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A sharp gas-bremsstrahlung beam capable of testing electromagnetic calorimeters
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Nuclear Instruments and Methods in Physics Research A 582 (2007) 489–496 www.elsevier.com/locate/nima
A sharp gas-bremsstrahlung beam capable of testing electromagnetic calorimeters T. Matsumuraa,, Y. Asanob, R. Chibac, T. Hashimotoc, A. Miurac, H. Shimizud, Y. Tajimac, H.Y. Yoshidac a
Department of Applied Physics, National Defense Academy, Hashirimizu, Yokosuka 239-8686, Japan b Synchrotron Radiation Research Institute/JASRI, SPring-8, Sayo, Hyogo 679-5198, Japan c Department of Physics, Yamagata University, 1-4-12 Koshirakawa, Yamagata 990-8560, Japan d Laboratory of Nuclear Science, Tohoku University, Mikamine, Sendai 982-0826, Japan Received 3 July 2007; received in revised form 8 August 2007; accepted 4 September 2007 Available online 12 September 2007
Abstract A sharp gas-bremsstrahlung beam with an energy up to 8 GeV is available at insertion device (ID) beamlines at SPring-8. As a practical use of the beam, a test experiment for a 3 3 PWO calorimeter has been performed at an ID beamline, BL11XU, to measure the position of the incident gas-bremsstrahlung beam impinging on the calorimeter. The obtained position resolution is qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffi ðð2:76 0:02Þ= E Þ2 þ ð0:30 0:04Þ2 mm, E in GeV, at the center of the calorimeter, which is consistent with the result previously obtained with a low-energy electron beam. Despite limited beam-energy information, the pencil-like photon beam generated on ID beamlines works well for a measurement of the position resolution of electromagnetic calorimeters. r 2007 Elsevier B.V. All rights reserved. PACS: 29.27.Fh; 29.40.Vj Keywords: Photon beam; Calorimeter; Lead tungstate; SPring-8
1. Introduction Test experiments with a photon or an electron beam are essentially important for the performance study of electromagnetic (EM) calorimeters to be used in Nuclear and Particle Physics experiments. In general, basic properties of an EM calorimeter, such as the energy and the position resolution, are evaluated in a test beam experiment after design optimization has been made with simulations. Such a beam test is usually conducted at a beamline dedicated for this purpose in electron or hadron accelerator-facilities. In Japan, beamlines for EM calorimeter tests in a few GeV region have been limited after the shutdown of the 12 GeV proton synchrotron at KEK in 2005. There are two Corresponding author. Tel.: +81 46 841 3810; fax: +81 46 844 5912.
E-mail address: [email protected] (T. Matsumura). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.09.006
facilities providing an electron/photon beam for EM calorimeter tests around this energy region. An electron and a positron beam are available at Laboratory of Nuclear Science, Tohoku University, for detector tests with the energy ranging from 100 to 800 MeV. The beam intensity and the momentum resolution are typically 3 kHz and sp =p1%, respectively. The LEPS beam at SPring-8 can also be utilized for the same purpose. The beam energy ranges from 1.5 to 2.4 GeV. It is a ‘‘clean’’ photon beam, which is produced by backward Compton scattering of UV laser light from 8 GeV stored electrons [1]. However, it is difficult to find a beam time for test experiments with the LEPS beam since long term experiments have been conducted on this beamline. A new electron-beamline for test experiments is planned at KEK, where electrons are generated by eþ e pair production of gas-bremsstrahlung photons coming from a 100 m straight section of the
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KEK-B accelerator. The momentum range is expected to be 0.5–3:4 GeV=c [2]. In this report, we discuss a sharp high-energy photon beam obtained in insertion-device (ID) beamlines at SPring-8. The size of the beam is 1 mm and the energy range is up to 8 GeV. The sharp photon beam enables us to perform precise measurement of the position resolution of EM calorimeters. In the following part of this report, at first, we discuss features of the high-energy photon beam in an ID beamline. Then, a test experiment with a PWO calorimeter is presented as a practical use of the photon beam at the ID beamline. Although the experiment was originally motivated from a view point of radiation safety [3], we focus on performance tests with the photon beam in this report. Finally, we discuss advantages and disadvantages of the high-energy photon beam on ID beamlines from a practical standpoint. 2. High energy photons on ID beamlines at SPring-8 SPring-8 is a synchrotron-radiation facility located in the West-Harima region in Japan [4]. The main accelerator of the facility is an 8 GeV electron storage-ring whose circumference is 1436 m. There are 44 straight sections in the storage ring. Each straight section in between two bending magnets is equipped with an insertion device such as an undulator or a wiggler to generate high-intensity synchrotron-radiation. The length of the straight section is typically about 16 m for ID beamlines. (There are some ID beamlines having a longer straight section.) A typical ID beamline is illustrated in Fig. 1. The beam divergences of the stored electrons are known to be very small at SPring-8. The average divergence of the beam at one of the ID
beamlines, BL11XU for example, has been estimated to be 50 mrad in the horizontal and 2 mrad in the vertical directions [3]. In the downstream part of the straight section, so-called ‘‘front-end’’, some slits or absorbers have been installed. A fixed mask and a movable X–Y slit are used for eliminating the unwanted power of synchrotron radiation. In addition, an absorber and a main shutter are placed in the front-end, which are used as a beam dump. The X–Y slit [5], which is placed about 29 m downstream of the straight section in the case of the BL11XU beamline, defines the beam size. It consists of two parts: an upstream slit and a downstream slit as illustrated in Fig. 2. The upstream slit defines the left and down side of the beam, looking downstream, and the downstream slit does the right and up side of the beam. The length of both slits is 500 mm. The typical aperture size of the X–Y slit is 1 1 mm2 (34:6 mrad 34:6 mrad in the angular unit) for a synchrotron radiation beam. There are slightly different aperture sizes depending on the ID beamlines. High-energy bremsstrahlung photons, besides synchrotron radiation, are generated at the ID beamlines due to the interaction of stored electrons and residual gasses in the straight section. The energy distribution of the gasbremsstrahlung photons ranges from 0 to 8 GeV, falling off with increasing energy as 1=E. The average emission angle of the bremsstrahlung photons is expected to be 64 mrad. Thus, the beam size of the gas-bremsstrahlung photons is defined by the aperture size of the X–Y slit. Using the gas-bremsstrahlung photons one can perform a test experiment for EM calorimeters in the optics hutch of an ID beamline.
