ARTICLE IN PRESS. Lifetime of MCP PMT. N. Kishimoto, M. Nagamine, K. Inami, Y. Enari, T. Ohshima

Transcription

1 Nuclear Instruments and Methods in Physics Research A 564 (26) Lifetime of MCP PMT N. Kishimoto, M. Nagamine, K. Inami, Y. Enari, T. Ohshima Department of Physics, Nagoya University, Chikusa, Nagoya , Japan Received 5 February 26; received in revised form 2 April 26; accepted 2 April 26 Available online 6 June 26 Abstract To examine the lifetime of a micro-channel plate photo-multiplier tube, we measured its basic performance, gain, quantum efficiency (QE), transit time spread and dark counts as a function of the integrated amount of irradiated photons up to 2:8 4 photons=cm 2, corresponding to a 4-year time duration under supposed super-kekb experimental conditions. The effect of the ion-feedback protection layer was clearly detected and the deterioration of the photo-cathode functioning was examined in terms of the QE spectrum on the light wavelength. r 26 Elsevier B.V. All rights reserved. PACS: 85.6.Ha; 29.4.Ka; Pw Keywords: MCP PMT; Lifetime; TOP counter. Introduction A micro-channel plate (MCP) photo-multiplier tube (PMT) generally provides excellent timing information as a photo-device. The transit time spread (TTS), the transit time fluctuation for single photo-electrons (p.e.), is typically a few ps. With the use of this superior performance, a specifically configured TOF counter exhibits a time resolution of s TOF 5 ps, as reported in Ref. []. To employ this superb timing property for practical use, its lifetime should be long enough under experimental conditions of high counting rate. In this regards, it is often stated that the MCP PMT is not durable under high counting rates or a large background circumstance. It is thus crucial to confirm its practical lifetime and to investigate ways to make the MCP PMT to have a sufficiently long lifetime. The relevant lifetime performance of the MCP PMT appears in terms of degradations of the quantum efficiency (QE), the multiplication gain (G), and TTS resolution ðs TTS Þ, and an increment of the dark counts ðn dark Þ; they could be attributed mostly to damage to the photo-cathode and the MCP-layers. Corresponding author. address: kenji@hepl.phys.nagoya-u.ac.jp (K. Inami). We have been developing the MCP PMT as a photodevice for a time-of-propagation (TOP) counter [2], a newly proposed K=p particle identification detector in terms of a Cherenkov ring-imaging counter, at a highluminosity B-factory. During the course of this R&D work, the lifetime of several MCP PMTs [3] were measured, as reported here. We hereafter refer to MCP PMT as PMT, for brevity. 2. PMTs and setup 2.. Integrated amounts of irradiated photons and output charges We assume a possible circumstance at a super-kekb factory [4] with a luminosity of 2 35 =cm 2 =s, by linearly extrapolating the current experimental condition at KEKB-Belle. The present luminosity of the KEKB collider is 2 34 =cm 2 =s, under which the background rate at the barrel region is about 792 khz, mostly of spent electrons. Our specific configuration of the TOP counter is expected to have a detected Cherenkov photon rate of 68 khz=cm 2 on a PMT. This corresponds to an integrated number of photons over a one-year period of N g ¼ 2: 3 photons=cm 2 =y; this number is taken to 68-92/$ - see front matter r 26 Elsevier B.V. All rights reserved. doi:.6/j.nima

2 N. Kishimoto et al. / Nuclear Instruments and Methods in Physics Research A 564 (26) be a time unit, one super-b year equivalent, in our lifetime evaluation. With a supposition of standard PMT performance to be G ¼ 2 6,QE¼ 2% and a photo-electron collection efficiency (CE) of ¼ 5%, the above N g constitutes 7 mc=cm 2 =y of integrated amounts of output charges from a PMT s anode Prevention layer from the ion-feedback One of the primary sources that shorten the lifetime of a PMT is positive ion-feedback. Positive ions could be produced in the multiplication process of secondary electrons by collisions with residual gas or MCP material, and accelerated backward by a high voltage. QE deterioration could be caused by their adsorption on the photocathode surface and by their damage on chemical bonds of the photo-cathode material. A bias-angle of the MCP is then introduced to reduce these feedback effects. Moreover, in order to protect the photo-cathode from these feedback-ions, a thin metal prevention layer is sometimes