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Carbon carbon detection: Diamond detectors and AMS
Nuclear Instruments and Methods in Physics Research B 268 (2010) 851–853
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Carbon carbon detection: Diamond detectors and AMS K.M. Wilcken a,*, S.P.H.T. Freeman a, A. Dougans a, S. Xu a, A. Galbiati b, K. Oliver b a b
Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, East Kilbride G75 0QF, UK Diamond Detectors Ltd., 16 Fleetsbridge Business Centre, Upton Road, Poole BH17 7AH, UK
a r t i c l e
i n f o
Article history: Available online 8 October 2009 Keywords: AMS CVD diamond detector SSAMS 14 C
a b s t r a c t 14
C ions (290 keV) have been detected with chemically vapour deposited diamond. Potential benefits of diamond detectors are radiation hardness, high charge collection and very fast response time/high bandwidth. Ó 2009 Elsevier B.V. All rights reserved.
1. Chemically vapor deposited diamond detectors and AMS
2. Experiment
Diamond has long been recognised as an attractive detector material because of its unique properties, including: radiation hardness; 5.5 eV wide band gap (low leakage current and no pnjunction needed); fast response/high bandwidth (typical pulse width is about 1 ns or below, and count rates up to 1010 ions/s have been reported [1]); high charge collection efficiency. Good reviews of the properties and physics of the diamond detectors are available, see e.g. Refs. [2,3] and references therein. High purity typeII natural diamonds (less than 10 ppm of nitrogen as an impurity) are required for detector applications, but such diamonds are rare and expensive. Recently, however, it has become possible to manufacture high purity (less than 5 ppb of nitrogen and boron) chemically vapor deposited (CVD) diamond in single and poly crystal forms. Both forms are suitable for detector applications: if only timing information is required, poly crystal diamond is appropriate and intrinsic timing resolution of 29 ps has been reported [1]; single crystal diamond is more appropriate for applications where energy information is essential. Typically diamond detectors are used in high-energy physics experiments and are often exposed to a high fluence of energetic particles. AMS, however, involves ion detection at low energy ([ 1 MeV/amu) and to date applications with diamond detectors have been few [4–6]. This earlier work was done with natural diamond detectors, although studies with CVD diamond are underway [7]. In this paper we report the first AMS measurements with a novel single crystal CVD diamond detector of 290 keV 14C ions.
A picture of the single crystal CVD diamond detector prototype is shown in Fig. 1. To avoid ion energy loss in traversing a surface contact electrode the electrodes (Ti/Pt/Au metallisation) are mounted on the sides of the crystal, instead of the front and back. This is important because 290 keV carbon ions are stopped in less than 0.4 lm diamond. For these early experiments a small surface area detector (0.4 4 mm) is used but large area detectors can also be manufactured. To minimize input capacitance, in order to reduce electronic noise, the detector was DC-coupled with Amptek A250CF thermoelectrically cooled preamplifier [9]. This signal was further amplified with Canberra 2015A spectroscopy amplifier and digitised and analysed. No high count rate experiments were performed and standard NIM electronics was sufficient. Detector bias was increased to 120 V (corresponding to mean field of 0.3 V/lm) when no more gain in charge collection was achieved. To test the single crystal CVD diamond detector it was mounted to our single-stage accelerator mass spectrometer (SSAMS) [8], replacing the standard passivated implanted planar silicon (PIPS) detector that is used for routine measurements.
* Corresponding author. Tel.: +44 1355 270177; fax: +44 1355 229898. E-mail address: [email protected] (K.M. Wilcken). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.10.047
3. Results and discussion We re-measured a set of 14C samples with the diamond detector to show that electron hole pairs can be effectively and reliably collected across the crystal with the side electrodes. These samples had been previously measured with the PIPS detector allowing easy comparison of the detectors. The data collected with both detectors are plotted against each other in Fig. 2a and b.
852
K.M. Wilcken et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 851–853
Fig. 1. The first prototype CVD single crystal diamond detector mounted on a 600 CF. The contact electrodes are on the crystal sides to reduce ion energy losses.
