THE qualities of liquid xenon (LXe) as an efficient

1800 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 52, NO. 5, OCTOBER 2005 Fast Timing Measurements of Gamma-Ray Events in Liquid Xenon Karl-Ludwig Giboni, Elena Aprile, Pawel Majewski, Kaixuan Ni, and Masaki Yamashita Abstract We have used newly developed metal-channel photomultipliers (PMTs) from Hamamatsu Photonics to detect the fast scintillation light from liquid xenon. The PMTs are designed to operate within the liquid and to efficiently detect the 175 nm Xe light. Their small transit time spread makes them ideal for fast timing applications. To evaluate the timing characteristics of a liquid xenon detector, we tested a gridded ionization chamber equipped with two such PMTs. Both PMTs observe the same interaction and share the detected scintillation light about equally. At zero field the measured timing resolution is 565 ps FWHM for 122 kev from 57 Co and 227 ps FWHM for 662 kev from 137 Cs. For electric fields up to 4 kv/cm drift field the timing resolution remained constant. This timing resolution opens the possibility of a time-of-flight (ToF) measurement between two LXe detectors separated by a distance of 10 20 cm, which is of great interest for background reduction in a Compton telescope for -ray astrophysics. Index Terms Liquid xenon (LXe), photomultiplier tube (PMT), scintillation, timing measurement. I. INTRODUCTION THE qualities of liquid xenon (LXe) as an efficient gamma-ray detector material have been known for many years, and are well documented in the literature [1], [2]. Until recently the attention has focused on the readout of the ionization electrons liberated in gamma-ray interactions. The prompt scintillation light emission of LXe has been used mostly as trigger signal [3]. With this approach detectors of good performance have been developed, but the complementary information contained in the scintillation light can lead to significant improvement. This additional information includes an independent energy measurement, very fast subnanosecond timing, and even particle identification by using pulse shape analysis. The reason for often neglecting the scintillation light in a Time Projection Chamber (TPC) design is understandable. Not only it is difficult to optimize a detector for simultaneous light and charge readout, but also the VUV light of xenon is easily absorbed on most surfaces in a typical ionization chamber. Windows transmitting the light out of the detector are cumbersome because of the presence of a vacuum cryostat Manuscript received March 7, 2005; revised June 1, 2005. This work was supported by a grant from the National Science Foundation to the Columbia Astrophysics Laboratory (Grant PHY-02-01740). P. Majewski acknowledges the support by the North Atlantic Treaty Organization under a grant awarded in 2003. The authors are with Physics Department and Astrophysics Laboratory, Columbia University, New York, NY, 10027 USA (e-mail: kgiboni@astro.columbia.edu). Digital Object Identifier 10.1109/TNS.2005.856763 around the liquid xenon detector. They also dramatically reduce the detected light by simple solid angle effects and total reflection. Recently, novel metal-channel photomultipliers (PMTs) were introduced by Hamamatsu Photonics Co., specifically optimized for use with liquid xenon detectors. These PMTs can be immersed in the liquid, since they do not compromise the purity of the liquid. With these PMTs it is possible to cover a large fraction of the surface in a liquid Xe chamber for efficient detection of the scintillation light. Due to short distance between photocathode and first dynode these PMTs also offer excellent timing characteristics. The full benefit of subnanosecond timing can be exploited in experiments requiring coincidence timing or time-of-flight (ToF) measurements. II. SCINTILLATION OF LIQUID XENON The light yield of liquid xenon for gamma-ray events is about 75% that of NaI. The energy to create one photon is [4], compared to the energy to create a free electron of [5]. For electron recoil the light has short decay constants of 2.2 ns, 27 ns with applied electric field [6], and 45 ns at zero field [7]. Xenon does not show