Fig. 1. Illustration of an ID beamline, BL11XU, for a calorimeter test [3]. Synchrotron radiation or gas-bremsstrahlung photons generated in the straight section (16.54 m) are collimated by the slits located at the front-end. For test beam experiments, calorimeters can be installed in between the beryllium window ð250 mmÞ and the monochromator placed in the optics hutch.
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Fig. 2. Illustration of the X–Y slit installed in the BL11XU beamline, consisting of the upstream and the downstream slits [5]. High energy photons generated at the straight section are collimated to be 1 1 mm2 with the slits. Each slit, which is made of Gridcop, has a vertical blade and a horizontal blade with a grazing-incidence angle of 1:57 . The left and lower side of the beam, looking downstream, are defined with the upstream slit and the right and upper side with the downstream slit. The upstream and the downstream slits are located at 28.45 m and 29.40 m downstream, respectively, from the center of the straight section. Note that the horizontal and vertical sizes are not to scale.
Fig. 3. (a) Illustration of the PWO calorimeter [3]. A copper plate was placed in front of the calorimeter to protect it from synchrotron radiation. (b) Enlarged illustration of the front face of the calorimeter.
3. Experiment 3.1. Experimental setup The experiment was carried out at BL11XU, which is an ID beamline at SPring-8 [6,7]. An undulator has been installed in the middle of the straight section (see Fig. 1). We set the gap of the undulator magnet to the maximum (50 mm) in order to suppress synchrotron radiation as much as possible. The aperture size of the X–Y slit was set to be 1 1 mm2 by remote control. An EM calorimeter, a 3 3 matrix of PbWO4 (PWO) crystals, was used for the experiment. The PWO calorimeter is illustrated in Fig. 3. The cross-sectional area of each crystal was 20 20 mm2 , and the length was 200 mm corresponding to 21:7X 0 , where X 0 denotes the radiation length. Nine photomultiplier tubes (PMTs), Hamamatsu R4125GMOD, were attached to the end of the crystals. The detail configuration of the PWO calorimeter was described in the papers [3,8]. The PWO calorimeter was placed just upstream of the monochromator in the optics hutch as shown in Fig. 1. The
distance from the center of the straight section to the calorimeter was 35.35 m. In order to install the calorimeter, we removed a beam pipe placed in between the beryllium window and the monochromator. The whole calorimeter was mounted on a movable stage which was able to slide both in the x and y directions with an accuracy of 20 mm. To protect the calorimeter from intense synchrotron radiation, an 8 mm thick copper plate ð0:56X 0 Þ was attached just in front of the calorimeter. The output signal from the central detector #5 was divided into two signals, one of which was used to make up the event trigger [3] with a discriminator. The analog signal from each PMT was digitized with a 12-bit ADC. The threshold energy of the trigger was set to be 0.5 GeV for energy measurements and 1.0 GeV for position measurements, respectively. The trigger rate was 1.1 kHz in the case of the threshold of 0.5 GeV with a beam current 96 mA of the storage ring. Live time of the data taking was about 60% in this trigger rate. 3.2. Energy calibration Fig. 4 shows pedestal-subtracted ADC spectra for all the PWO crystals. The photon beam was injected onto the center of the central crystal, in which the main part of shower energy was deposited. According to an EGS4 Monte-Carlo simulation [9], 74% of the incident photon energy is deposited in the central crystal and a fraction of about 18% is in the peripheral crystals. Hence, 92% of the photon energy is deposited in the calorimeter. The remaining part of the energy leaks out to the sides (7%) and the rear (1%) of the calorimeter [8]. Thus the simulation gives a ratio, 80%, of the energy deposited in the central crystal to that in the whole calorimeter. And actually, the experimental data show the ratio consistent with the simulation result. Relative energy calibration for the signals of nine crystal detectors was made by using the movable stage so as to
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where E dep denotes the energy deposit in the central crystal; free parameters N, s0 and C represent a normalization factor, the energy resolution of the central crystal at 1 GeV and a conversion factor from the ADC channel # to the energy deposit in the unit of GeV/channel, respectively. Fixed parameters E min and E max denote borders of an integral region of convolution; we set as E min ¼ 2 GeV and E max ¼ 5:92 GeV. After the relative calibration of each crystal was made, the energies deposited in all the crystals were summed up. Then, for absolute energy calibration, the summed energy was divided by the factor 0.92 that corresponds to the fraction of energy deposited in the calorimeter. Fig. 5 shows the energy distribution of the gas-bremsstrahlung photons obtained after the energy calibration.
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inject the photon beam onto the center of each crystal. An end-point method was employed for the ADC spectrum in which the end point corresponds to the maximum energy of the bremsstrahlung photons, 8 GeV. Since the fraction of the energy deposit in the central crystal is 74%, the maximum energy deposit in the crystal is expected to be 5.92 GeV. Therefore, we set the ADC value at the end point to be 5.92 GeV. For this purpose, the following function was fitted to the edge region of the ADC spectrum: Z E max 1 ðE dep ADC CÞ2 N exp (1) dE dep , 3=2 2s20 E dep E min E dep
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Fig. 5. Energy distribution of gas-bremsstrahlung photons measured with the PWO calorimeter.