placed on the surface of the first MCP-layer at its photo-cathode side. However, it inevitably introduces transmission inefficiency for photo-electrons, typically about 4%, and accordingly causes a CE reduction Characteristics of MCP PMT Seven MCP PMTs were tested: two were manufactured by Hamamatsu Photonics Co. (HPK), and the others were purchased from Budker Institute of Nuclear Physics (BINP). All PMTs had a multi-alkali photo-cathode, and were two MCP-layer type; half of the PMTs were equipped with a prevention layer and the other half were not. They were labeled by w and wo, respectively. Their characteristics are listed in Table Setup Fig. shows the setup for the lifetime measurement. PMTs were irradiated by a blue LED ðl 4 nmþ, triggered by ns-wide pulses with repetition rates of either f ¼ or 5 khz from a pulse generator. The light intensity of the LED was calibrated by a QE well-known vacuum tube (HPK H37-4) at every periodical performance measurement. The output charges of the PMTs were evaluated from the ADC distributions. The observed number of photo-electrons of a PMT was N p:e: ¼ 22 per pulse, corresponding to an output charges of S Q ¼ 22 mc=cm 2 =day under the standard condition. During continuous irradiation, we periodically measured the performance of the PMTs for single photons generated by a light-pulser (PLP), a Hamamatsu picosec light-pulser (PLP-2-SLDH-4), from once a day to once a week. The PLP yielded light of l ¼ 48 nm with a duration of 5 ps and a jitter of ps. The integrated number of irradiated photons per cm 2 over a certain time period was calculated to be N g ¼ X ðn i g Zi Þf i Dt i () i where n i g is the number of irradiated photons on a PMT per pulse and per cm 2 at the ith measurement, Z i is the calibration factor for the LED intensity, f i is the LED lightning frequency, and Dt i is the time duration of the ith irradiation. Correspondingly, the integrated amounts of Table Characteristics of MCP PMTs HPK a BINP a -w -wo -w -2w -3w -4wo -5wo Window Fused quartz Borosilicate glass diameter (mm) of effective area 8 QE ¼ 4 nm D b ðmmþ MCP aperture (%) L b (mm).24.3 Bias-angle (deg.) 3 5 Gap (mm)/voltage (V) between Photo-cathode and first MCP-layer 2./97.7/3.2/34 First and second MCP-layers.5/94.7/245 Second MCP-layer and anode./48.3/3.2/ HV supplied Gain ð 6 Þ s TTS (ps) Dark counts (khz) CE (%) All PMTs have a multi-alkali photo-cathode and two MCP-layers. a PMTs with and without an ion-feedback prevention layer are labeled by w and wo, respectively, in the individual tube numbers. b D and L are the diameter and thickness of the MCP plate, respectively.

3 26 ARTICLE IN PRESS N. Kishimoto et al. / Nuclear Instruments and Methods in Physics Research A 564 (26) 24 2 Signal output MCP-PMTs ND filter Laser Calibration PMT LED PLP head Pulse Generator Relative Gain Black Box Fig.. Schematic drawing of the setup. Integrated irradiation (x 3 photons/cm 2 ) output charges from the PMTs anode is X ¼ e X ðn i p:e: f i þ n i dark ÞGi Dt i (2) i Q where e is the electron charge, n i p:e: is the detected number of photo-electrons per pulse and per cm 2 at the ith measurement, and G i is the gain. 3. Measurements Fig. 2. Relative gain variations under LED irradiation for all PMTs. The marks indicate HPK-w ðþ, HPK-wo ðþ, BINP-w ð Þ, BINP-2w ðmþ, BINP-3w ð.þ, BINP-4wo ðnþ and BINP-5wo ð&þ. (a) before after (b) (c) after (+2V) 3.. Gain The gain is the multiplicated number of charges, evaluated at the peak channel of ADC distribution under single-photon irradiation. Fig.2 shows the relative variations of G as a function of N g for all PMTs; their initial gains are listed in Table. All PMTs show a similar behavior, especially, at N g 45 2 photons=cm 2. They were quite stable within the observed time period over 4 9 super-b years, and no obvious difference between PMTs with and without the prevention layers could be found. On the other hand, during the first quarter of one super-b year, the gains of both HPKs and BINP-w dropped greatly, while the other BINPs did not. As long as a PMT affords to increase its high voltage (HV), the gain-drop can be recovered, as shown in Fig. 3. The solid histogram is the ADC distribution at the beginning of irradiation, the shaded one is at the end stage, and the dotted one was obtained by increasing HV by 2 V after irradiation, where the 2th channel corresponds to G ¼ 6 with an offset at the th channel. Table 2 lists