self absorption of the light, and at the high purity levels required for charge transport, the attenuation length for light is much larger than 1 m. It seems to be a near ideal scintillator, but practically it is difficult to construct a detector efficiently detecting simultaneously light and charge, because the light is emitted in the vacuum ultraviolet (VUV) at 175 nm. VUV light is easily absorbed on the surface of most construction materials. Additionally, the refractive index of xenon is high and much light is lost due to total reflection by passing through the liquid gas interface or through a window. Note that the equivalent of optical grease used to couple PMTs to scintillators and windows in the visible range is not known for the VUV at cryogenic temperatures. In the last years several developments changed the situation. It was established that Teflon (PTFE) is a very good diffuse reflector with more than 88% reflectivity [8]. It can be used inside the liquid xenon without compromising the high purity requirement. Furthermore new PMTs were developed based on a metal-channel structure with a very compact envelope supporting the environmental conditions encountered in a LXe detector. Without additional windows to the exterior the losses due to total internal reflection are eliminated and the design of the detector is simplified. It is now feasible to cover a large fraction of the detector s surface with active photocathode material. Much of the remaining surface can be made reflective by using PTFE maximizing the light collection. 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GIBONI et al.: FAST TIMING MEASUREMENTS OF GAMMA-RAY EVENTS IN LIQUID XENON 1801 Fig. 2. Photograph of the custom developed PMT base. Fig. 1. Photograph of the Hamamatsu R9288 PMT. III. PHOTOMULTIPLIERS FOR USE IN LIQUID XENON During the last years Hamamatsu Photonics Co. developed a new line of PMTs based on a metal-channel multiplying structure. The most obvious advantage of these tubes is the short length, about 35 mm for a 5 cm diameter PMT. The short length is an invaluable asset considering that the volume around the tube in a detector is filled with costly LXe, which cannot be used as active volume. Materials used for the tube envelope are quartz, glass, and nickel coated stainless steel, all compatible with the purity requirement of the LXe. The envelope of the tube is designed to withstand the cold temperatures and high pressures (up to 3.5 atm). After their initial introduction, some of the metal-channel tubes were further optimized, specifically for the use in LXe. Initial versions of the 5 cm diameter tube (R6041 mod.) were plagued with a low quantum efficiency at 175 nm specified to 5% 8%, and a collection efficiency around only 50%. In the new model R9288 1 this was corrected. Now the is specified above 20%, and the collection efficiency has grown to 70%. A photo of this PMT is shown in Fig. 1. The R9288 has been extensively used in several different LXe detectors by our group as well as other groups, i.e., experiment. 2 For operation in the liquid also the HV divider connecting to the PMT has to be clean not to spoil the purity, and its components have to withstand the low temperature. Based on a circuit design from Hamamatsu, we developed a custom printed circuit board. On a 0.5 mm thick ceramic substrate standard surface mount, resistors and capacitors are soldered to the metalized traces. In newer versions, the resistors are directly printed on the ceramic with techniques used for hybrid electronic circuits. This base is shown in Fig. 2. After soldering the components and pins for the connections, the Rosin based solder flux is removed, and the circuit is cleaned with alcohol. This PMT base does not show any deteriorating influence on the liquid xenon purity [9]. 