3.3. Position reconstruction There are several methods of reconstructing the position of an incident photon from its energies deposited in the
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where xi and yi are the central position of the ith crystal of the calorimeter, and E i denotes the energy deposit in it. In order to check the relation between the center of gravity (xG ; yG ) and the incident position (x; y), we measured the distribution of the center of gravity for eight different points in the range from y ¼ 0 mm to y ¼ 11 mm by moving the stage on which the calorimeter was placed. Fig. 6 shows the distributions of the center of gravity at 5 GeV for the different incident positions (0, 3, 5, 7 mm). Here we used events within the energy range of 50 MeV around 5 GeV as the events corresponding to 5 GeV (the events of a certain energy are selected in the way). Shifting the incident position from the center to downward results in a relative increase of the energy deposit in the crystal #2, located just below the central crystal #5 (see Fig. 3). Hence, the mean value of the center of gravity decreases with the shift. Fig. 7 represents the mean values of the center of gravity as a function of the incident beam position. The maximum gradient is seen near the edge of the central crystal ðy ¼ 10 mmÞ. This behavior is attributed to the fact that the deposited energy due to electromagnetic showers falls exponentially as the lateral distance from the incident beam position increases [10]. The relation can be reproduced with a function, yG ¼ p0 tan p1 ðy p2 Þ,
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Fig. 6. Distributions of the center of gravity yG at 5 GeV for different beam positions y ¼ ð0 mm; 3 mm; 5 mm; 7 mmÞ.
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nine PWO crystals. In this report we employ a center of gravity method in order to compare the result with that obtained in the past experiment in which the same PWO crystals were used. The center of gravity for the position in both x and y directions (xG , yG ) is defined as P9 P9 E i xi i¼1 E i yi xG ¼ Pi¼1 ; y ¼ , (2) P9 G 9 E i¼1 i i¼1 E i
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Fig. 7. Relation between the position given with the center of gravity and the incident position for 5 GeV photons. Dotted curve shows the function (3) with parameters fitted to the data. Note that y ¼ 10 mm corresponds to the bottom edge of the central crystal. Table 1 Parameters of the function yG ¼ p0 tan p1 ðy p2 Þ E (GeV)
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0:06 0:02 0:11 0:02 0:09 0:02 0:09 0:02 0:09 0:02 0:08 0:02
where p0 , p1 and p2 denote free parameters. The function is fitted to each ðy; yG Þ plot separately for six different incident energies from 2 to 7 GeV in 1 GeV steps. Note that the data more than 2 GeV were used in the analysis since the energy threshold in the position measurement was set to be 1 GeV. The fitted function for 5 GeV is also superimposed in Fig. 7. The parameters p0 and p1 for several energies are summarized in Table 1. The parameters remain constant within errors for the incident energy greater than 4 GeV, while they have small energy dependence for the energy less than 3 GeV. The small dependence at low energies could arise from trigger-biased events taking place near the crystal edge, resulting in a small shift to the center. The non-zero values of the parameter p2 are originated from misalignment of the calorimeter, which is 0.09 mm on average. The inverse function of Eq. (3) with the obtained parameters gives the incident position of the beam. Fig. 8 shows the reconstructed positions for 2 and 7 GeV photons injected on the center of the central crystal. The narrower
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Fig. 8. Distributions of the reconstructed position of the beam injected on the center of the calorimeter for 2 and 7 GeV photons. Dashed curves show the result of a fit with function (4).
distribution for the 7 GeV case is due to better position resolution of the PWO calorimeter.
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The position resolution at the center of the calorimeter is deduced from the reconstructed position distribution shown in Fig. 8. The beam size at the calorimeter position is estimated to be 1:2 1:2 mm2 , which is given with the location of the X–Y slit and the calorimeter. Here we assume that gas bremsstrahlung photons are generated at the center of the straight section. The estimated beam profile on the calorimeter has a square shape with sharp edges, which correspond to y ¼ þ0:60 mm and y ¼ 0:62 mm in y direction. This small asymmetry with respect to y ¼ 0 comes from the difference of the location of the upstream slit (28.45 m from the center of the straight section) and the downstream slit (29.40 m). To extract the resolution from the position distribution, we introduce a convolution of a Gaussian, representing fluctuations due to the position resolution, with a step function, showing the distribution of the beam profile. We employ a convolution function " ! !# N y ymin y ymax erf pffiffiffi erf pffiffiffi ðD ymax ymin Þ, D 2s y 2sy (4) where the fixed parameters, ymax ð¼ þ0:60 mmÞ and ymin (¼ 0:62 mmÞ, represent the upper and lower edge of the beam profile, sy denotes the position resolution of the PWO calorimeter in y direction and N is a normalization factor. The fitted curves are superimposed in Fig. 8. The energy dependence of the position resolution at the center is shown as filled circles in Fig. 9. The position resolution becomes better as the incident photon energy increases because of larger photo-electron statistics at high energies. This dependence is parametrized as vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ! u u 2:76 0:02 2 t pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (5) sy ðmmÞ ¼ þ ð0:30 0:04Þ2 . EðGeVÞ
position resolution (mm)
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Fig. 9. Energy dependence of the position resolution at the center of the calorimeter. Filled circles show the result of the present experiment, and open squares show the result obtained in the previous experiment with a low-energy electron-beam for the same PWO calorimeter [8]. The curve represents the function (5) fitted to the present data. Note that the temperatures were different between two experiments: 25 C for the present experiment, and 13 C for the previous experiment.