There is good agreement with high level samples and blanks. Compared with the large area 150 mm2 PIPS detector the diamond
(a)
130 y = 1.0023x 120
Diamond [ pMC ]
110 100
is small and consequently the measurement statistics are poor. Also, the small area diamond detector can induce scatter to the data following from the sensitivity to beam energy. However, the goodness of agreement suggests good beam energy stability. The samples had been previously much sputtered too. Diamond detector spectra with 290 keV 14C ions from blank and Ox-II samples are presented in Fig. 3. The darkcount rate of the detector is also shown to visualise the contribution of electronic noise to the spectrum; only a small fraction of 14C events are lost in gating to dissociate the low level noise. As shown in Fig. 2b measurements of the organic blank samples were in the range of 0.09– 0.17% of modern carbon corresponding to 51–56 kyrs BP. This is comparable what is achieved using the standard PIPS detector. Average energies required to create a single electron–hole pair in silicon and in diamond, and an electron–ion pair in propane typically used in gas ionization chambers, are presented in Table 1. That it requires more energy to create an electron–hole pair in diamond than in silicon is advantageous for the charge separation due to the lower charge carrier density, assuming a big enough signal is produced. However, when comparing diamond to gas ionization chambers (using propane) only half of the energy is required to create an electron–hole pair. This raises an interesting possibility of a solid state diamond dE/dx detector, which would in principle produce twice the signal and separation with a given energy compared to a gas ionization chamber. In the earlier work Steier et al. [4,6] passed ions through a front contact layer, but observed energy losses even additional to the expected contact layer loss. These losses seem to correlate with the ionization density and are potentially because of low charge carrier collection efficiency at close proximity to the contact electrode. In our case ions enter the diamond crystal directly and therefore the situation is slightly different. However, additional losses compared
90 80
120 OXII Blank darkcount
70 100
50 50
60
70
80
90
100 110 120 130
PIPS [ pMC ]
(b) 0.28
y = 1.0023x
Intensity [ counts ]
60 80 60 40
0.24 20
Diamond [ pMC ]
0.2 0.16
0
50
150
200
250
300
Energy [ channel ]
0.12
Fig. 3. Comparison of darkcount, blank and Ox-II spectra with diamond detector. Low level noise is well separated from 14C events and is gated out in software.
0.08 0.04 0
100
Table 1 Average energies required to create an electron–hole pair in silicon and in diamond, and an electron–ion pair in propane.
0
0.04 0.08 0.12 0.16
0.2
0.24 0.28
PIPS [ pMC ] Fig. 2. Measurements of the same samples using diamond and PIPS detectors for (a) high and (b) low level 14C samples.
Average energy required for an ion–electron pair (eV) Silicon Diamond Propane
3.6 13 26
K.M. Wilcken et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 851–853
to the signal from the PIPS detector are observed and cannot be explained simply by the factor of four lower charge carrier production. 4. Conclusions It has been shown that CVD diamond detector can be used to measure low energy (290 keV) 14C ions. This is non-trivial because the range of these low energy ions is very short. Also, the signal is smaller than from silicon based detectors because more energy is required to create charge carries, and these are collected from a long distance to the side electrodes. The electrode design crucially both minimizes the ion energy losses and exploits the radiation hardness of the diamond. A very thin implanted contact layer would probably cause close to negligible energy losses but would most likely damage quite easily under a high fluence of heavy ions. Prospects for CVD diamond detectors in AMS look good. A possible application might be positive ion radiocarbon AMS, where both energy resolution and high count rate are required from the detector due to the large 14N interference. Alternatively, the side electrode could be segmented making a solid state dE/dx detector,
853
