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LSO/LYSO Crystals for Future HEP Experiments

Because of their high stopping power (X0 = 1.14 cm), fast (t = 40 ns) and bright (4 times of BGO) scintillation and good radiation hardness, cerium doped silicate based heavy crystal scintillators (LSO and LYSO) have attracted a broad interest in the high energy physics community pursuing precision electromagnetic calorimeter in severe radiation environment. We present in this paper current status of large size LSO and LYSO crystals adequate for HEP applications. The optical and scintillation properties and their radiation hardness are discussed.


Introduction
In the last two decades, cerium doped silicate based heavy crystal scintillators have been developed for the medical industry. As of today, mass production capabilities of lutetium oxyorthosilicate (Lu 2 (SiO 4 )O, LSO) [1] and lutetium-yttrium oxyorthosilicate (Lu 2(1−x) Y 2x SiO 5 , LYSO) [2,3] are established. Because of their high stopping power (> 7 g/cm 3 ), high light yield (200 times of PWO) and fast decay time (40 ns) this material has also attracted a broad interest in the physics community pursuing precision electromagnetic calorimeter, such as the proposed SuperB forward endcap calorimeter [4], the KLOE experiment [5] and the Mu2e experiment [6]. Our initial investigation on large size (2.5 × 2.5 × 20 cm) LSO/LYSO crystals also shows that this new generation of heavy crystal scintillators has a superb radiation hardness against γrays [7], neutrons [8] and charged hadrons [9]. They are thus an excellent material to be used in a severe radiation environment, such as the proposed high luminosity large hadron collider (HL-LHC). The main obstacles of using these crystals in the experimental physics are two fold: the availability of high quality crystals in sufficiently large size and the high cost associated with their high melting point (∼2,000 • ). This report is a part of an on-going R&D program to further develop this material to be of practical use at the HL-LHC. Table 1 lists basic properties of heavy crystal scintillators: NaI(Tl), CsI(Tl), BaF 2 , CeF 3 , bismuth gemanade (Bi 4 Ge 3 O 12 or BGO), lead tungstate (PbWO 4 or PWO) and LSO [10]. As shown in the table, all crystals, except CeF 3 , have either been used in, or actively pursued for, high energy and nuclear physics experiments. Figure 1 is a photo showing twelve crystal samples. In addition to samples listed in Table 1 pure CsI, CsI(Na), LYSO as well as LaCl 3 and LaBr 3 are also shown in this photo although the last two are not yet in mass production stage. Samples are arranged in an order of their density, or radiation length. All non-hygroscopic samples are wrapped with white Tyvek paper as reflector. Hygroscopic NaI, CsI, LaBr 3 and LaCl 3 are sealed in packages with two ends made of quartz plates of 3 or 5 mm thick to avoid   Intensity (a.u.) Transmittance (%) Figure 2. The excitation (red) and emission (blue) spectra (left scale) and the transmittance (green) spectra (right scale) are shown as a function of wavelength for ten crystal scintillators.

