Pulse height non-linearity in LaBr3:Ce crystal for gamma ray spectrometry and imaging

Pani R.1, Cinti M.N.1, Pellegrini R.1, Bennati P2.,Ridolfi S.1, Orsolini Cencelli V.3, De Notaristefani F.3, Fabbri A.4, Navarria F.5,Lanconelli N.5, Moschini G.6 , Boccaccio P.7 and R.Scafè8

1 INFN and the Department of Experimental Medicine, “Sapienza” University of Rome.

2. INFN and EDEMOM PhD school of Microelectronics, Rome III University , Rome Italy.

3. INFN and Department of Physics “Roma III” University, Rome, Italy

4. INFN and Department of Electronic Engineering “Roma III” University, Rome, Italy

5. INFN and the Department of Physics, University of Bologne, Bologne Italy.

6. INFN and the Department of Physics, University of Padua, Padua Italy.

7 INFN and Legnaro National Laboratory, Legnaro, Italy

8 INFN and ENEA, Casaccia, Rome Italy

The LaBr3:Ce crystal represents a new class of scintillator with very high performances in gamma ray spectroscopy. The high light output (63000 ph/MeV) and the fast light pulse decay of 16 ns allow for high counting rate, excellent energy and time resolution results, with respect to most diffused scintillators. In order to take full advantage of LaBr3 scintillation performances, a corresponding improvement of photodetector characteristics would be suitable, in particular, for what concerns an higher QE. On the contrary non-linearity effects on pulse height distribution, due to very high peak currents induced in the PMT by the fast and intense light pulse of LaBr3:Ce are expected. So a reduction of number of dynodes or modification regarding the voltage divider (tapered divider) is suitable to limit this phenomenon.

In this work the response in term of linearity of the pulse height distribution and energy resolution of five PMTS are studied. These PMTs have different configurations, performances and purpose built. In fact two of them are Multi Anode PMT, for position sensitive application, and the other three PMTs are strictly dedicated for spectroscopy applications.

In particular the investigated PMTs were: a 3 inch Æ R6231MOD PMT (QE 30%@380nm), equipped with a modified bleeder by Saint Gobain; a 1inch x 1 inch R7600U-200 PMT, with Ultra BiAlkali (UBA) photocathode (QE=42% @380 nm); a 1inch x 1 inch R7600U (QE=22% @380 nm); a 2inch x 2 inch H8500 MA-PMT (QE=27% @380 nm) and finally a 2 inch x 2 inch MA-PMT (H8500C-100 MOD8), a new prototype developed recently, equipped with a Super Bialkali (SBA) photocathode, with quantum efficiency (QE) of about 38% @ 380 nm. The outputs of the two MA-PMTs, coming from 8x8 anode array, were read in a short circuit configuration. A tapered divider was utilized for both R7600 PMTs while the H8500C-100 MOD8 MA-PMT presents a reduced number of dynode stage respect to the H8500 standard MA-PMT. All the PMTs are manufactured by Hamamatsu.

About the scintillation crystal, a lanthanum tri-bromide crystal, with size Æ12.5 mm x 12.5 mm, was coupled to Hamamatsu R7600U PMT and R6231MOD PMT. Subsequently a continuous LaBr3:Ce crystal, with detection area of 51 mm x 51 mm, 4.0 mm thickness and a glass window of 3.0 mm was coupled to all PMTs except for couple of R7600U, due to their small dimensions. This crystal is optimized for imagin application and it presents an entrance surface polished with white coating and the edges have a black coating.

For comparison a NaI:Tl continuous crystal, with the same detection area of LaBr one, was also utilized. The study was made in the energy range of 32 – 1330 keV

Analyzing the calibration curve (energy as a function of the full energy peak channel in the spectra) obtained for each configuration crystal-PMT and radioactive sources, a problem of pulse height no linearity, in particular at low energy was shown. This effect was also evident comparing the single spectrum for different radioactive source. Detailing the analysis, a specific fragmentation of the calibration curve in subsets was necessary to calibrate correctly the detector and as consequence to evaluate the energy resolution.

After this procedure ,the energy resolution values are expected to be equal for the same crystal independently of the utilized PMT , inside the range of variability introduced by the different quantum efficiency and charge collection in the first dynode stage. So the non-linearity effect seems to be a consequence of the characteristics of the lathanum bromide.

The same behavior was also verified for both H8500 PMTs in multiwire readout configuration, dedicated for imaging application .