- Email: [email protected]
Development of CVD diamond detector for time-of-flight measurements
Nuclear Instruments and Methods in Physics Research B 317 (2013) 710–713
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Development of CVD diamond detector for time-of-flight measurements S. Michimasa a,⇑, M. Takaki a, M. Dozono b, S. Go a, H. Baba b, E. Ideguchi c, K. Kisamori a, H. Matsubara b, H. Miya a, S. Ota a, H. Sakai b, S. Shimoura a, A. Stolz d, T.L. Tang a, H. Tokieda a, T. Uesaka b, R.G.T. Zegers d a
Center for Nuclear Study, University of Tokyo, RIKEN Campus, Wako, Saitama 351-0198, Japan RIKEN Nishina Center, Wako, Saitama 351-0198, Japan c RCNP, Osaka University, Ibaraki, Osaka 567-0047, Japan d National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824, USA b
a r t i c l e
i n f o
Article history: Received 20 March 2013 Received in revised form 9 August 2013 Accepted 14 August 2013 Available online 25 October 2013 Keywords: Diamond detector Time resolution Time-of-flight measurement
a b s t r a c t The paper describes recent developments of diamond detector at CNS and discusses the timing signal transfer system suitable for optimizing its performance. The diamond detector is well known to have good properties as a radiation detector such as fast response and good radiation hardness. Consequently, it is an excellent candidate for serving as a high-resolution thin timing detector. CNS and NSCL/MSU started a collaboration to develop diamond detectors, and manufactured a detector with a size of 28 28 mm2 and thickness of 200 lm. An irradiation experiment of the detector was performed by using a 32-MeV a beam to check its basic performance. The CNS detector had almost 100% efficiency and a timing resolution of 27 ps (r). Jitter in long optical-fiber signal-transfer lines was examined and found to be 10.7 ps (r). Diamond detectors and signal transfer system using a optical fiber enable us to provide timeof-flight measurements with extremely high resolution. Ó 2013 Elsevier B.V. All rights reserved.
1. Motivation A detector with a fast response to radiation is important for nuclear physics experiments, and studies on materials for timing detectors are actively conducted. In particular, for high-resolution spectroscopy experiments with the SHARAQ spectrometer [1,2], a timing detector with very good resolution is critical. Such a timing detector is important time-of-flight (TOF) mass measurements, (p; n) experiments in inverse kinematics and for general-purpose beam monitoring. As a solution of a timing detector satisfying the requirements for those experiments, a CVD diamond detector is under development. Diamond has attractive properties for a radiation detector, and already has a long history in radiation applications. However, the use of diamond detectors was limited due to constraints on the price and size of the natural materials. The chemical vapor deposition (CVD) technique [3–5] has made it possible to create highquality diamond material with significant size, and has provided opportunities of practical use of diamond radiation detectors [6]. The properties of diamond are summarized in Table 1, together with those of silicon for comparison. The large displacement energy of diamond indicates its inherent radiation hardness. Its small ⇑ Corresponding author at: Center for Nuclear Study, University of Tokyo, RIKEN Campus, Wako, Saitama 351-0198, Japan. Tel.: +81 484644225. E-mail address: [email protected] (S. Michimasa). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.08.055
dielectric constant and large band gap result in low detector capacitance and low leakage current, and thus a low noise level is expected. Its large mobility and saturation velocity of charge carriers promise very good timing resolution of signal responses, combined with a large breakdown field. Based on the outstanding properties of diamond, we aim at developing a CVD diamond detector as a thin and large-area counter with extremely good timing resolution. The detector development has been performed by the collaboration of CNS and NSCL/ MSU. In the paper, we describe recent developments of CVD diamond detectors at CNS. 2. CNS CVD diamond detector A diamond detector with cathode strips was manufactured in 2011. Its photograph and cathode design are illustrated in Fig. 1. The diamond material is polycrystalline CVD (pCVD) crystal which was commercially available from Diamond Detectors Ltd. Its size and thickness are 30 30 mm2 and 0.2 mm, respectively. The cathode is 28 28 mm2. The detector has one pad on one side (Side A), and 4 strips the other side (Side B). On Side A, 4 readout wires are bonded to the corners. On Side B, 2 readout wires are bonded to both edges of each strip. The four strips are 9 mm, 5 mm, 5 mm, 9 mm in width, respectively, since the center strips receive higher beam rate. In total, the detector has 12 readouts in order to deduce hit position and timing. The cathode has multi-layer structure of
S. Michimasa et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 710–713
711
Table 1 Properties of diamond and silicon. Physics properties at 300 K
Diamond
Silicon
Band gap (eV) Breakdown field (V/m)
5:5
1:12
Resistivity (X cm) Electron mobility (cm2/V/s) Hole mobility (cm2/V/s) Saturation velocity (km/s) Dielectric constant Displacement Energy (eV/atom) Energy to create an e-h pair (eV) Thermal conductivity (W/cm/K) Lattice constant (Å)
107
3 105 11
> 10 1800 1200 220 5:7 43 13 20 3:57
2:3 105 1500 600 82 11:9 13–20 3:6 1:27 5:43
9 mm
5 mm 5 mm
9 mm
30 mm
Side A
28 mm
Side B
Fig. 1. Photographs and cathode pattern of a manufactured diamond detector.