Properties of Scintillating Crystals
The solid black dots are the theoretical limit of the transmittance.
and LaCl 3 which are a cylinder with a length of 1.5 X 0 and areas at two ends equaling to 1.5×1.5 X 2 0 to match the 2 inch diameter of the PMT cathode. Figure 2 shows a comparison of the transmittance, emission and excitation spectra as a function of wavelength for ten samples. The solid black dots in these plots are the theoretical limit of the transmittance, which was calculated by using corresponding refractive index as a function of wavelength taking into account multiple bounces between the two parallel end surfaces and assuming no internal absorption [11]. Most samples, except LaBr 3 and LaCl 3 , have their transmittance approaching the theoretical limits, indicating a negligible internal absorption. The poor transmittance measured for LaBr 3 and LaCl 3 samples is probably due to scattering centers inside these samples.
It is interesting to note that BaF 2 , BGO, NaI(Tl), CsI(Tl) and PbWO 4     spectra well within the transparent region showing no obvious self-absorption effect. The UV absorption edge in the transmittance spectra of LSO, LYSO, CeF 3 , LaBr 3 and LaCl 3 , however, cuts into the emission spectra and thus affects crystal's light output. This self-absorption effect is more serious in long crystal samples used in high energy and nuclear physics experiment as discussed for LSO and LYSO crystals [12]. It is well known that a good light response uniformity is crucial to maintain a small constant term for a crystal calorimeter. Figure 3 shows the 137 Cs γ-ray pulse hight spectra measured by a Hamamatsu R1306 PMT with bi-alkali cathode for twelve crystal samples. Also shown in these figures are the corresponding FWHM energy resolution (E.R.). γ-rays spectroscopy with a few percents resolution is required to identify isotopes for the homeland security application. It is clear that only LaBr 3 approaches this requirement. All other crystals do not provide sufficient energy resolution at low energies.     function of wavelength, it should be taken out to directly compare crystal's light output. Figure 5 shows typical quantum efficiency as a function of wavelength for a PMT with bialkali cathode (Hamamatsu R1306) and a PMT with multi-alkali cathode (Photonis 2254B), a Si APD (Hamamatsu S8664) and a Si PIN PD (Hamamatsu S2744). The emission spectra of LSO/LYSO, BGO and CsI(Tl) crystals are also shown in these figures. Table 2 summarized numerical result of the emission weighted average quantum efficiency for several readout devices. The light output values in Table 1 are listed with the PMT quantum efficiency taken out. The light output of LSO and LYSO crystals is a factor of 4 and 200 of that of BGO and PbWO 4 respectively. Scintillation light yield from crystal scintillators may also depends on the temperature.  Table 1.

LSO/LYSO Crystal Calorimeter
Large size LSO and LYSO crystals are routinely grown in industry [12]. Figure 7 shows four long crystal samples of 2.5 × 2.5 × 20 cm 3 from CTI Molecular Imaging (CTI), Crystal Photonics, Inc. (CPI), Saint-Gobain Ceramics & Plastics, Inc. (Saint-Gobain) and Sichuan Institute of Piezoelectric and Acousto-optic Technology (SIPAT). Figure 8 shows the spectra of 0.51 MeV γ-rays from a 22 Na source observed with coincidence triggers [12]. The readout devices used are a Hamamatsu R1306 PMT (Left) and two Hamamatsu S8664-55 APDs (Right). The FWHM resolution for the 0.51 MeV γ-ray is about 12% for the PMT readout, which can be compared to 15% for the BGO sample. With the APD readout, the γ-ray peaks are clearly visible. The energy equivalent readout noise was less than 40 keV for these long LSO and LYSO samples.
LSO/LYSO crystals is also found to be much more radiation hard than other crystals commonly used in high energy and nuclear physics experiment, such as BGO, CsI(Tl) and PbWO 4 [13]. Their scintillation mechanism is not damaged by γ-ray irradiation. Radiation damage in LSO and LYSO crystals recovers very slow under room temperature but can be completely cured by thermal annealing at 300 • C for ten hours. The energy equivalent γray induced readout noise was estimated to be about 0.2 MeV and 1 MeV respectively in a      [7]. Figure 9 (Left) shows an expanded view of the longitudinal transmittance spectra for three samples before and after several steps of the γ-ray irradiation with integrated dose of 10 2 , 10 4 and 10 6 rad. Also shown in the figure is the corresponding numerical values of the photo-luminescence weighted longitudinal transmittance (EW LT ). Figure 9 (Right) shows the normalized average light output as a function of integrated dose for five long samples from various vendors. It is interesting to note that all samples show consistent radiation resistance with degradations of both the light output and transmittance at 10 to 15% level after γ-ray irradiation with an integrated dose of 1 Mrad.

Summary
In a brief summary, LSO/LYSO crystals are an excellent material for a total absorption electromagnetic calorimeter for a future high-energy physics experiment in a severe radiation environment. Assuming the same APD-based readout scheme used for the CMS PWO calorimeter, the expected energy resolution of a LSO/LYSO crystal based electromagnetic calorimeter would be where the stochastic term is dominated intrinsically by the fluctuation of shower leakage as indicated by GEANT simulations. This represents a fast calorimeter over a large dynamic range with very low noise. Thanks also to the LSO/LYSO low temperature coefficient, such a calorimeter is less demanding to the experimental environment. Because of the crystals excellent radiation hardness, a LSO/LYSO crystal calorimeter is capable of making precision measurement for electrons, photons and jets and thus provides a great physics discovery potential in a severe radiation environment, like the HL-LHC.