Scintillating crystals for calorimetry and other applications.

Transcription

1 1 Scintillating crystals for calorimetry and other applications. Egidio Longo a a Dipartimento di Fisica Università di Roma La Sapienza e INFN Sezione di Roma P.le A. Moro Roma, Italy Scintillating crystals are the state of the art for electromagnetic calorimeters and are exploited in many different applications. The paper presents some recent developments and results in different fields of application. 1. INTRODUCTION Scintillating crystals have been known since 1900, and are widely used as detectors since In the last decades, the discovery and the development of new scintillating crystals has been driven by basic research needs and has required major technological efforts. High-energy physics has played a major role in the development of new scintillators on an industrial scale and with affordable costs. Examples are BGO (Bi 4 Ge 3 O 12, CsI, and PWO (PbWO 4 ), introduced to fulfill precise requirements from high-energy experiments and then widely exploited in other applications, as medical imaging. The reasons for the success of scintillating crystals in high-energy physics are many: they provide an excellent energy resolution over a wide energy range, with high detection efficiency for low energy electrons and photons. Their structural compactness eases the mechanical assembly with minimal external supporting structures, facilitating hermetic coverage. Their natural tower structure allows fine transverse granularity and provides straightforward algorithms for event reconstruction and particle identification. In the 70s, the Crystal Ball experiment [1], made of 732 NaI crystals, first demonstrated the feasibility of large-scale 4π detectors using scintillating crystals. The study of the complex Charmonium spectrum and its comparison with potential models in QCD by the detection of radiative decays resulting in monochromatic photons of few hundred MeV [2] was an impressive demonstration of the power of high resolution calorimeters. A few years later, thanks to the development of the heavy scintillator, BGO, a 4π crystal detector for the precise detection of high energy electromagnetic particles was fit in a multi-purpose experiment for a collider: the L3 detector [3] using more than BGO crystals was successfully operated at LEP, detecting photons and electrons in the energy range from 100 MeV to 100 GeV. In the last decade, the continuous R&D on crystals allowed the requirements coming from new collider and CP-violation detectors to be faced: fast response, radiation hardness and increasing number of channels. The CMS Collaboration, which will operate at LHC from year 2007, is building a calorimeter made of more than PWO crystals [4]. 2. KEY POINTS ON THE SCINTILLA- TION MECHANISM The main point to be kept in mind is that the scintillation mechanism is based on the presence of defects and impurities in the crystal lattice, generating local electronic energy levels in the energy band gap. The centers are of three main types: luminescence centers, directly responsible for the photon emission, in competition with the quenching centers, dissipating the excited energy via thermal interactions, and finally the so called traps, metastable levels which can capture the electrons for a while, afterward releasing them to either the valence band through a radiationless transition, or the conduction band by absorption of thermal energy. The detailed pattern of these levels characterizes the scintillation properties of

2 2 each specific crystal. As the relative abundance of these levels can be controlled to a large extent by the purity of the raw material, by the growing procedure or by doping, scintillating crystals can be tuned to follow the requirements of the different specific applications. Scintillation can be seen as a three-step process: excitation from the ground to the excited state, relaxation though the different energy levels of the excited state and finally radiative emission, at a wavelength substantially longer than the excitation wavelength (Stokes shift). The number of electron-hole pairs is given by the deposited energy divided by the energy required to produce an electron-hole pair, always larger than the band gap E g : N eh = E dep βe g with β > 1. The number of emitted photons is given by the number of pairs multiplied by the efficiency S of transfer to the luminescent centers and by the radiative efficiency Q of the luminescence centers, so that at the end the number of emitted photons per unit deposited energy is proportional to SQ and inversely proportional to βe g : then the basic recipe for an high light yield is to have Q and S as high as possible and βe g as low as possible, with the medium transparent to the emission wavelength. In principle, the dimension parameters, namely the radiation length and the Molière radius, drive the selection of the basic crystal. But then, crystal performance can be fine tuned, keeping in mind that the light yield, the response time and the radiation hardness are strictly related to the optimization of defect and impurity concentration, stoichiometry control and doping, so that the optimization process is sometime long and uncertain. The final step is the match of the detector characteristics with the read-out, by selecting and optimizing suitable photodetectors and by developing dedicated electronics able to exploit the performance of the crystals. 3. RECENT ACHIEVEMENTS ON HIGH ENERGY CRYSTAL CALO- RIMETERS Recent results were reported by two experiments using PWO crystal calorimeters: BTeV, to study the B physics at Tevatron by means of γ, π 0 and η detection, and CMS, with the aim to detect electrons and photons in the Higgs search at LHC BTeV calorimeter In the BTeV electromagnetic calorimeter crystals will be read-out by photomultipliers. Recent results were presented from a test-beam made in Protvino [5], with a matrix of five by five crystals exposed to an electron beam up to 45 GeV. A stochastic term of 1.8%/E(GeV) was achieved. All the contributions to the resolution are well reproduced by Monte Carlo simulation, including the constant contribution coming from the nonuniformity of the light collection along the crystal axis, as shown in Fig. 1 Figure 1. Comparison of BTeV PWO calorimeter energy resolution with MC simulation: I includes shower fluctuations, II includes also non uniformity in light collection and III the photon statistics contribution. Interesting results were presented in this Seminar on the radiation resistance of the crystals,