The present result can be compared with the past experimental data obtained at KEK-Tanashi with an electron beam below 1 GeV [8]. The same PWO calorimeter was used in both the experiments. The position resolutions evaluated with the electron beam are also plotted in Fig. 9. Coefficients of Eq. (5) obtained by the past experiment are 2:6 0:1 for the first term and 0:4 0:6 for the second term, respectively. The present result for the position resolution is consistent with the result previously obtained with the low-energy electron beam, indicating that the estimated beam size of the bremsstrahlung photons at the calorimeter is reasonable. A small discrepancy of the
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coefficient in the first term between the two experiments could arise from the difference of detector temperatures: 25 C for the present experiment, and 13 C for the previous experiment. The estimated number of photoelectrons is 1.6/MeV at 13 C [8] and light yield of PWO crystals decreases with temperature as 1:3%= C [11]. Note that the PWO crystals used in the present experiment were undoped crystals produced more than 10 years ago. The light output was about 13, compared with the value of 9=MeV reported by the CMS group [12]. The smaller number of photoelectrons 1.6/MeV is due to the small effective area, 15 mm , of the PMT viewing the PWO crystal directly. 4. Discussions From the test experiment using the PWO calorimeter, we found some advantages and disadvantages for practical use of the gas-bremsstrahlung photon beam for a performance test of EM calorimeters. As for the disadvantages, a major problem is that beam energy information is limited because there is no tagging device on the beamlines. Only the maximum end-point of the energy distribution, 8 GeV, can be used as the energy information. For this reason, the linearity of output signals to incident photon energies would not be tested with the beam, although the linearity is one of the most important properties of calorimeters. Another small problem is that the available space for an experimental setup is limited to be 1 m because of a monochromator placed close to the entrance of the optics hutch. Hence, a test experiment for a large size of calorimeters might be difficult. Protection against synchrotron radiation is also an issue to be concerned. In the experiment we used a copper plate with a thickness of 8 mm ð0:56X 0 Þ to protect the calorimeter from synchrotron radiation. The copper plate behaves at the same time as a converter for the bremsstrahlung photons. About 35% of the photons are converted to eþ e pairs in the copper plate, which could be problematic for some test experiments because the average position of shower generations is generally different between photons and electrons. We have evaluated the width of the position distribution with or without the copper plate using a Monte-Carlo simulation based on the Geant4 toolkit [13], where the energy fluctuation due to photo-electron statistics is taken into account. The energy threshold was set to be 1 GeV for the event samples in the simulation so as to meet the requirement for the experimental data. The result shows no difference between both cases; the widths for 7 GeV photons are 1:08 0:03 mm with the copper plate and 1:07 0:03 mm without the plate, respectively. For 2 GeV photons they are 1:89 0:02 mm in both cases. This means that the effect of the copper plate is negligibly small on the measurement of the position resolution since the eþ e pairs generated in the copper plate are emitted in very forward angles in this energy region. Of course, the problem can be solved
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Fig. 10. Beam-profile distribution in the area of 2 2 cm2 measured with the PWO calorimeter at 7 GeV. Note that the position resolution of the PWO calorimeter, which has been estimated to be 1.1 mm, gives a dominant contribution to the width of the distribution.
directly by installing a thin veto counter for charged particles behind the absorber if necessary. In contrast with these disadvantages, there are some advantages in calorimeter tests at the ID beamlines. The most attractive feature of the beam is its sharpness. The beam-profile distribution measured with the PWO calorimeter at 7 GeV is shown in Fig. 10, where the dominant contribution of the width comes from the position resolution of the calorimeter, s ¼ 1:1 mm at 7 GeV. As shown in the figure, almost no beam-halo components are observed. This feature enables us the precise measurement of the position resolution of EM calorimeters. For further beam collimation, it is possible to reduce the aperture size of the X–Y slit by remote control, for example 0:5 0:5 mm2 , although we set the aperture to be 1 1 mm2 in the present experiment. Beam intensity of 1 kHz with the energy threshold above 0.5 GeV and the aperture size of 1 1 mm2 , is generally suitable for the test experiment, where the data can be taken without significant loss with a simple data acquisition system. For these reasons, we can conclude that the high-energy photon beam obtained at ID beamlines is useful for performance tests, especially for a study of position resolution of EM calorimeters. 5. Summary A sharp bremsstrahlung photon beam obtained in ID beamlines at SPring-8 has been discussed from a point of view of a tool for a test experiment of EM calorimeters. As a practical use of the photon beam, an experiment has been performed with a 3 3 PWO
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calorimeter. The position resolution is estimated to be ffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðð2:76 0:02Þ= EðGeVÞÞ2 þ ð0:30 0:04Þ2 mm, which is consistent with the result obtained in the previous experiment using a low-energy electron beam. This indicates that the estimated beam size of the bremsstrahlung photon at the calorimeter, 1:2 1:2 mm2 , is reasonable. The energy information of the photon beam is limited because of no tagging devices on the beamline. Nevertheless, the sharp pencil-like photon beam generated at ID beamlines is useful to evaluate the position resolution of EM calorimeters. Acknowledgements The authors would like to thank M. Oura (RIKEN) and S. Takahashi (JASRI) for a discussion about beamline components at SPring-8. The present work was supported in part by the Ministry of Education, Science, Sports and Culture of Japan with the Grant-in-Aid for Scientific Research Nos. 09640334 and 15340069.
