quench levels for estimation of low level H-3 activity by liquid scintillation counting using TDCR based Hidex 300SL and LKB Wallac Quantulus1220 INTRODUCTION  Widely used Liquid Scintillation Counter (LSC) LKB Wallac Quantulus 1220 and the newly introduced TDCR based Hidex 300SL were calibrated for H-3 estimation.  Samples spiked with H-3 with varying levels of quench were quantified using Hidex 300SL LSC and the results obtained were compared with LKB Wallac Quantulus 1220.  The difference between two LSC systems is mainly the use of two different quench indicating parameters (QIP) namely Spectral Quench Parameter of External Standard (SQP (E)) in Quantulus 1220 and Triple to Dual Coincidence Ratio (TDCR) in Hidex 300SL.  These parameters are used to correct quench associated with samples. The present study was carried out to determine the correlation of these two QIPs with the counting efficiencies to arrive at the precise activities. MATERIALS AND METHODS  LKB Wallac Quantulus 1220 and Hidex 300SL liquid scintillation analyzers.  3 H capsule (E max -18.6keV) Standard (Packard).  Scintillation cocktail: Quicksafe-400 (Zinsser Analytic).  Nitromethane (Merck, Germany) as chemical quencher.  Scintillator : sample ratio (12:8).  H-3 quenched standards ~82.5 Bq (or ~10.3 Bq/ml) were prepared.  Counting time for both the instruments 500 min.  In TDCR, triple and double coincidences are measured and the ratio of these coincidences is calculated. RST : Triple Coincidence Counts; RS : Double Coincidence Counts; RT : Double Coincidence Counts; ST : Double Coincidence Counts; TDCR: Triple Double Coincidence Ratio C t : triple coincidence counts C d : double coincidence counts C all : all coincidence counts  Calibration curve of Quench Indicating Parameter (TDCR value for Hidex and SQP(E) for Quantulus) vs %counting efficiency obtained. Fig: 1 TDCR vs % Efficiency Fig: 2 SQP(E) vs % Efficiency  Quantified H-3 samples of different activity range varying from 1.4 Bq/ml to 0.27 Bq/ml using both the instruments. RESULTS  Hidex 300SL, LSC gives consistent results for all quench levels whereas in case of Quantulus 1220, as the quench increases, there is a sharp rise in the percentage deviation while quantifying activity as depicted in Table 1. Table 1: Calculated 3 H activity using Quantulus and Hidex 300SL LSCs along with % deviation w.r.t. reference activity.  For scintillator : sample, 10:1 composition (low quench levels or low water load) the TDCR value is proportional to percentage efficiency.  The proportionality factor is ~1 whereas for mixing ratio of scintillator : sample, 12:8 composition (higher quench levels or high water load), the proportionality factor does not remain 1. CONCLUSION In Hidex 300SL LSC TDCR method, quench affects the quantification of H-3 activity to a lesser extent compared to the Quantulus 1220 liquid scintillation counting. TDCR is an absolute standardization method. For low water load TDCR value is proportional to percentage efficiency (proportionality factor is ~1). For higher water load TDCR is linearly proportional to percentage efficiency with some intercept. Reference 3 H activity (Bq/ml) SQP(E) Value TDCR value Calculated 3 H activity (Bq/ml) % deviation w.r.t. ref. activity Quantulus LSC Hidex 300SL LSC QuantulusHidex 300SL 1.41835.10.2741.461.34-3.944.96 0.69838.30.2710.730.70-5.01.4 0.40836.90.2650.430.41-5.15-2.5 0.27837.80.2630.290.28-6.86-3.7