the relative gains under the above different conditions, as well as N g and P Q over the full tested periods QE at l ¼ 48 nm QE was measured as the ratio of the number of detected signals to the number of irradiated single photons of PLP, whereof the CE was factored out. Fig. 4 shows the relative variations of QE as a function of N g. In order to examine the dependence on the LED frequency, the pulse rate was initially set at khz and changed later to 5 khz. No event (d) (e) (f) ADC (count/.25pc) Fig. 3. ADC distributions for single photons for (a) BINP-5wo, (b) BINP- w, (c) BINP-2w, (d) HPK-w, (e) HPK-wo and (f) BINP-4wo. The solid, shaded and dotted histograms are before and after irradiation, and by supplying a 2 V higher HV at the end. One hundred and twenty ADC channel corresponds to G ¼ 6 with an offset at the th channel. specific different behavior was found between the two cases. Without the prevention layer, QE rapidly dropped to less than 5% within a tenth of a super-b year and reached a few % at one year. Even with the prevention layer equipped, BINP-2w and -3w exhibited a sharp drop, reducing to QEo5% at one half year; BINP-w showed a slightly slow drop, but reached 6% at one year. Only HPK-w exhibited a gentle slope. It became around 9% after 4 super-b years l dependence of QE We prepared a monochrometer system, as illustrated in Fig. 5, to measure the QEðlÞ-spectrum. Using a Si photodiode (PD), HPK S337-BQ, as the standard devise, QE was measured by supplying +2 V on the first MCP-

4 N. Kishimoto et al. / Nuclear Instruments and Methods in Physics Research A 564 (26) Table 2 Relative gains at the end of irradiation and at the time when HV was increased by 2 V Relative gain HPK BINP -w -wo -w -2w -3w -4wo -5wo At the end of measurement By increasing HV by 2 V N g ð 3 photons=cm 2 Þ P Q ðmc=cm2 Þ Relative Q.E..5 λ = 4 nm Q.E. (%) 2 - Q.E. ratio Integrated irradiation (x 3 photons /cm 2 ) Fig. 4. Relative QE variation under LED irradiation. The marks denote HPK-w ðþ, HPK-wo ðþ, BINP-w ð Þ, BINP-2w ðmþ, BINP-3w ð.þ, BINP-4wo ðnþ and BINP-5wo ð&þ. (a) λ (nm) λ (nm) Fig. 6. (a) QEðlÞ after full irradiation, and (b) QEðlÞ ratio between after and before irradiation. The solid line in (a) is the initial QEðlÞ of HPK-w, and the dotted line is QEðlÞ of an another BINP-w PMT, not irradiated. The marks denote HPK-w ðþ, HPK-wo ðþ, BINP-w ð Þ, BINP-2w ðmþ, BINP-3w ð.þ, BINP-4wo ðnþ and BINP-5wo ð&þ. (b) pico-ampere meter A MCP-PMT or Si Photodiode Aperture φ5mm λ-cut filter cm Black Box Halogen lamp Monochrometer Fig. 5. Schematic drawing of a monochrometer system for the QEðlÞ measurement. The Halogen lamp was a Shimazu Co., AT-G; the monochrometer was Shimazu, SPG-2S ranging over l ¼ 229 nm with a l resolution of 7 nm at a mm-wide slit; the standard SiPD was an HPK S337-BG with an effective area of mm 2 and a sensitive l-range of 9-, nm; the pico-ampere meter was a Keithley 6487, with a current resolution of fa. The filter, Y52, cut down light at lo52 nm. layer surface, while keeping the photo-cathode at V, with the following formula: QEðlÞ ¼ I PMT QE I PD ðlþ (3) PD where I PMT and I PD were the photo-currents of the PMT and the photo-diode (PD), respectively, measured by a pico-ampere meter. QE PD ðlþ was the QE known at certain l for the standard SiPD. With this system, l was varied from 25 to 85 nm. Fig. 6(a) shows the measured QEðlÞ spectra after irradiation, where the QEðlÞs of HPK-w that was specified by HPK at its delivery, and of an another BINP-w that was not irradiated are indicated by the solid and dotted curves, respectively, as the references of the initial QEðlÞ for HPK and BINP. Because of the different window materials, QE at a short l region exhibited a different behavior between HPK and BINP. The l cutoff was 6 nm for fused quartz of HPK and 3 nm for borosilicate of BINP. The QEðlÞ ratio between after and before the irradiation is plotted in Fig. 6(b). It is common over all PMTs that the degradation of the QE ratio is much larger at longer l than at shorter l.hpkw, which exhibits the smallest degradation among the seven PMTs, reduces the QE ratio to 3% at l7 nm, and 95% at 3 nm. It yields a 3-times difference in the QE ratio between the two l regions. For other PMTs showing very sharp drops, times differences in the QE ratio appear between 3 and 7 nm. The larger are the QE ratio drops, the stronger is the degradation that appears at longer l.