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Nuclear Instruments and Methods in Physics Research A 582 (2007) 489–496 www.elsevier.com/locate/nima
A sharp gas-bremsstrahlung beam capable of testing electromagnetic calorimeters T. Matsumuraa,, Y. Asanob, R. Chibac, T. Hashimotoc, A. Miurac, H. Shimizud, Y. Tajimac, H.Y. Yoshidac a
Department of Applied Physics, National Defense Academy, Hashirimizu, Yokosuka 239-8686, Japan b Synchrotron Radiation Research Institute/JASRI, SPring-8, Sayo, Hyogo 679-5198, Japan c Department of Physics, Yamagata University, 1-4-12 Koshirakawa, Yamagata 990-8560, Japan d Laboratory of Nuclear Science, Tohoku University, Mikamine, Sendai 982-0826, Japan Received 3 July 2007; received in revised form 8 August 2007; accepted 4 September 2007 Available online 12 September 2007
Abstract A sharp gas-bremsstrahlung beam with an energy up to 8 GeV is available at insertion device (ID) beamlines at SPring-8. As a practical use of the beam, a test experiment for a 3 3 PWO calorimeter has been performed at an ID beamline, BL11XU, to measure the position of the incident gas-bremsstrahlung beam impinging on the calorimeter. The obtained position resolution is qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffi ðð2:76 0:02Þ= E Þ2 þ ð0:30 0:04Þ2 mm, E in GeV, at the center of the calorimeter, which is consistent with the result previously obtained with a low-energy electron beam. Despite limited beam-energy information, the pencil-like photon beam generated on ID beamlines works well for a measurement of the position resolution of electromagnetic calorimeters. r 2007 Elsevier B.V. All rights reserved. PACS: 29.27.Fh; 29.40.Vj Keywords: Photon beam; Calorimeter; Lead tungstate; SPring-8
1. Introduction Test experiments with a photon or an electron beam are essentially important for the performance study of electromagnetic (EM) calorimeters to be used in Nuclear and Particle Physics experiments. In general, basic properties of an EM calorimeter, such as the energy and the position resolution, are evaluated in a test beam experiment after design optimization has been made with simulations. Such a beam test is usually conducted at a beamline dedicated for this purpose in electron or hadron accelerator-facilities. In Japan, beamlines for EM calorimeter tests in a few GeV region have been limited after the shutdown of the 12 GeV proton synchrotron at KEK in 2005. There are two Corresponding author. Tel.: +81 46 841 3810; fax: +81 46 844 5912.
E-mail address: [email protected] (T. Matsumura). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.09.006
facilities providing an electron/photon beam for EM calorimeter tests around this energy region. An electron and a positron beam are available at Laboratory of Nuclear Science, Tohoku University, for detector tests with the energy ranging from 100 to 800 MeV. The beam intensity and the momentum resolution are typically 3 kHz and sp =p1%, respectively. The LEPS beam at SPring-8 can also be utilized for the same purpose. The beam energy ranges from 1.5 to 2.4 GeV. It is a ‘‘clean’’ photon beam, which is produced by backward Compton scattering of UV laser light from 8 GeV stored electrons [1]. However, it is difficult to find a beam time for test experiments with the LEPS beam since long term experiments have been conducted on this beamline. A new electron-beamline for test experiments is planned at KEK, where electrons are generated by eþ e pair production of gas-bremsstrahlung photons coming from a 100 m straight section of the
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KEK-B accelerator. The momentum range is expected to be 0.5–3:4 GeV=c [2]. In this report, we discuss a sharp high-energy photon beam obtained in insertion-device (ID) beamlines at SPring-8. The size of the beam is 1 mm and the energy range is up to 8 GeV. The sharp photon beam enables us to perform precise measurement of the position resolution of EM calorimeters. In the following part of this report, at first, we discuss features of the high-energy photon beam in an ID beamline. Then, a test experiment with a PWO calorimeter is presented as a practical use of the photon beam at the ID beamline. Although the experiment was originally motivated from a view point of radiation safety [3], we focus on performance tests with the photon beam in this report. Finally, we discuss advantages and disadvantages of the high-energy photon beam on ID beamlines from a practical standpoint. 2. High energy photons on ID beamlines at SPring-8 SPring-8 is a synchrotron-radiation facility located in the West-Harima region in Japan [4]. The main accelerator of the facility is an 8 GeV electron storage-ring whose circumference is 1436 m. There are 44 straight sections in the storage ring. Each straight section in between two bending magnets is equipped with an insertion device such as an undulator or a wiggler to generate high-intensity synchrotron-radiation. The length of the straight section is typically about 16 m for ID beamlines. (There are some ID beamlines having a longer straight section.) A typical ID beamline is illustrated in Fig. 1. The beam divergences of the stored electrons are known to be very small at SPring-8. The average divergence of the beam at one of the ID
beamlines, BL11XU for example, has been estimated to be 50 mrad in the horizontal and 2 mrad in the vertical directions [3]. In the downstream part of the straight section, so-called ‘‘front-end’’, some slits or absorbers have been installed. A fixed mask and a movable X–Y slit are used for eliminating the unwanted power of synchrotron radiation. In addition, an absorber and a main shutter are placed in the front-end, which are used as a beam dump. The X–Y slit [5], which is placed about 29 m downstream of the straight section in the case of the BL11XU beamline, defines the beam size. It consists of two parts: an upstream slit and a downstream slit as illustrated in Fig. 2. The upstream slit defines the left and down side of the beam, looking downstream, and the downstream slit does the right and up side of the beam. The length of both slits is 500 mm. The typical aperture size of the X–Y slit is 1 1 mm2 (34:6 mrad 34:6 mrad in the angular unit) for a synchrotron radiation beam. There are slightly different aperture sizes depending on the ID beamlines. High-energy bremsstrahlung photons, besides synchrotron radiation, are generated at the ID beamlines due to the interaction of stored electrons and residual gasses in the straight section. The energy distribution of the gasbremsstrahlung photons ranges from 0 to 8 GeV, falling off with increasing energy as 1=E. The average emission angle of the bremsstrahlung photons is expected to be 64 mrad. Thus, the beam size of the gas-bremsstrahlung photons is defined by the aperture size of the X–Y slit. Using the gas-bremsstrahlung photons one can perform a test experiment for EM calorimeters in the optics hutch of an ID beamline.