Fig. 3. Diagram of signal processing circuits for the irradiation experiment.
of 32-MeV a particles in the diamond detector corresponds to that of 320-AMeV 12N isotopes. Therefore, this experiment was aimed in particular to examine a capability of the diamond detectors for intermediate-energy RI beams. The beam intensity was limited to be approximately 1000 counts/s and the beam spot size was typically 5 mm in diameter at the diamond detector. To optimize for fast signals from diamond detectors, low-noise RF amplifiers (CIVIDEC Broadband Diamond Amplifier) were used as preamplifiers. Fig. 4 is a snapshot showing the outputs: Yellow line shows the signal from Side A; cyan and magenta lines show signals from the edges of the first strip on Side B, respectively. The scale of the horizontal axis is 20 ns/div for all signals, and the vertical scales are 100 mV/div (yellow) and 50 mV/div (cyan and magenta), respectively. The signals from Side A are slower than those from Side B due to the larger capacitance of Side A. A typical signal from Side B has rise time of 700 ps and decay time of 10 ns. Therefore, the striped design is effective not only for a stability in high counting rate but also for an improvement of the time response. In the experiment, we also obtained charge information and measured the TOF between a diamond detector and plastic scintillator. The TOF was measured by using a Time-to-Amplitude Converter (TAC) and a 13-bit Analog-to-Digital Converter (ADC). The timing resolution of this system was estimated to be 6.5 ps (r) by measuring a constant time difference created by a pulser signal. To estimate the detection efficiency of the diamond detector, we checked the coincidence ratio of signals from the diamond detector and from the plastic scintillator. We deduced the
10-nm titanium and 100-nm gold. The titanium layer is important for ohmic contact between a diamond and a gold layer. SMA connectors are used to reduce signal strength loss in a radio frequency region ( 10 GHz).
3. Irradiation experiments of CNS diamond detector To examine the basic performance of the detector, we have performed an irradiation experiment. The experimental setup and diagram of signal processing circuits are shown in Figs. 2 and 3, respectively. In operation, 400 V was applied to the pad (Side A). The setup was placed in the E7b course in RIKEN accelerator facility, and an a beam of 32 MeV was used. The energy deposit
α
Fig. 2. Detector setup of irradiation experiment by a beam.
Fig. 4. Snapshot of oscilloscope showing the outputs: Yellow line shows a signal from Side A (100 mV/div); cyan and magenta lines show from the edges of the first strip on Side B (50 mV/div), respectively. The scale of the horizontal axis is 20 ns/ div. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
S. Michimasa et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 710–713
Detection efficiency (%)
Detection efficiency
100
Charge collection depth
50
0
0
200
100
100 200 300 400 500
Charge collection depth (μm)
(a)
Bias voltage (V)
(b)
6000 Time resolution
Yield (counts/ch)
712
3000
0 0 100 200 -200 -100 Time difference TL - TR(ps)
Fig. 5. (a) Detection efficiency for 32-MeV a beam as a function of bias voltage. The collection depth estimated from charge of a pulse signal is shown in the same graph. (b) Time difference spectrum between both edges of a strip of the diamond detector. We estimate to be the time resolution to be 27 ps from the spectrum.