3 3 studied by exposing the matrix to high intensity electron and pion beams. This study shows that the damage is not due to a reduction of the scintillation, and there is a dependence on the dose rate. It has also been shown that even after a total adsorbed dose of 2.5 Mrad at a dose rate of 100 krad/hr, crystals are still working, in spite of a reduction of a factor three in the light output CMS ECAL progresses The CMS ECAL Collaboration presented several recent items of progress, many of them also reported to this Seminar. A new electronics design was validated together with the cooling system during 2003 test beam at CERN, achieving resolution figures close to the design ones. It has been demonstrated that the radiation damage monitoring system is able to reach the required precision. Moreover, the systematic measurements routinely performed on mass production crystals has allowed the accumulation of statistics on several parameters, contributing to a deeper understanding of the crystal characteristics and indicating some unexpected solutions to some of the open problems, first of all for the precalibration of the detector New electronics and cooling The CMS PWO crystals are read by avalanche photodiodes (APDs). The front-end electronics is required to have 90dB dynamic range and to be radiation hard, as it will be placed on the detector. The solution is a multi-gain preamplifier with 40 ns shaping time, coupled to 12-bit ADCs, sampling the signal every 25 ns. The new design has been developed in the 0.25 µm CMOS IBM technology. The generation of trigger primitives, which are sent off-detector synchronously to the Level-1 trigger, and the pipelining of the full crystal data awaiting a Level-1 accept, is also included in the new front-end electronics design. This solution, combined with the availability of optical data links at 800 Mb/s, is cost effective thanks to the reduction of the optical data links, being one per channel in the previous design, to three per trigger tower (five by five crystals). The power consumption, including the trigger logic, is slightly below 3 W per channel. This dissipated heat must be effectively extracted, as crystals must operate at a nominal temperature of 18 C. As both the light yield and the photodetector gain depend on the temperature at the level of few percent per degree, the specification for the constant term of the resolution translates to a temperature stabilization of better than 0.05 C around the nominal value. For this purpose, a chilled-water cooling system is embedded in the grids separating the crystals from the electronics and in the mechanical structure supporting the electronics. In summer 2003, the system was tested for the first time with one module of 400 crystals equipped with fake electronics dissipating a total power of 1.2 kw, according to the figure given above. The extreme case was tested by switching on and off the dissipating power. In order to also check the contribution from convection, the test was repeated with the module at three different orientations, namely 12, 3 and 6 o clock. The maximum average variation, measured on the crystal surface and on the APD capsules, was C, very close to the specifications. In summer 2003, a full system test of the new 0.25 µm electronics, including the front end, the trigger and the optical control, was performed with 50 channels exposed to an electron testbeam at four energies between 25 and 100 GeV [6]. The resolution as a function of the energy is shown in Fig. 2, for electrons impinging in a 4 x 4 mm 2 window at the center of the crystal, integrating over a three by three crystal matrix, after the event-by-event subtraction of a coherently varying baseline. The total remaining noise per channel is less than 45 MeV. Data are fitted with the standard parameterization of the resolution and the fit result is σ E = 2.93% E(GeV) E(GeV) 0.4%, close to the specifications of the technical proposal. Note that the difference in the stochastic term with respect to BTeV results is due to the use of different photodetectors.