Fig. 1. Illustration of an ID beamline, BL11XU, for a calorimeter test [3]. Synchrotron radiation or gas-bremsstrahlung photons generated in the straight section (16.54 m) are collimated by the slits located at the front-end. For test beam experiments, calorimeters can be installed in between the beryllium window ð250 mmÞ and the monochromator placed in the optics hutch.
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horizontal blade
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Fig. 2. Illustration of the X–Y slit installed in the BL11XU beamline, consisting of the upstream and the downstream slits [5]. High energy photons generated at the straight section are collimated to be 1 1 mm2 with the slits. Each slit, which is made of Gridcop, has a vertical blade and a horizontal blade with a grazing-incidence angle of 1:57 . The left and lower side of the beam, looking downstream, are defined with the upstream slit and the right and upper side with the downstream slit. The upstream and the downstream slits are located at 28.45 m and 29.40 m downstream, respectively, from the center of the straight section. Note that the horizontal and vertical sizes are not to scale.
Fig. 3. (a) Illustration of the PWO calorimeter [3]. A copper plate was placed in front of the calorimeter to protect it from synchrotron radiation. (b) Enlarged illustration of the front face of the calorimeter.
3. Experiment 3.1. Experimental setup The experiment was carried out at BL11XU, which is an ID beamline at SPring-8 [6,7]. An undulator has been installed in the middle of the straight section (see Fig. 1). We set the gap of the undulator magnet to the maximum (50 mm) in order to suppress synchrotron radiation as much as possible. The aperture size of the X–Y slit was set to be 1 1 mm2 by remote control. An EM calorimeter, a 3 3 matrix of PbWO4 (PWO) crystals, was used for the experiment. The PWO calorimeter is illustrated in Fig. 3. The cross-sectional area of each crystal was 20 20 mm2 , and the length was 200 mm corresponding to 21:7X 0 , where X 0 denotes the radiation length. Nine photomultiplier tubes (PMTs), Hamamatsu R4125GMOD, were attached to the end of the crystals. The detail configuration of the PWO calorimeter was described in the papers [3,8]. The PWO calorimeter was placed just upstream of the monochromator in the optics hutch as shown in Fig. 1. The
distance from the center of the straight section to the calorimeter was 35.35 m. In order to install the calorimeter, we removed a beam pipe placed in between the beryllium window and the monochromator. The whole calorimeter was mounted on a movable stage which was able to slide both in the x and y directions with an accuracy of 20 mm. To protect the calorimeter from intense synchrotron radiation, an 8 mm thick copper plate ð0:56X 0 Þ was attached just in front of the calorimeter. The output signal from the central detector #5 was divided into two signals, one of which was used to make up the event trigger [3] with a discriminator. The analog signal from each PMT was digitized with a 12-bit ADC. The threshold energy of the trigger was set to be 0.5 GeV for energy measurements and 1.0 GeV for position measurements, respectively. The trigger rate was 1.1 kHz in the case of the threshold of 0.5 GeV with a beam current 96 mA of the storage ring. Live time of the data taking was about 60% in this trigger rate. 3.2. Energy calibration Fig. 4 shows pedestal-subtracted ADC spectra for all the PWO crystals. The photon beam was injected onto the center of the central crystal, in which the main part of shower energy was deposited. According to an EGS4 Monte-Carlo simulation [9], 74% of the incident photon energy is deposited in the central crystal and a fraction of about 18% is in the peripheral crystals. Hence, 92% of the photon energy is deposited in the calorimeter. The remaining part of the energy leaks out to the sides (7%) and the rear (1%) of the calorimeter [8]. Thus the simulation gives a ratio, 80%, of the energy deposited in the central crystal to that in the whole calorimeter. And actually, the experimental data show the ratio consistent with the simulation result. Relative energy calibration for the signals of nine crystal detectors was made by using the movable stage so as to
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Fig. 4. Pedestal subtracted ADC distributions for all the crystals. Bremsstrahlung photons were injected on the center of the central crystal, #5.
where E dep denotes the energy deposit in the central crystal; free parameters N, s0 and C represent a normalization factor, the energy resolution of the central crystal at 1 GeV and a conversion factor from the ADC channel # to the energy deposit in the unit of GeV/channel, respectively. Fixed parameters E min and E max denote borders of an integral region of convolution; we set as E min ¼ 2 GeV and E max ¼ 5:92 GeV. After the relative calibration of each crystal was made, the energies deposited in all the crystals were summed up. Then, for absolute energy calibration, the summed energy was divided by the factor 0.92 that corresponds to the fraction of energy deposited in the calorimeter. Fig. 5 shows the energy distribution of the gas-bremsstrahlung photons obtained after the energy calibration.
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inject the photon beam onto the center of each crystal. An end-point method was employed for the ADC spectrum in which the end point corresponds to the maximum energy of the bremsstrahlung photons, 8 GeV. Since the fraction of the energy deposit in the central crystal is 74%, the maximum energy deposit in the crystal is expected to be 5.92 GeV. Therefore, we set the ADC value at the end point to be 5.92 GeV. For this purpose, the following function was fitted to the edge region of the ADC spectrum: Z E max 1 ðE dep ADC CÞ2 N exp (1) dE dep , 3=2 2s20 E dep E min E dep
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Fig. 5. Energy distribution of gas-bremsstrahlung photons measured with the PWO calorimeter.