detection efficiency of the diamond detector as a number ratio of coincident signals to the plastic scintillator installed downstream. Fig. 5(a) shows measured detection efficiency as a function of bias voltage. The detector efficiency became almost 100% at 200 V. We analyzed the charge distributions of the pulse signal from the pad (Side A) to estimate the collection depth of a pCVD crystal. The collection depth is an indication of how many charge carriers are lost due to nonconformity of the diamond crystal. The collection depth is calculated from the integrated charges of pulse signals obtained by the QDCs, where the amplification factor of preamplifier was assumed to be the catalog value of 40 dB. The collection depth has a positive correlation with the detector efficiency, as shown in the same figure. At 200 V, the collected charge was estimated to be 70% of the total of electron–ion pairs generated by the beam. The deduced collection depth is 190 lm at 400 V and is comparable to the physical thickness of 200 lm. Thus the carrier loss in this diamond detector is considered to be very small, and this detector is considered to suitable for use as a radiation detector. For full collection of the charge, bias voltage of more than 400 V is necessary, which corresponds to 1/ 5 of the breakdown field. The long-term stability of the detector under strong electric field is still an open question. To deduce the timing resolution of diamond detectors, the time difference between signal pulses from both edges of a strip of the diamond detector is shown in Fig. 5 (b). The width of the spectrum pffiffiffi indicates 2 times of the timing resolution under a condition of the small beam spot size. The actual beam spot size on diamond detector is estimated to be 5 mm. The time resolution of this diamond detector is evaluated to be 27 ps (r). To investigate the position dependence of the timing, we installed the diamond detector in the final focal plane of the SHARAQ 0.5 (a) Setup
CRDC 450
Diamond 300
Time difference TL- TR (ns)
(b) Position dependence 0.25
0
-0.25
spectrometer as shown in Fig. 6(a). The beam impinged on the detectors was 14N at 200 A MeV. The hit position could be obtained by the CRDC tracking detectors [7,8] with 500 lm resolution. Fig. 6(b) shows the correlation plot of the hit position and the time difference in both edges of a strip of the diamond detector. The gradient of the correlation corresponds to the transfer velocity on the cathode of the diamond detector, and was estimated to be 9.5 ps/ mm. The velocity was similar to an estimated value of 6.3 ps/mm by considering its effective dielectric constant. Therefore, the timing resolution of the diamond detector can be improved by using information on the hit position. In the figure, the time resolution was estimated to be 30 ps from the vertical width of the correlation locus. The resolution is consistent with the result in Fig. 5(b).
4. Signal transfer system for long TOF measurement By using two diamond detectors, we will be able to measure TOF with extremely good resolution. By combining the high-resolution TOF measurement with the high-resolution momentum measurement of the SHARAQ, precise mass measurements of radioactive nuclei far from stability are possible. To realize the experiment in the SHARAQ, we have to measure the time of flight between the BigRIPS F3 focal plane and the final focus S2 of the SHARAQ spectrometer, which are 100 m apart. Prior to this work, the amount of timing jitter in the signal transfer line along the SHARAQ beamline was an open question. We measured the jitter in several transfer cables. Fig. 7 shows a circuit diagram for an optical fiber. Tested cables were embedded in the delay line part in the figure. In the test, we built the circuit with full attentions in time resolution of pulse processing modules. As the Time-to-Digital Converter (TDC), a TC842 module of Agilent Technologies, which has a dynamic range of 20 s and a time resolution of 5 ps, was used. A discriminator was designed and manufactured by Iwatsu to obtain extremely fast response with time resolution of 10 ps. These circuit modules were connected by low-attenuation and high-frequency RF cables. The time spectrum obtained by using the optical system is shown in Fig. 8 (a). where the delay line was an optical fiber of 200 m long. The used optical transfer system consisted of RPN1110 and RPN-1130 units provided by REPIC. The estimated jitter
Transfer velocity 9.5 ps/mm
CRDC
-0.5 -20
-10 0 10 Hit position (mm)
20
Fig. 6. (a) Setup of the diamond detector in the final focal plane of the SHARAQ spectrometer. (b) Correlation plot of hit position and time difference T L T R . The transfer velocity in the diamond detector was estimated to be 9.5 ps/mm from gradient of the correlation.
Fig. 7. Circuit diagram for jitter test of transfer cables in a case of a optical fiber. Tested cables are embedded in the delay line part in the figure.