4 Amplitude (GeV) Amplitude (GeV) Figure 3: The reconstructed signal of a 100 GeV electron using the sum of nine crystals. The average amplitude is calibrated to the incident beam momentum. Figure 4: The reconstructed signal of a 100 GeV electron using the sum of 25 crystals. The average amplitude is calibrated to the incident beam momentum. 4 "(E)/E (%) past few years to study the signal and monitor relative variations. In 2002 and 2003 a set of crystals was studied, resulting in an average α of 1.5, with a spread of 5% [7] as required by the "(E)/E (%) aforementioned 1.2 arguments Stochastic = 2.93 ± 0.21 % Constant = 0.40 ± 0.03 % Noise( # 9 ) = 129 ± 2 MeV E beam (GeV) Figure 2. Energy resolution integrating over a three by three crystal matrix Figure 5: The resolution obtained from 4 runs of different energies summed over nine crystals Radiation damage monitor An important issue for this calorimeter is the monitoring of the changes in crystal transparency due to radiation damage, and recovery, in the hostile LHC radiation environment. With LHC operating at high luminosity, crystal damage saturates in a few hours, and partially recovers during the stops of the collider with a similar time scale. On the basis of the specifications, crystals are expected to suffer a signal reduction of less than 6%. To maintain a few per mill precision in the calibration, this 6% variation must be corrected with a precision of the order of 5%. For this purpose, a laser monitor at four different wavelengths (from 440 to 800 nm) has been developed. However, under irradiation the laser response (R) varies less than the electron response (S). An ( exponential ) α S R law can model this difference: S 0 = R 0. The main concern is whether the parameter α is the same for all the crystals. Several irradiation tests with high intensity electron and pion beams were performed in the Calibration One of the most important issues for the CMS calorimeter will be the capability to calibrate the 0.8 detector to a precision of few per mill, to keep at high energy the excellent resolution provided by 0.6 the small intrinsic fluctuations in the crystal light and in the energy containment. Several calibration 0.4 methods will be available in situ, based on azimuthal symmetry of the energy flow, on the E/p 0.2 comparison for the electrons coming from W decays, on the Z invariant mass reconstruction from electron decays and other physical processes. However, at the start-up, an intercalibration will (GeV) be needed for triggering purposeseand beam to allow a fast detector debugging and understanding. Unfortunately, because of the tight schedule, no systematic 6: test The resolution beam precalibration obtained fromwill 4 runs be possible of dif- Figure ferent for the energies wholesummed detector. overfor 25 crystals. the initial intercalibration one has to rely on the LY measurements systematically performed by radioactive sources on all crystals during their qualification. In spite of the very low energies involved in these measurements and of the differences in the read-out systems, a comparison of the LY with the crystal response in the test-beam shows that the error on the calibration constants obtained in this way is only 4.5 % Systematic studies of crystal properties In the mean time, data has been accumulated on thousands of crystals during their qualification. Among other parameters, the longitudinal transmission at 360 nm (LT 360 ) is measured for all the crystals, as it is related to the radiation hardness: actually, this measurement is an estimation of the presence of a kind of traps responsible under irradiation for the formation of color centers at higher wavelengths, in the region of the light emission. Recently an impressive correlation was found between LT 360 and the LY measured by the radioactive source. This is a relevant ef-

5 5 fect, as the LT measurement is more stable and reliable with respect to the LY. By a linear fit to the correlation plot, a LY value can be predicted from LT 360. Just taking the average of these two LY estimates, the spread of the ratio of this corrected LY to the one obtained from the test beam is reduced to 4.08 %, a small but significant improvement [8]. However, the explanation of this effect has some interest in itself: actually, a simple change in transmission cannot explain the effect if the emission spectra of all the crystals are identical. The correlation with LT 360 is much higher than with the transmission in the region of maximum emission. A possible explanation is that the same centers causing radiation damage under irradiation act as traps for the electrons generated in the excitation. Trapped electrons do not contribute any more to the scintillation process, thus reducing the light yield. A simple analytical model reproduces well this correlation, also providing an explanation for the observed spread in the intrinsic LY of the crystals. 4. CRYSTAL DETECTORS FOR MEDI- CAL IMAGING Crystals are widely used as gamma detectors for medical imaging. A typical application is in the Positron Emission Tomography (PET), where a positron emitter linked to some body function or metabolic by-product is injected in the patient: the coincidence detection of two collinear gammas of 511 kev (produced in the subsequent electronpositron annihilation) in a ring of crystals pointing to the patient s body, allows the determination of the so called line of response, a straight line connecting the centers of the two hit crystals. The intersection of several lines of response gives the position of the emitter in the plane of the ring. The resolution of this technique (few mm for a ring diameter of 1 m) is limited by several physics effects, such as the finite range of the positron before the annihilation, the acollinearity resulting from an annihilation in flight or the parallax error, caused by late interactions of the photons in the crystal: assigning the photon position to the front face of the crystal gives a wrong line of response. A new technique has been proposed to reduce the parallax error, by using a tower made by two crystals with different response time, optically coupled. By pulse shape discrimination, early interactions along the tower axis can be distinguished from late ones. For this purpose, an important achievement is the development of LuYAP (Ce-doped mixed lutetium and yttrium aluminum perovskite crystals) [9], two to three times faster than LSO, currently used for PET. However, the growth of this particular phase is critical, due to the narrow perovskite region of stability, requiring an extremely defined temperature and molar fraction of the components. So, high precision in the stoichiometry is required, together with a stable heating system with good control of thermal leaks, to keep the melt temperature in the range of 3 C around the melting point close to 2000 C. This process has been systematically investigated by Crystal Clear Collaboration, born at CERN in the frame of the R&D program for LHC, and nowadays fully devoted to the screening of new possible scintillating crystals for all kind of applications. 5. LOW TEMPERATURE DETECTORS In several physics domains related to the search for rare signal, the detection of small signals is often crucial. Typical cases are the searches for double beta decays (DBD) or for signals of the interactions of dark matter. In these applications crystals are used as bolometers, measuring the deposited energy by phonon detection. The advantage is that the elementary excitation is in the range of 10 mev, very small with respect of energies involved in scintillation, of the order of ev, thus allowing the detection of signals with very low thresholds. However, signals must emerge from thermal noise, so that bolometers are better operated at very low temperature, in the region of mk. This class of detectors can also provide a powerful tool to fight the background if at the same time a scintillation signal is produced and detected (typically by a second bolometer thermally decoupled from the main crystal): for instance, in the double beta decay, the electrons