3.3. Position reconstruction There are several methods of reconstructing the position of an incident photon from its energies deposited in the
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where xi and yi are the central position of the ith crystal of the calorimeter, and E i denotes the energy deposit in it. In order to check the relation between the center of gravity (xG ; yG ) and the incident position (x; y), we measured the distribution of the center of gravity for eight different points in the range from y ¼ 0 mm to y ¼ 11 mm by moving the stage on which the calorimeter was placed. Fig. 6 shows the distributions of the center of gravity at 5 GeV for the different incident positions (0, 3, 5, 7 mm). Here we used events within the energy range of 50 MeV around 5 GeV as the events corresponding to 5 GeV (the events of a certain energy are selected in the way). Shifting the incident position from the center to downward results in a relative increase of the energy deposit in the crystal #2, located just below the central crystal #5 (see Fig. 3). Hence, the mean value of the center of gravity decreases with the shift. Fig. 7 represents the mean values of the center of gravity as a function of the incident beam position. The maximum gradient is seen near the edge of the central crystal ðy ¼ 10 mmÞ. This behavior is attributed to the fact that the deposited energy due to electromagnetic showers falls exponentially as the lateral distance from the incident beam position increases [10]. The relation can be reproduced with a function, yG ¼ p0 tan p1 ðy p2 Þ,
(3)
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Fig. 6. Distributions of the center of gravity yG at 5 GeV for different beam positions y ¼ ð0 mm; 3 mm; 5 mm; 7 mmÞ.
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nine PWO crystals. In this report we employ a center of gravity method in order to compare the result with that obtained in the past experiment in which the same PWO crystals were used. The center of gravity for the position in both x and y directions (xG , yG ) is defined as P9 P9 E i xi i¼1 E i yi xG ¼ Pi¼1 ; y ¼ , (2) P9 G 9 E i¼1 i i¼1 E i
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Fig. 7. Relation between the position given with the center of gravity and the incident position for 5 GeV photons. Dotted curve shows the function (3) with parameters fitted to the data. Note that y ¼ 10 mm corresponds to the bottom edge of the central crystal. Table 1 Parameters of the function yG ¼ p0 tan p1 ðy p2 Þ E (GeV)
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2:34 0:02 2:42 0:02 2:47 0:02 2:45 0:02 2:46 0:02 2:48 0:02
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0:06 0:02 0:11 0:02 0:09 0:02 0:09 0:02 0:09 0:02 0:08 0:02
where p0 , p1 and p2 denote free parameters. The function is fitted to each ðy; yG Þ plot separately for six different incident energies from 2 to 7 GeV in 1 GeV steps. Note that the data more than 2 GeV were used in the analysis since the energy threshold in the position measurement was set to be 1 GeV. The fitted function for 5 GeV is also superimposed in Fig. 7. The parameters p0 and p1 for several energies are summarized in Table 1. The parameters remain constant within errors for the incident energy greater than 4 GeV, while they have small energy dependence for the energy less than 3 GeV. The small dependence at low energies could arise from trigger-biased events taking place near the crystal edge, resulting in a small shift to the center. The non-zero values of the parameter p2 are originated from misalignment of the calorimeter, which is 0.09 mm on average. The inverse function of Eq. (3) with the obtained parameters gives the incident position of the beam. Fig. 8 shows the reconstructed positions for 2 and 7 GeV photons injected on the center of the central crystal. The narrower
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Fig. 8. Distributions of the reconstructed position of the beam injected on the center of the calorimeter for 2 and 7 GeV photons. Dashed curves show the result of a fit with function (4).
distribution for the 7 GeV case is due to better position resolution of the PWO calorimeter.
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The position resolution at the center of the calorimeter is deduced from the reconstructed position distribution shown in Fig. 8. The beam size at the calorimeter position is estimated to be 1:2 1:2 mm2 , which is given with the location of the X–Y slit and the calorimeter. Here we assume that gas bremsstrahlung photons are generated at the center of the straight section. The estimated beam profile on the calorimeter has a square shape with sharp edges, which correspond to y ¼ þ0:60 mm and y ¼ 0:62 mm in y direction. This small asymmetry with respect to y ¼ 0 comes from the difference of the location of the upstream slit (28.45 m from the center of the straight section) and the downstream slit (29.40 m). To extract the resolution from the position distribution, we introduce a convolution of a Gaussian, representing fluctuations due to the position resolution, with a step function, showing the distribution of the beam profile. We employ a convolution function " ! !# N y ymin y ymax erf pffiffiffi erf pffiffiffi ðD ymax ymin Þ, D 2s y 2sy (4) where the fixed parameters, ymax ð¼ þ0:60 mmÞ and ymin (¼ 0:62 mmÞ, represent the upper and lower edge of the beam profile, sy denotes the position resolution of the PWO calorimeter in y direction and N is a normalization factor. The fitted curves are superimposed in Fig. 8. The energy dependence of the position resolution at the center is shown as filled circles in Fig. 9. The position resolution becomes better as the incident photon energy increases because of larger photo-electron statistics at high energies. This dependence is parametrized as vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ! u u 2:76 0:02 2 t pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (5) sy ðmmÞ ¼ þ ð0:30 0:04Þ2 . EðGeVÞ
position resolution (mm)
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Fig. 9. Energy dependence of the position resolution at the center of the calorimeter. Filled circles show the result of the present experiment, and open squares show the result obtained in the previous experiment with a low-energy electron-beam for the same PWO calorimeter [8]. The curve represents the function (5) fitted to the present data. Note that the temperatures were different between two experiments: 25 C for the present experiment, and 13 C for the previous experiment.