S. Michimasa et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 710–713
713
Fig. 8. Jitters measured by using (a) a 200-m optical fiber delay and (b) a 16-m metal coaxial cable delay.
effect in the transfer system was 11.7 ps (r). It is noted that the value included the time resolution of the TDC. As a baseline for comparison, Fig. 8(b) shows a jitter effect of a metal coaxial cable (RG58A/U, 16 m). The jitter effect in the metal cable was estimated to be 250 ps (r), and was 10 times worse than that of the optical fiber. Furthermore, the time spectrum of the metal cable was disturbed from a Gaussian shape, while the one with an optical fiber kept a Gaussian shape. A Gaussian shape in time distribution is important for deducing reliable TOF from the time spectrum. As a conclusion the optical fiber transfer system is suitable to TOF measurement at SHARAQ by using diamond detectors. 5. Summary A diamond detector was developed by CNS aimed at TOF measurements in the SHARAQ beamline. The CNS diamond detector was manufactured from a commercial polycrystalline CVD diamond plate, and was used in irradiation experiments. The experimental tests were performed to study the basic performance of the detector. The CNS detector showed good performance: almost 100% efficiency and a time resolution of 27 ps (r) for light particles. Therefore it is found to be suitable for intermediate-energy light ion beams around Z 7. To estimate the jitter effect in a long signal transfer, we measured the jitters in long delay lines by using a sophisticated timing system. The jitter effect of an optical fiber was
10.7 ps (r), and it was extremely stable in time measurement. These performances of a diamond detector and signal transfer system was sufficient to perform experiments of unstable nuclear masses, with 300-keV mass resolution in the mass range of A 50. Accordingly experiments for pf-shell nuclei will be possible when RIBF delivers proper primary beams. Acknowledgments The authors are grateful for the continuous support from the Center for Nuclear Study, the University of Tokyo and from the RIKEN Nishina Center for Accelerator-Based Science. The present work was partly supported by a Grant-in-Aid for Scientific Research (No. 22740150) by the Ministry of Educations, Culture, Sports, Science and Technology. References [1] [2] [3] [4] [5] [6] [7] [8]
T. Uesaka et al., Prog. Theor. Exp. Phys. 2012 (2012) 03C007. S. Michimasa, et al., in: Proceedings of this Conference, 2013. W.G. Eversole, U.S. patent Nos. 3030187 and 3030188 (1958). J.C. Angus, H.A. Will, W.S. Stanko, J. Appl. Phys. 39 (1968) 2915. B.V. Deryaguin et al., J. Cryst. Growth 2 (1968) 380. The RD42 Collabration, Nucl. Instr. Meth. A 435 (1999) 194. H. Tokieda, Mater Thesis, the University of Tokyo, 2010. S. Michimasa, et al., CNS annual report 2007, CNS-REP- 80 (2009) 58.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Development of CVD diamond detector for time-of-flight measurements S. Michimasa a,⇑, M. Takaki a, M. Dozono b, S. Go a, H. Baba b, E. Ideguchi c, K. Kisamori a, H. Matsubara b, H. Miya a, S. Ota a, H. Sakai b, S. Shimoura a, A. Stolz d, T.L. Tang a, H. Tokieda a, T. Uesaka b, R.G.T. Zegers d a
Center for Nuclear Study, University of Tokyo, RIKEN Campus, Wako, Saitama 351-0198, Japan RIKEN Nishina Center, Wako, Saitama 351-0198, Japan c RCNP, Osaka University, Ibaraki, Osaka 567-0047, Japan d National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824, USA b
a r t i c l e
i n f o
Article history: Received 20 March 2013 Received in revised form 9 August 2013 Accepted 14 August 2013 Available online 25 October 2013 Keywords: Diamond detector Time resolution Time-of-flight measurement
a b s t r a c t The paper describes recent developments of diamond detector at CNS and discusses the timing signal transfer system suitable for optimizing its performance. The diamond detector is well known to have good properties as a radiation detector such as fast response and good radiation hardness. Consequently, it is an excellent candidate for serving as a high-resolution thin timing detector. CNS and NSCL/MSU started a collaboration to develop diamond detectors, and manufactured a detector with a size of 28 28 mm2 and thickness of 200 lm. An irradiation experiment of the detector was performed by using a 32-MeV a beam to check its basic performance. The CNS detector had almost 100% efficiency and a timing resolution of 27 ps (r). Jitter in long optical-fiber signal-transfer lines was examined and found to be 10.7 ps (r). Diamond detectors and signal transfer system using a optical fiber enable us to provide timeof-flight measurements with extremely high resolution. Ó 2013 Elsevier B.V. All rights reserved.
