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Photon Array for studies with Radioactive Ion and Stable beams

The Photon Array for the studies with Radioactive Ion and Stable beams PARIS collaboration aims to design a calorimeter dedicated to the detection of gamma-rays in an energy range between 100 keV up to 50 MeV, as part of the detectors for SPIRAL 2. The tough requirements in terms of energy resolution, gamma-ray detection efficiency and time resolution match with the high-performances of Lanthanum Bromide (LaBr3:Ce), as a scintillator to be part of this calorimeter. However, the high-cost of this material limits its use for large-size apparatus and an alternative is the Phosphorous Sandwich (Phoswich), where a LaBr3:Ce crystal is joined to another one, like Sodium Iodide (NaI:Tl) or Cesium Iodide (CsI:Na), and then coupled to a single photodetector.. Such a device must meet criteria including high-quality crystal with homogeneous light-response and an optimized coupling between the two scintillators and the Photodetector. The two materials must also differ by their scintillation decay times in order to separate their individual contribution to the output signal by an adapted reading system. The RDD Department of IPN Orsay studied several samples of LaBr3:Ce as “single” crystal and in Phoswich configuration with NaI:Tl and CsI:Na respectively. All of them were purchased to the Saint-Gobain company and were coupled to several Photomultipliers (PMT). The experimental studies aim to determine energy resolution together with the light-response and the intrinsic time resolution for these devices.
Light yield and energy resolution : 

Each PMT coupled to a scintillator is equipped with a basis providing the anode and last dynode signals. The read-out system allows separating the events originating from the two crystals coupled to the same Phoswich and consists of a standard spectroscopic chain. The dynode signal feeds a Cremat CR-113 charge preamplifier whose output is sent to an ORTEC shaping amplifier (LaBr3:Ce shaping time set to 0.5µs, NaI:Tl/CsI:Na shaping time 3µs). A CAMAC peak-sensing analog-to-digital ADC digitizes and records the pulse-height signal. A charge to digital converter (QDC) measures the charge of the PMT anode signal and discriminates the light produced in the LaBr3:Ce from that produced in slower coupled scintillators. Data were analyzed off-line with the ROOT package.

Single LaBr3:Ce and Phoswiches were irradiated with radioactive sources (22Na, 137Cs, 60Co and 207Bi) which provide energy peaks in a range between 511 and 1770 keV. Figure 1 (figure 1 of Giulia’s paper) shows the 137Cs spectrum measured with a Phoswich, made of a 50.8x50.8x50.8 mm LaBr3:Ce joined with a 50.8x50.8x152.4 NaI:Tl, coupled to a Hamamatsu R7723-100 PMT. The LaBr3:Ce energy resolution is deduced from a Gaussian fit on data after removing events with a NaI:Tl contribution to the signal and gives 4.61% FWHM, which is about 17% worst than a single LaBr3:Ce single. The loss of scintillation light at the crystal interface and during the transport in the NaI:Tl crystal explains this discrepancy. The light response homogeneity is studied by coupling scintillators to a Photonis XP5300 PMT and by irradiating the scintillators with a collimated 137Cs source. Figure 2 (Figure 3 of Giulia’s paper) shows the light-yield as a function of the source position. A large variation of light-yield is observed for the Phoswich LaBr3:Ce/CsI:Na which is explained by low-quality crystals. More results and further details can be found in the poster, presented at the 2012 NDIP conference, and the corresponding NIM paper.

Time resolution :

Coincidence time resolution technique is then used to determine the intrinsic time resolution of set-ups made of a single LaBr3:Ce or a LaBr3:Ce/NaI:Tl Phoswich, coupled to a PMT.

First, a cylindrical 25.4x25.4 mm LaBr3:Ce crystal is coupled to a Photonis XP20D0 PMT, equipped with a basis which provides the anode and last dynode signal. Then time measurements by irradiation with a 22Na and a 60Co source are performed in coincidence with a cylindrical 25.4x25.4 mm plastic detector PilotU, coupled to a Photonis XP2020 PMT equipped with two-splitted anode outputs. The XP20D0 PMT and one of the XP2020 anode outputs fed a 4-channels Enertec 7174 Constant Fraction Discriminator (CFD) which delivers in output a signal for the START and the STOP of an ORTEC 467 Time Amplifier Converter (TAC). The TAC output signal amplitude is proportional to the delay between the two signals and is digitized by a CAMAC peak-sensing ADC. For each detector, a second CFD output is sent to a Lecroy 465 Coincidence module which generates gate(s) and the trigger. The PilotU energy is measured by integration of the second XP2020 anode signal in a QDC, while that of the LaBr3:ce crystal is given by a spectroscopic chain similar as those used for energy resolution measurements. Data are analyzed off-line with the ROOT package, by gating the PilotU energy on the Compton front-edge, and selecting several energy windows in the LaBr3:Ce. Figure 4 shows the coincidence time resolution as a function of the energy measured in the LaBr3:Ce crystal for the runs with a 22Na and a 60Co respectively. The shift between the two curves is due to the difference of the plastic intrinsic time resolution which is of 226 +/- 13 ps and 150 +/- 7 ps at 340 and 820keV respectively. The slope of the quadratic function fitted on the data points give the intrinsic time resolution for the (LaBr3:Ce + XP20D0) which is of 4079 +/- 250 ps.sqrt(E). The corresponding values at 511 and 1332keV are of 180 +/- 11 and 112 +/- 7ps respectively and agree well with those of a previous publication.

This last detector is then used for reference for further coincidence measurements with a 50.8x50.8x101.6 mm single LaBr3:Ce crystal and the LaBr3:Ce/NaI:Tl Phoswich. For this last device, the pure LaBr3:Ce events are selected by the same method than used for energy resolution studies. Coupled with a Hamamatsu R9779 or a Hamamatsu R7723-100, the intrinsic time resolutions of the single LaBr3:Ce are of 236 +/-20 ps and 229 +/- 18 ps respectively at 1332 keV, showing that equivalent time performances to a standard fast PMT could be reached with an enhanced quantum-efficiency PMT, but with a higher transient time spread. For the Phoswich coupled with the Hamamatsu R7723-100, the intrinsic time resolution is of 241 +/- 13 ps at 1332 keV, indicating that the time performances of the LaBr3:Ce crystal are not significantly affected in a Phoswich structure.

Contact : J. Bettane
Links :

Publications and reports :

“Energy Resolution of LaBr3:Ce in a Phoswich configuration with CsI:Na and NaI:Tl scintillator crystals”, G. Hull, B. Genolini, M. Josselin, I. Matea, J. Peyre, J. Pouthas, T. Zerguerras, Nucl. Instr. & Meth. A, Vol. 695, pp. 350-353, Dec. 2012, doi:10.1016/j.nima.2011.10.023



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