6 6 (with an energy in the region of MeV) will produce at the same time phonon and light signals, while the alpha particles from residual radioactive decays (one of the main sources of background) are expected to release a negligible signal in light. In the case of WIMP interactions, the signal is produced by nuclear recoil, without scintillation, contrary to the main background, given by beta and gamma radioactive decays. In this case, however, the expected signals are in the region of 10 kev: taking into account that the fraction of energy going into scintillation is a few percent, one expects only tens of photons, so that a high detection efficiency is required to make the anticoincidence effective. Nevertheless, the CRESST collaboration, detecting 1% of the deposited energy on a CaWO 4 crystal read by a sapphire light detector, was able to achieve a rejection factor of 98% in the region of 10 to 20 kev, and 99.7% above 15 kev [10]. This coincidence technique is also studied by the CUORE collaboration, preparing an experiment at the Gran Sasso Laboratory for the search of the neutrino-less DBD in the 130 Te isotope, present with 34% natural abundance, using crystals of TeO 2. The neutrino-less DBD would produce a signal of MeV. A small scintillation signal has been reported for TeO 2 crystals at 20 mk [11]. The hope is now to improve this scintillation by activating the crystal with some suitable dopant. In this particular application, doping must face delicate problems, like the dangerous presence of radioactive elements in the dopants, or the modification induced by doping in the thermal characteristics of TeO 2 crystals. 6. CONCLUSIONS widens the potential of the detectors, leading to new applications in different domains. ACKNOWLEDGEMENTS I would like to thank my colleagues I. Dafinei, M. Diemoz and C. Seez for many useful comments and suggestions. REFERENCES 1. M. Oreglia et al., Phys. Rev. D25 (1982) J.E. Gaiser et al., Phys. Rev. D34 (1986) B. Adeva et al., Nucl. Instr. and Meth. A 289 (1990) CMS ECAL TDR CERN/LHCC 97-33, also available from: 5. V.A. Batarin et al., Nucl. Instr. and Meth. A 510 (2003) G. Dewhirst and R. Bruneliere, CMS RN 2004/004, available from: 7. A. Van Lysebetten and P. Verrecchia, CMS RN 2004/001, available from: 8. L. Barone et al, CMS RN 2004/005, available from: 9. A. Annenkov, A. Fedorov, M. Korzhik, P. Lecoq and O. Missevitch, Nucl. Instr. and Meth. A 527 (2004) P. Meunier et al., Appl. Phys. Lett. 75 (1999) P. de Marcillac et al., A study of scintillating crystals and bolometers at 20 mk 10th Int. Workshop on Low Temperature Detectors, Genoa Crystals calorimeters still represent the state of the art for electromagnetic calorimetry in High Energy Physics. New projects under development and construction are working to exploit the high resolutions provided by these detectors in unprecedented domains and environments. So far, test results confirm the feasibility of these extreme target performances. The continuous investigation of crystal properties, mandatory to fulfill these requirements,