1. Motivation A detector with a fast response to radiation is important for nuclear physics experiments, and studies on materials for timing detectors are actively conducted. In particular, for high-resolution spectroscopy experiments with the SHARAQ spectrometer [1,2], a timing detector with very good resolution is critical. Such a timing detector is important time-of-flight (TOF) mass measurements, (p; n) experiments in inverse kinematics and for general-purpose beam monitoring. As a solution of a timing detector satisfying the requirements for those experiments, a CVD diamond detector is under development. Diamond has attractive properties for a radiation detector, and already has a long history in radiation applications. However, the use of diamond detectors was limited due to constraints on the price and size of the natural materials. The chemical vapor deposition (CVD) technique [3–5] has made it possible to create highquality diamond material with significant size, and has provided opportunities of practical use of diamond radiation detectors [6]. The properties of diamond are summarized in Table 1, together with those of silicon for comparison. The large displacement energy of diamond indicates its inherent radiation hardness. Its small ⇑ Corresponding author at: Center for Nuclear Study, University of Tokyo, RIKEN Campus, Wako, Saitama 351-0198, Japan. Tel.: +81 484644225. E-mail address: [email protected] (S. Michimasa). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.08.055
dielectric constant and large band gap result in low detector capacitance and low leakage current, and thus a low noise level is expected. Its large mobility and saturation velocity of charge carriers promise very good timing resolution of signal responses, combined with a large breakdown field. Based on the outstanding properties of diamond, we aim at developing a CVD diamond detector as a thin and large-area counter with extremely good timing resolution. The detector development has been performed by the collaboration of CNS and NSCL/ MSU. In the paper, we describe recent developments of CVD diamond detectors at CNS. 2. CNS CVD diamond detector A diamond detector with cathode strips was manufactured in 2011. Its photograph and cathode design are illustrated in Fig. 1. The diamond material is polycrystalline CVD (pCVD) crystal which was commercially available from Diamond Detectors Ltd. Its size and thickness are 30 30 mm2 and 0.2 mm, respectively. The cathode is 28 28 mm2. The detector has one pad on one side (Side A), and 4 strips the other side (Side B). On Side A, 4 readout wires are bonded to the corners. On Side B, 2 readout wires are bonded to both edges of each strip. The four strips are 9 mm, 5 mm, 5 mm, 9 mm in width, respectively, since the center strips receive higher beam rate. In total, the detector has 12 readouts in order to deduce hit position and timing. The cathode has multi-layer structure of
S. Michimasa et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 710–713
711
Table 1 Properties of diamond and silicon. Physics properties at 300 K
Diamond
Silicon
Band gap (eV) Breakdown field (V/m)
5:5
1:12
Resistivity (X cm) Electron mobility (cm2/V/s) Hole mobility (cm2/V/s) Saturation velocity (km/s) Dielectric constant Displacement Energy (eV/atom) Energy to create an e-h pair (eV) Thermal conductivity (W/cm/K) Lattice constant (Å)
107
3 105 11
> 10 1800 1200 220 5:7 43 13 20 3:57
2:3 105 1500 600 82 11:9 13–20 3:6 1:27 5:43
9 mm
5 mm 5 mm
9 mm
30 mm
Side A
28 mm
Side B
Fig. 1. Photographs and cathode pattern of a manufactured diamond detector.
Fig. 3. Diagram of signal processing circuits for the irradiation experiment.
of 32-MeV a particles in the diamond detector corresponds to that of 320-AMeV 12N isotopes. Therefore, this experiment was aimed in particular to examine a capability of the diamond detectors for intermediate-energy RI beams. The beam intensity was limited to be approximately 1000 counts/s and the beam spot size was typically 5 mm in diameter at the diamond detector. To optimize for fast signals from diamond detectors, low-noise RF amplifiers (CIVIDEC Broadband Diamond Amplifier) were used as preamplifiers. Fig. 4 is a snapshot showing the outputs: Yellow line shows the signal from Side A; cyan and magenta lines show signals from the edges of the first strip on Side B, respectively. The scale of the horizontal axis is 20 ns/div for all signals, and the vertical scales are 100 mV/div (yellow) and 50 mV/div (cyan and magenta), respectively. The signals from Side A are slower than those from Side B due to the larger capacitance of Side A. A typical signal from Side B has rise time of 700 ps and decay time of 10 ns. Therefore, the striped design is effective not only for a stability in high counting rate but also for an improvement of the time response. In the experiment, we also obtained charge information and measured the TOF between a diamond detector and plastic scintillator. The TOF was measured by using a Time-to-Amplitude Converter (TAC) and a 13-bit Analog-to-Digital Converter (ADC). The timing resolution of this system was estimated to be 6.5 ps (r) by measuring a constant time difference created by a pulser signal. To estimate the detection efficiency of the diamond detector, we checked the coincidence ratio of signals from the diamond detector and from the plastic scintillator. We deduced the
10-nm titanium and 100-nm gold. The titanium layer is important for ohmic contact between a diamond and a gold layer. SMA connectors are used to reduce signal strength loss in a radio frequency region ( 10 GHz).