1 Specially developed models by Hamamatsu Photonics Company. [Online] Available: http://www.hamamatsu.com/ 2 [Online] Available: http://meg.web.psi.ch/ The short flight path of the electrons in the metal-channel construction is very similar across all the active surface of the cathode. Thus the PMT has a very small transit time spread (TTS), i.e., good timing characteristics. The TTS is specified to be less than 750 ps FWHM obtained with full photocathode illumination. This value characterizes the timing response for single photoelectrons. With a large number of photoelectrons better timing can be achieved, and the resolution improves with. IV. EXPERIMENTAL SET UP To evaluate the timing resolution a gridded ionization chamber, shown schematically in Fig. 3, was used. With this chamber the light and charge signals from gamma-ray events can be observed simultaneously. The active volume is defined by a Teflon cylinder of 5 cm inner diameter with two stretched metal meshes at the ends, one serving as cathode and one as anode. An additional mesh serves as shielding grid 3 mm in front of the anode. Finally, one more mesh was required to prevent crosstalk from the strong PMT pulses on the anode of the ionization chamber. Two Hamamatsu PMTs R9288, view the chamber through the meshes from both ends. The active volume between cathode and shielding grid is 2 cm long. The surfaces in this chamber are either Teflon reflector or active photocathode. According to simulations the PMTs see about 19% of the light in the chamber directly, and an additional 48% are observed after one or more reflections on the Teflon walls. A photo of the chamber with PMTs removed is shown in Fig. 4. The chamber is mounted horizontally in a stainless steel vessel, i.e., the PMTs and their bases are fully immersed in liquid. The chamber vessel is placed in a vacuum cryostat and cooled by a mixture of liquid nitrogen alcohol. The cryostat and the Xe handling and purification system were used in earlier experiments with LXe detectors and are described elsewhere [9]. Electric fields up to 4 kv/cm can be applied to the drift volume. The chamber is illuminated with external gamma ray sources. The PMT signals are externally fed into fast amplifiers (KN2104 by Kaizu, Japan) with 500 MHz bandwidth and discriminators. The logic signals from the discriminators are used as Start and Stop for a Time to Amplitude Converter (ORTEC TAC 566) with resolution. For each gamma-ray event the TAC signal as well as the pulse height of the two PMTs are recorded, and the charge signal from the ionization measurement is digitized. In each experiment the light and charge collection in the ionization chamber is determined. The measured electron

1802 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 52, NO. 5, OCTOBER 2005 Fig. 3. Schematic of the LXe chamber and signals readout. Dimensions are in cm. that no signal is attenuated by impurities. Measuring the light collection at zero electric field and using 122 kev line from 6 photoelectrons per kev was observed. Fig. 4. Photograph of the LXe chamber. The two PMTs are not shown. The chamber is mounted upside down in the chamber vessel. lifetime best characterizes the purity level. For the present tests this lifetime is higher than 200, compared with a maximum drift time of about 10 in the 2 cm long chamber. Since the purity requirements for efficient charge collection are much stronger than for light collection in the LXe it was made certain V. RESULTS OF TIMING MEASUREMENT Before measurement with LXe, the time measurement electronics was tested without requiring the complex operation of purifying the xenon, filling and recuperating it from the chamber. For this purpose the two PMTs were first tested with a small plastic scintillator sandwiched between the tubes. The scintillator was irradiated with 662 and 1275 kev rays from a and sources. With a source the coincidence timing distribution has a FWHM of 340 ps per PMT, taking into account that two identical, independent signals, the Start and the Stop, contributed to the resolution measurement. The corresponding value for the 1275 kev line of is 224 ps FWHM, showing the expected scaling of the timing resolution with. Compared to this fast plastic scintillator response, the LXe coincidence timing is expected to be slower, since most of the light is produced with decay constants up to 30 ns. Only the fastest component of the Xe light actively contributes to the timing measurement. Most of the photons will come from the 2 ns decay component which is about 25% of the total Xe light.