5 28 ARTICLE IN PRESS N. Kishimoto et al. / Nuclear Instruments and Methods in Physics Research A 564 (26) TTS TTS was measured, as shown in Fig. 7, and is listed in Table 3: In general, it did not deteriorate with N g, but remained stable keeping s TTS ¼ 3245 ps. This was because regardless of the gain-drop within the observed time period, G ¼ Oð 6 Þ was maintained so as to hold s TTS at its saturated value [6] Dark counts 8 We measured the dark counts, rather than the dark current, whose pulse-heights were higher than the threshold of the discriminator. It was set at 2 mv, which corresponded to =ð22þ of a single photo-electron s pulse-height. Fig. 8(a) shows the resulting behavior on n dark, where the vertical axis is on the logarithmic scale. BINP-5wo and HPK-wo showed a steep drop of 2 orders of magnitude at a quarter of the super-b year, while BINP- w, -2w and -3w dropped 2 orders of magnitude over 7 9 years. On the other hand, HPK-w did not exhibit an apparent decrease. The origin of dark counts could be electrons, thermally emitted from the photo-cathode and multiplicated through the same process as the ordinary signals, so that the n dark behavior would resemble the QE variation. Fig. 8(b) shows n dark vs. QEðlÞ at l ¼ 48 nm; n dark decreased along with a QE reduction. n dark also decreased even with QE unchanged at the beginning of the measurement; it could be an aging appearance N g vs. P Q N g is a quantity independent from the deterioration of the PMT performance, while P Q is a quantity that depends strongly on the performance, such as G and QE. Fig. 9 indicates the relation between N g and P Q for individual PMTs. When no deterioration of both G and QE occurred, their relation had a linear shape, such as that of HPK-w. On the other hand, when the PMT performances degraded, their relation would curve to show shallower inclinations. For instance, BINP-w had a high performance on a product of G and QE for the first 2 super-b years, but it gradually deteriorated during the next year, which resulted in a 25-times smaller G QE during the following consecutive years. The reason why BINP-w had Dark count (Kcps) (a) Integrated irradiation (x 3 photons /cm 2 ) TTS (ps) Integrated irradiation (x 3 photons /cm 2 ) Dark count (Kcps) (b) -2 Q.E. (%) at 4 nm Fig. 7. TTS variation under LED irradiation. The marks denote HPK-w ðþ, HPK-wo ðþ, BINP-w ð Þ, BINP-2w ðmþ, BINP-3w ð.þ, BINP-4wo ðnþ and BINP-5wo ð&þ. Fig. 8. (a) Dark count variation under LED irradiation, and (b) n dark vs. QEðlÞ at l ¼ 48 nm. The marks denote HPK-w ðþ, HPK-wo ðþ, BINP- w ð Þ, BINP-2w ðmþ, BINP-3w ð.þ, BINP-4wo ðnþ and BINP-5wo ð&þ. Table 3 TTS before and after irradiation HPK BINP -w -wo -w -2w -4wo -5wo s TTS s TTS

6 N. Kishimoto et al. / Nuclear Instruments and Methods in Physics Research A 564 (26) Integrated Charge (mc/cm 2 ) a higher G QE performance than that of HPK-w in the beginning is related to their higher CE and G, as can be seen in Table. Some performances are suited to be plotted as a function of P Q rather than N g, as discussed in the next section. 4. Discussions and summary 4.. Gain-drop and its recovery The gain decrease could be attributed to a drop in the secondary electron s emission rate, d, of the hydrogenated MCP surface due to a large bombardment of a multiplicated number of secondary electrons. The gain is generally expressed as G ¼ d n (4) where n is the number of collisions for the secondary emission. d increases with the bombardment energy, E, under relevant operational condition: it is roughly proportional to the high voltage between two MCP-layers. To give actual values, d 2; 2:5 and3ate 6; 75 and 25 ev, respectively. It deteriorates by Dd ¼ ð522þ%, depending on E, after an irradiation of 9 electrons=cm 2 6 mc=cm 2 [5]. On the other hand, the fact that an increase of HV MCP by 2 V recovers the gain reduction indicates Dd6% and % at E 6 and 3 ev, respectively. Although we do not know exactly what value E or n takes in our PMTs, the above evaluation is fairly consistent with data in Ref. [5] QE and P Q QE deterioration would be caused by positive ionfeedback, predominantly produced in the last stage of secondary electron multiplication at the anode side of the second MCP-layer. Thus, the deterioration should be examined in terms of P Q, rather than N g, as plotted in Fig. : It is newly found that QE drops in proportion to P Q for BINP-w s. Integrated irradiation (x 3 photons/cm 2 ) Fig. 9. N g vs. P Q. The marks denote HPK-w ðþ, HPK-wo ðþ, BINP-w ð Þ, BINP-2w ðmþ, BINP-3w ð.