3. Irradiation experiments of CNS diamond detector To examine the basic performance of the detector, we have performed an irradiation experiment. The experimental setup and diagram of signal processing circuits are shown in Figs. 2 and 3, respectively. In operation, 400 V was applied to the pad (Side A). The setup was placed in the E7b course in RIKEN accelerator facility, and an a beam of 32 MeV was used. The energy deposit
α
Fig. 2. Detector setup of irradiation experiment by a beam.
Fig. 4. Snapshot of oscilloscope showing the outputs: Yellow line shows a signal from Side A (100 mV/div); cyan and magenta lines show from the edges of the first strip on Side B (50 mV/div), respectively. The scale of the horizontal axis is 20 ns/ div. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
S. Michimasa et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 710–713
Detection efficiency (%)
Detection efficiency
100
Charge collection depth
50
0
0
200
100
100 200 300 400 500
Charge collection depth (μm)
(a)
Bias voltage (V)
(b)
6000 Time resolution
Yield (counts/ch)
712
3000
0 0 100 200 -200 -100 Time difference TL - TR(ps)
Fig. 5. (a) Detection efficiency for 32-MeV a beam as a function of bias voltage. The collection depth estimated from charge of a pulse signal is shown in the same graph. (b) Time difference spectrum between both edges of a strip of the diamond detector. We estimate to be the time resolution to be 27 ps from the spectrum.
detection efficiency of the diamond detector as a number ratio of coincident signals to the plastic scintillator installed downstream. Fig. 5(a) shows measured detection efficiency as a function of bias voltage. The detector efficiency became almost 100% at 200 V. We analyzed the charge distributions of the pulse signal from the pad (Side A) to estimate the collection depth of a pCVD crystal. The collection depth is an indication of how many charge carriers are lost due to nonconformity of the diamond crystal. The collection depth is calculated from the integrated charges of pulse signals obtained by the QDCs, where the amplification factor of preamplifier was assumed to be the catalog value of 40 dB. The collection depth has a positive correlation with the detector efficiency, as shown in the same figure. At 200 V, the collected charge was estimated to be 70% of the total of electron–ion pairs generated by the beam. The deduced collection depth is 190 lm at 400 V and is comparable to the physical thickness of 200 lm. Thus the carrier loss in this diamond detector is considered to be very small, and this detector is considered to suitable for use as a radiation detector. For full collection of the charge, bias voltage of more than 400 V is necessary, which corresponds to 1/ 5 of the breakdown field. The long-term stability of the detector under strong electric field is still an open question. To deduce the timing resolution of diamond detectors, the time difference between signal pulses from both edges of a strip of the diamond detector is shown in Fig. 5 (b). The width of the spectrum pffiffiffi indicates 2 times of the timing resolution under a condition of the small beam spot size. The actual beam spot size on diamond detector is estimated to be 5 mm. The time resolution of this diamond detector is evaluated to be 27 ps (r). To investigate the position dependence of the timing, we installed the diamond detector in the final focal plane of the SHARAQ 0.5 (a) Setup
CRDC 450
Diamond 300
Time difference TL- TR (ns)
(b) Position dependence 0.25
0
-0.25
spectrometer as shown in Fig. 6(a). The beam impinged on the detectors was 14N at 200 A MeV. The hit position could be obtained by the CRDC tracking detectors [7,8] with 500 lm resolution. Fig. 6(b) shows the correlation plot of the hit position and the time difference in both edges of a strip of the diamond detector. The gradient of the correlation corresponds to the transfer velocity on the cathode of the diamond detector, and was estimated to be 9.5 ps/ mm. The velocity was similar to an estimated value of 6.3 ps/mm by considering its effective dielectric constant. Therefore, the timing resolution of the diamond detector can be improved by using information on the hit position. In the figure, the time resolution was estimated to be 30 ps from the vertical width of the correlation locus. The resolution is consistent with the result in Fig. 5(b).