GIBONI et al.: FAST TIMING MEASUREMENTS OF GAMMA-RAY EVENTS IN LIQUID XENON 1803 Fig. 5. Cs scintillation light spectrum of summed signals from two PMTs. Fig. 7. Time resolution as a function of the deposited -ray energy. Fig. 6. Distribution of the difference of the integrated pulses from both PMTs over their sum. Vertical lines select region of events sharing equal amount of energy to both PMTs. Fig. 8. Time resolution dependence on the applied electric field. For measurements with LXe we used external -ray sources. To remove events from -ray interactions in the dead LXe volume outside the Teflon structure, we require that a charge signal is simultaneously recorded with the light signal for accepted events. An example of the light spectrum obtained from with lines indicating energy range of interest is shown in Fig. 5. Alternatively, especially when no electric field was applied, a requirement that both PMTs roughly see similar light amplitudes rejected the background from outside the chamber volume. Fig. 6 shows the distribution of the difference of the PMTs signal over their sum. The lines indicate the cut to select events with equal signal amplitude on each PMT. The shape of the distribution is caused by the geometry and structure of the multimesh chamber. The timing resolution is 517 ps FWHM for the selected data. Again, this value takes into account that two PMTs contributed to the measurement, each of them adding quadratically a timing uncertainty. We also note that the energy of the gamma ray is approximately shared equally between the two PMTs. In applications like Positron Emission Tomography (PET), the arrival time of a 511 kev gamma ray is measured against a signal from another gamma ray. This means, all the 511 kev are used for one timing signal only. Practically, in such applications the resolution is expected to be better. Finally we maximized the light output of the chamber, by mounting the PMTs closer to the ends of the cylindrical structure and covering remaining steel rings surfaces with a Teflon reflector. After these modifications the observed light signals Fig. 9. Timing distribution obtained with Cs and cuts shown in Figs. 5 and 6. were twice as large. The additional light resulted in a timing resolution better by a factor, as expected, since in our compact chamber the additional travel time of reflected photons is negligible. Fig. 7 shows the measured values for the timing resolution versus source energy using: (122 kev), (511 kev), and (662 kev). The obtained values of timing are respectively: 567, 277, and 228 ps FWHM. As example, the timing distribution for (662 kev) is shown in Fig. 9. It is a well-established effect that the amount of light in a liquid Xe detector is reduced by about 50% when an electrical field is applied. In the electrical field some of the charges start to drift toward the anode. Thus, fewer electrons recombine with the ions, which produce a large part of the scintillation light.

1804 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 52, NO. 5, OCTOBER 2005 Despite the fact that we observed a timing resolution consistent with dependence, varying the electric field in our chamber does not significantly change the timing. Fig. 8 shows that the resolution is almost constant between 0 and 3 kv/cm although the light amplitude is reduced by about 50% at the high fields. We attribute this result to the different processes producing scintillation light. The recombination of electrons produces the light with two longer decay constants (25 and 35 ns), whereas the light used for the timing measurement is produced via direct excitation with the 2.2 ns decay constant. This component, however, is not decreased by the external field. VI. DISCUSSION AND RESULTS The subnanosecond timing resolution of liquid xenon with the new metal-channel PMTs has substantial benefits for many experiments. One such experiment is the ongoing MEG experiment [10], where coincidence timing values of 120 ps are expected with 52.8 MeV gamma rays. Our results are fully compatible with the results obtained so far by the MEG group. For nonaccelerator physics, we discuss two applications which require good timing. A. LXe PET The application of LXe for PET has been considered since some time [11] [13], and several groups continue to work on this application. Timing resolution is mainly used to establish a coincidence between two detectors on opposite sites of the object under study, detecting positron annihilation gammas. A good timing resolution helps to suppress background by eliminating random coincidences of two independent annihilation events. Since each detector sees the full energy of 511 kev a resolution below 300 ps is achievable, comparing favorably with 5 10 ns of commercial BGO systems. B. LXe Advanced Compton Telescope The Columbia group has pioneered the application of a LXeTPC for imaging MeV rays from astrophysical sources. A first prototype Compton telescope was developed and successfully tested in the laboratory and in the near space environment with a series of balloon-borne experiments. The advantage of a LXeTPC to reduce the dominating -ray background through Compton kinematics reconstruction can be dramatically enhanced by adding a ToF capability. The power of TOF for effective background reduction has been clearly demonstrated in the first and only space-based Compton telescope, the COMPTEL instrument [14] on the Compton Gamma-Ray Observatory. COMPTEL was based on two scintillator arrays, a liquid scintillator and NaI, separated by 1.5 m, which significantly reduced the geometrical acceptance. Based on the fast timing capability shown for LXe, an advanced Compton Telescope design with LXeTPCs separated by only 10 20 cm appears very promising. 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