þ, BINP-4wo ðnþ and BINP-5wo ð&þ. Relative Q.E After-pulse and ion-feedback Integrated Charge (mc/cm 2 ) λ = 4 nm Fig.. Relative QE vs. P Q. The marks denote HPK-w ðþ, HPK-wo ðþ, BINP-w ð Þ, BINP-2w ðmþ, BINP-3w ð.þ, BINP-4wo ðnþ and BINP- 5wo ð&þ. Positive ions were accelerated by HV to feedback and collide with the photo-cathode; as a result they ejected secondary electrons out of the photo-cathode. Such electrons passed the ordinary multiplication process and could be detected as after-pulses, as can be seen in Fig.. We observed the pulses by 6 ns after the signal for HPKwo under irradiation with 6 photons=pulse. Time duration, T after, between the signal and the afterpulse is the sum of the transit time of the signals and the drift time, t d, of the positive ions over the distance,, between the photo-cathode and the anode side of the second MCP-layer. A simple estimate provides T after ¼ t d þ (5) rffiffiffiffiffiffi 2m t d ¼ (6) ev where m is the positive ion mass and V is the HV applied over. Here, t d comprises two parts: one is between the two MCP-layers, and the other is between the first MCP-layer and the photo-cathode. With V 3 kv, T after ¼ 5:4 and 7.2 ns are calculated for H þ and H þ 2 or Heþ, respectively: accordingly, the ions are likely to be H þ. The output pulse of a single photo-electron was 2 mv, so that the after-pulses in Fig. comprised 3 secondary electrons. Taking into account of CE6%, it indicates that 22 secondary electrons were produced at the photo-cathode. The after-pulse appeared at a somewhat similar frequency with the signal rate. A single ion among the feedbacked ones might reach to the photocathode with some probability with a sufficient energy to yield an after-pulse. In this case, the positive ion was accelerated to 3 kev and produced 22 secondary electrons: 7 secondary electrons/kev. Fig. 2 is for HPK-w: after-pulses can be seen at ns after the signals, and the former pulse-height is 5-times smaller than that of the signals. Positive ions are assumed in this case to be stopped at the prevention layer, and then accelerated again to the photo-cathode. Eqs. (5) and (6) provide T after ¼ :7 and 5 ns with V ¼ kv between the

7 2 ARTICLE IN PRESS N. Kishimoto et al. / Nuclear Instruments and Methods in Physics Research A 564 (26) 24 2 the bonding of the molecules or lead to their attachment on the surface, and result in a larger work-function and QE spectrum suppressed at longer l. It can be seen that after irradiation the l-cutoff shifted to 8 and 7 nm for HPK and BINP, respectively, corresponding to workfunctions of.6 and.8 ev BINP vs. HPK Fig.. After-pulses in HPK-wo, irradiating with 6 photons=pulse. HPK-w can be expected to function without any serious deterioration of its performance for more than super-b years under the supposed conditions, while BINP-w exhibited, even a prevention layer equipped, a much shorter lifetime. This may be explained by two reasons. Firstly, the fact that CE for BINP-w, 4 6%, is much larger than that of HPK-w, 37%, indicates that the higher transmission for the photo-electrons would also hold for positive ions. That is, the prevention layer might not effectively function for the ions at BINP-w, compared to HPK-w. The other concerns the bias-angle: the smaller angle of 5 at BINP-w would not be sufficient to suppress the feedback, compared to 3 at HPK-w PMT vacuum Fig. 2. After-pulses in HPK-w, detecting with 6 photons=pulse. first MCP-layer