4. Signal transfer system for long TOF measurement By using two diamond detectors, we will be able to measure TOF with extremely good resolution. By combining the high-resolution TOF measurement with the high-resolution momentum measurement of the SHARAQ, precise mass measurements of radioactive nuclei far from stability are possible. To realize the experiment in the SHARAQ, we have to measure the time of flight between the BigRIPS F3 focal plane and the final focus S2 of the SHARAQ spectrometer, which are 100 m apart. Prior to this work, the amount of timing jitter in the signal transfer line along the SHARAQ beamline was an open question. We measured the jitter in several transfer cables. Fig. 7 shows a circuit diagram for an optical fiber. Tested cables were embedded in the delay line part in the figure. In the test, we built the circuit with full attentions in time resolution of pulse processing modules. As the Time-to-Digital Converter (TDC), a TC842 module of Agilent Technologies, which has a dynamic range of 20 s and a time resolution of 5 ps, was used. A discriminator was designed and manufactured by Iwatsu to obtain extremely fast response with time resolution of 10 ps. These circuit modules were connected by low-attenuation and high-frequency RF cables. The time spectrum obtained by using the optical system is shown in Fig. 8 (a). where the delay line was an optical fiber of 200 m long. The used optical transfer system consisted of RPN1110 and RPN-1130 units provided by REPIC. The estimated jitter
Transfer velocity 9.5 ps/mm
CRDC
-0.5 -20
-10 0 10 Hit position (mm)
20
Fig. 6. (a) Setup of the diamond detector in the final focal plane of the SHARAQ spectrometer. (b) Correlation plot of hit position and time difference T L T R . The transfer velocity in the diamond detector was estimated to be 9.5 ps/mm from gradient of the correlation.
Fig. 7. Circuit diagram for jitter test of transfer cables in a case of a optical fiber. Tested cables are embedded in the delay line part in the figure.
S. Michimasa et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 710–713
713
Fig. 8. Jitters measured by using (a) a 200-m optical fiber delay and (b) a 16-m metal coaxial cable delay.
effect in the transfer system was 11.7 ps (r). It is noted that the value included the time resolution of the TDC. As a baseline for comparison, Fig. 8(b) shows a jitter effect of a metal coaxial cable (RG58A/U, 16 m). The jitter effect in the metal cable was estimated to be 250 ps (r), and was 10 times worse than that of the optical fiber. Furthermore, the time spectrum of the metal cable was disturbed from a Gaussian shape, while the one with an optical fiber kept a Gaussian shape. A Gaussian shape in time distribution is important for deducing reliable TOF from the time spectrum. As a conclusion the optical fiber transfer system is suitable to TOF measurement at SHARAQ by using diamond detectors. 5. Summary A diamond detector was developed by CNS aimed at TOF measurements in the SHARAQ beamline. The CNS diamond detector was manufactured from a commercial polycrystalline CVD diamond plate, and was used in irradiation experiments. The experimental tests were performed to study the basic performance of the detector. The CNS detector showed good performance: almost 100% efficiency and a time resolution of 27 ps (r) for light particles. Therefore it is found to be suitable for intermediate-energy light ion beams around Z 7. To estimate the jitter effect in a long signal transfer, we measured the jitters in long delay lines by using a sophisticated timing system. The jitter effect of an optical fiber was
10.7 ps (r), and it was extremely stable in time measurement. These performances of a diamond detector and signal transfer system was sufficient to perform experiments of unstable nuclear masses, with 300-keV mass resolution in the mass range of A 50. Accordingly experiments for pf-shell nuclei will be possible when RIBF delivers proper primary beams. Acknowledgments The authors are grateful for the continuous support from the Center for Nuclear Study, the University of Tokyo and from the RIKEN Nishina Center for Accelerator-Based Science. The present work was partly supported by a Grant-in-Aid for Scientific Research (No. 22740150) by the Ministry of Educations, Culture, Sports, Science and Technology. References [1] [2] [3] [4] [5] [6] [7] [8]
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