Speaker
Dr
Raffaele Scafè
(Casaccia Research Center, ENEA, Rome, Italy and INFN, University of Rome “La Sapienza”, Rome, Italy)
Description
LaBr3:Ce represents a very promising scintillation crystal for spectrometric and
imaging devices, thanks to its good scintillation properties as the high light
yield (61000 photons per MeV) which allows excellent energy resolution values (3%
FWHM at 662keV). It is attracting the scientific community for the potential
improvement of PET and SPECT instrumentation. First measurements of energy
resolution values obtained coupling large crystal samples (from 0.5 to 1 inch
diameter) to PMTs show unexpected worse values than NaI:Tl for photon energies less
than 100keV. On the contrary values at higher energies are close to the limit of
statistical PMT contribution. In addition the scintillation light detection by
standard PMT is affected by the high dynode current due to the fast and intense
scintillation pulses from LaBr3:Ce (about ten times higher than NaI:Tl). The recent
availability of large area Si-APDs (Avalanche Photodiode) and the progresses in
LaBr3:Ce growing technique have suggested an additional study of light output
performed by this photodetector. This coupling is also promising because of the
good quantum efficiency (about 60%) of APD at the LaBr3:Ce emission wavelength
(380nm), compared with that of bialkali photocathodes (about 25%).
In this work we report, compare and discuss the results of measurements performed
using three different scintillators, available in our laboratories. A 12.7mm
diameter by 12.7mm thickness LaBr3:Ce crystal and a 2.0x2.0x10.0mm3 pixel of a
YAP:Ce array have been used to evaluate the APD response at short wavelength.
Results are compared to those obtained with CsI:Tl that is considered as the gold
standard crystal for APD measurements. Two CsI:Tl crystals sized 9.0x9.0x10.6mm3
and 2.9x2.9x5.0mm3 are used for comparison with LaBr3:Ce and YAP:Ce respectively.
Results obtained by APD and PMT readout are presented. Hamamatsu Si-APDs (5x5mm2
and 10x10mm2 active areas) are used as solid state light detectors. PMT
measurements employ an Hamamatsu R6231 and a Thorn EMI 9765B05 with 2 and 3 inch
photocathode diameter respectively.
A study of the energy resolution has been focused in the energy range from some
tens of keV to 662keV to investigate the LaBr3:Ce response at photon energies below
100keV showed by Dorenbos as the limit of better energy resolution of this
scintillator with respect to the competitors. This energy range is particularly
important for nuclear medicine imaging.
Spectra obtained at 22°C with YAP:Ce and LaBr3:Ce crystals coupled to the 25mm2 APD
show the 81keV peak from 133Ba well resolved from the low-energy noise. The energy
resolution values measured at 122keV coupling the PMT to the YAP:Ce and LaBr3:Ce
crystals (27.0% and 7.5% respectively) improve by about 15% when using APDs. The
APD readout of CsI:Tl crystals (9.0x9.0x10.6mm3 and 2.9x2.9x5.0mm3) gives energy
resolution values of 15.6% and 10.5% respectively.
Author
Dr
Raffaele Scafè
(Casaccia Research Center, ENEA, Rome, Italy and INFN, University of Rome “La Sapienza”, Rome, Italy)
Co-authors
Dr
Giorgia Iurlaro
(Casaccia Research Center, ENEA, Rome, Italy)
Dr
Livia Montani
(Casaccia Research Center, ENEA, Rome, Italy)
Dr
Maria Nerina Cinti
(Dept. of Exp.Medicine and INFN, University of Rome “La Sapienza”, Rome, Italy)
Dr
Roberto Pani
(Dept. of Exp.Medicine and INFN, University of Rome “La Sapienza”, Rome, Italy)
Dr
Rosanna Pellegrini
(Dept. of Exp.Medicine and INFN, University of Rome “La Sapienza”, Rome, Italy)