and the photo-cathode for H þ and H þ 2 or He þ, respectively. H þ is again likely to be the positive ions. With CE ¼ 37%, 7 secondary electrons are inferred to have been ejected by a kev ion: it agrees well with the evaluation made for HPK-wo, mentioned above. Comparison between HPK-wo and -w indicates that the prevention layer reduced the after-pulse appearance rate from 55% to 3% of the signals under 6 photons=pulse irradiation. These numbers of the after-pulse rates became % and.5%, respectively, in the case of single-photon irradiation while keeping their pulse-heights unchanged. It also decreased the number of ejected secondary electrons from 22 to 7. Resultantly, it can be stated that the positive ion-feedback effect could be weakened by a factor of 5 6 by equipping the prevention layer QEðlÞ degradation Multi-alkali photo-cathode (NaKSbCs) generally has a work-function of.4 ev [6], which corresponds to l 9 nm. Accordingly, both HPK and BINP exhibit their maximum l-cutoff at around 9 nm at the beginning of irradiation. Bombardment by positive ions would cut The vacuum pressure should be one of the important items for suppressing positive ion production. Unfortunately, the achieved pressures have not been announced for all PMTs. We believe that an improvement of their vacuum could result in longer lifetimes for both PMTs of HPK and BINP. In summary, we measured the MCP PMTs performance changes by irradiating photons of at maximum 2:8 4 =cm 2, equivalent to 4 super-b years. A gain-drop happened, but was not essential, since the TTS resolutions were unaffected by it, and whereupon the gain could be recovered by increasing HV. The most essential effect appeared as QE degradation, which could not be recovered. The prevention layer worked effectively for HPK-w to protect the photo-cathode from positive ion-feedback. Some other elements to prolong the lifetime are discussed in the text. An HPK-w demonstrated clearly superior performance. A larger set of HPK-w s would be tested to statistically confirm its behavior. Also, the testing would be carried out over a longer time period. For HPK-w, the detection efficiency over l ¼ 328 nm photons became 87% after the 4 super-b years irradiation, but it was recovered to 95% by increasing HV by 2 V. Among the basic performances, QE is the most essential to control the lifetime of the MCP PMT usage: we set our criterion for QE not to degrade to less than 6% of the initial value, at which K=p separation ability of the TOP counter would deteriorate to 75% of the original one. So that, HPK-w exhibits acceptable performance within 4 super-b years; it could be expected to extend to years of super-b experiment (see, Fig. 4).

8 N. Kishimoto et al. / Nuclear Instruments and Methods in Physics Research A 564 (26) Acknowledgments We would like to thank Professors Y. Tihkonov, A. Bondar, G. Fedotovich, B. Shwartz, A.Onuchin, E. Kravechenko for their many valuable discussions and granting us the rights to own BINP PMTs. This work is supported by a Grant-in-Aid for Science Research on Priority Area, Mass Origin and Supersymmetry Physics, from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [] K. Inami, et al., Nucl. Instr. and Meth. A 56 (26) 33. [2] M. Akatsu, et al., Nucl. Instr. and Meth. A 44 (2) 24; T. Ohshima, Nucl. Instr. and Meth. A 453 (2) 33; T. Ohshima, ICFA Instr. Bull. 2 (2)2 (5pp.); M. Hirose, et al., Nucl. Instr. and Meth. A 46 (2) 326; S. Matsui, et al., Nucl. Instr. and Meth. A 463 (2) 22; Y. Enari, et al., Nucl. Instr. and Meth. A 494 (22) 43; T. Hokuue, et al., Nucl. Instr. and Meth. A 494 (22) 436; Y. Enari, et al., Nucl. Instr. and Meth. A 547 (25) 49. [3] M. Akatsu, et al., Nucl. Instr. and Meth. A 528 (24) 763. [4] S. Hashimoto (Ed.), et al., Letter of Intent for KEK Super B Factory, KEK-REPORT-24-4, June 24. [5] A. Authinarayanan, R.W. Dudding, Adv. Electron. Electron Phys. A 4 (976) 67. [6] C. Ghosh, Phys. Rev. B 22 (98) 972; B.K. Singh, et al., Nucl. Instr. and Meth. A 454 (2) 364.