The present result can be compared with the past experimental data obtained at KEK-Tanashi with an electron beam below 1 GeV [8]. The same PWO calorimeter was used in both the experiments. The position resolutions evaluated with the electron beam are also plotted in Fig. 9. Coefficients of Eq. (5) obtained by the past experiment are 2:6 0:1 for the first term and 0:4 0:6 for the second term, respectively. The present result for the position resolution is consistent with the result previously obtained with the low-energy electron beam, indicating that the estimated beam size of the bremsstrahlung photons at the calorimeter is reasonable. A small discrepancy of the
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coefficient in the first term between the two experiments could arise from the difference of detector temperatures: 25 C for the present experiment, and 13 C for the previous experiment. The estimated number of photoelectrons is 1.6/MeV at 13 C [8] and light yield of PWO crystals decreases with temperature as 1:3%= C [11]. Note that the PWO crystals used in the present experiment were undoped crystals produced more than 10 years ago. The light output was about 13, compared with the value of 9=MeV reported by the CMS group [12]. The smaller number of photoelectrons 1.6/MeV is due to the small effective area, 15 mm , of the PMT viewing the PWO crystal directly. 4. Discussions From the test experiment using the PWO calorimeter, we found some advantages and disadvantages for practical use of the gas-bremsstrahlung photon beam for a performance test of EM calorimeters. As for the disadvantages, a major problem is that beam energy information is limited because there is no tagging device on the beamlines. Only the maximum end-point of the energy distribution, 8 GeV, can be used as the energy information. For this reason, the linearity of output signals to incident photon energies would not be tested with the beam, although the linearity is one of the most important properties of calorimeters. Another small problem is that the available space for an experimental setup is limited to be 1 m because of a monochromator placed close to the entrance of the optics hutch. Hence, a test experiment for a large size of calorimeters might be difficult. Protection against synchrotron radiation is also an issue to be concerned. In the experiment we used a copper plate with a thickness of 8 mm ð0:56X 0 Þ to protect the calorimeter from synchrotron radiation. The copper plate behaves at the same time as a converter for the bremsstrahlung photons. About 35% of the photons are converted to eþ e pairs in the copper plate, which could be problematic for some test experiments because the average position of shower generations is generally different between photons and electrons. We have evaluated the width of the position distribution with or without the copper plate using a Monte-Carlo simulation based on the Geant4 toolkit [13], where the energy fluctuation due to photo-electron statistics is taken into account. The energy threshold was set to be 1 GeV for the event samples in the simulation so as to meet the requirement for the experimental data. The result shows no difference between both cases; the widths for 7 GeV photons are 1:08 0:03 mm with the copper plate and 1:07 0:03 mm without the plate, respectively. For 2 GeV photons they are 1:89 0:02 mm in both cases. This means that the effect of the copper plate is negligibly small on the measurement of the position resolution since the eþ e pairs generated in the copper plate are emitted in very forward angles in this energy region. Of course, the problem can be solved
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Fig. 10. Beam-profile distribution in the area of 2 2 cm2 measured with the PWO calorimeter at 7 GeV. Note that the position resolution of the PWO calorimeter, which has been estimated to be 1.1 mm, gives a dominant contribution to the width of the distribution.
directly by installing a thin veto counter for charged particles behind the absorber if necessary. In contrast with these disadvantages, there are some advantages in calorimeter tests at the ID beamlines. The most attractive feature of the beam is its sharpness. The beam-profile distribution measured with the PWO calorimeter at 7 GeV is shown in Fig. 10, where the dominant contribution of the width comes from the position resolution of the calorimeter, s ¼ 1:1 mm at 7 GeV. As shown in the figure, almost no beam-halo components are observed. This feature enables us the precise measurement of the position resolution of EM calorimeters. For further beam collimation, it is possible to reduce the aperture size of the X–Y slit by remote control, for example 0:5 0:5 mm2 , although we set the aperture to be 1 1 mm2 in the present experiment. Beam intensity of 1 kHz with the energy threshold above 0.5 GeV and the aperture size of 1 1 mm2 , is generally suitable for the test experiment, where the data can be taken without significant loss with a simple data acquisition system. For these reasons, we can conclude that the high-energy photon beam obtained at ID beamlines is useful for performance tests, especially for a study of position resolution of EM calorimeters. 5. Summary A sharp bremsstrahlung photon beam obtained in ID beamlines at SPring-8 has been discussed from a point of view of a tool for a test experiment of EM calorimeters. As a practical use of the photon beam, an experiment has been performed with a 3 3 PWO
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calorimeter. The position resolution is estimated to be ffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðð2:76 0:02Þ= EðGeVÞÞ2 þ ð0:30 0:04Þ2 mm, which is consistent with the result obtained in the previous experiment using a low-energy electron beam. This indicates that the estimated beam size of the bremsstrahlung photon at the calorimeter, 1:2 1:2 mm2 , is reasonable. The energy information of the photon beam is limited because of no tagging devices on the beamline. Nevertheless, the sharp pencil-like photon beam generated at ID beamlines is useful to evaluate the position resolution of EM calorimeters. Acknowledgements The authors would like to thank M. Oura (RIKEN) and S. Takahashi (JASRI) for a discussion about beamline components at SPring-8. The present work was supported in part by the Ministry of Education, Science, Sports and Culture of Japan with the Grant-in-Aid for Scientific Research Nos. 09640334 and 15340069.
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