which would obviously do away with entrance window and gas all together. In principle this would allow very simple detector design and could be used, for example, in 36Cl detection. Less energy is required to create an electron–hole pair in diamond than is needed to create one in an ionization chamber using propane, and thus bigger signal and better separation should be achievable with the same given particle energy. References [1] E. Berdermann, K. Blasche, P. Moritz, H. Stelzer, B. Voss, Diam. Relat. Mater. 10 (2001) 1770. [2] R.J. Tapper, Rep. Prog. Phys. 63 (2000) 1273. [3] H. Pernegger, Phys. Stat. Sol. (a) 203 (2006) 3299. [4] P. Steier, R. Golser, W. Kutschera, V. Liechtenstein, A. Priller, C. Vockenhuber, S. Winkler, Nucl. Instrum. Meth. Phys. Res. B 223-224 (2004) 205. [5] V.Kh. Liechtenstein, N.V. Eremin, R. Golser, W. Kutschera, A.A. Paskhalov, A. Priller, P. Steier, C. Vockenhuber, S. Winkler, Nucl. Instrum. Meth. Phys. Res. A 521 (2004) 203. [6] P. Steier, V.Kh. Liechtenstein, D. Djokicˇ, R. Golser, A. Wallner, A.G. Alexeev, V.S. Khrunov, W. Kutschera, Nucl. Instrum. Meth. Phys. Res. A 590 (2008) 221. [7] P. Steier, et al., manuscript in preparation. [8] S.P.H.T. Freeman, A. Dougans, P. Naysmith K.M. Wilcken, S. Xu, Nucl. Instrum. Meth. Phys. Res. B, these proceedings. [9] Amptek Inc.,14 De Angelo Drive, Bedford, MA 01730, USA.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Carbon carbon detection: Diamond detectors and AMS K.M. Wilcken a,*, S.P.H.T. Freeman a, A. Dougans a, S. Xu a, A. Galbiati b, K. Oliver b a b
Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, East Kilbride G75 0QF, UK Diamond Detectors Ltd., 16 Fleetsbridge Business Centre, Upton Road, Poole BH17 7AH, UK
a r t i c l e
i n f o
Article history: Available online 8 October 2009 Keywords: AMS CVD diamond detector SSAMS 14 C
a b s t r a c t 14
C ions (290 keV) have been detected with chemically vapour deposited diamond. Potential benefits of diamond detectors are radiation hardness, high charge collection and very fast response time/high bandwidth. Ó 2009 Elsevier B.V. All rights reserved.
1. Chemically vapor deposited diamond detectors and AMS
2. Experiment
Diamond has long been recognised as an attractive detector material because of its unique properties, including: radiation hardness; 5.5 eV wide band gap (low leakage current and no pnjunction needed); fast response/high bandwidth (typical pulse width is about 1 ns or below, and count rates up to 1010 ions/s have been reported [1]); high charge collection efficiency. Good reviews of the properties and physics of the diamond detectors are available, see e.g. Refs. [2,3] and references therein. High purity typeII natural diamonds (less than 10 ppm of nitrogen as an impurity) are required for detector applications, but such diamonds are rare and expensive. Recently, however, it has become possible to manufacture high purity (less than 5 ppb of nitrogen and boron) chemically vapor deposited (CVD) diamond in single and poly crystal forms. Both forms are suitable for detector applications: if only timing information is required, poly crystal diamond is appropriate and intrinsic timing resolution of 29 ps has been reported [1]; single crystal diamond is more appropriate for applications where energy information is essential. Typically diamond detectors are used in high-energy physics experiments and are often exposed to a high fluence of energetic particles. AMS, however, involves ion detection at low energy ([ 1 MeV/amu) and to date applications with diamond detectors have been few [4–6]. This earlier work was done with natural diamond detectors, although studies with CVD diamond are underway [7]. In this paper we report the first AMS measurements with a novel single crystal CVD diamond detector of 290 keV 14C ions.
A picture of the single crystal CVD diamond detector prototype is shown in Fig. 1. To avoid ion energy loss in traversing a surface contact electrode the electrodes (Ti/Pt/Au metallisation) are mounted on the sides of the crystal, instead of the front and back. This is important because 290 keV carbon ions are stopped in less than 0.4 lm diamond. For these early experiments a small surface area detector (0.4 4 mm) is used but large area detectors can also be manufactured. To minimize input capacitance, in order to reduce electronic noise, the detector was DC-coupled with Amptek A250CF thermoelectrically cooled preamplifier [9]. This signal was further amplified with Canberra 2015A spectroscopy amplifier and digitised and analysed. No high count rate experiments were performed and standard NIM electronics was sufficient. Detector bias was increased to 120 V (corresponding to mean field of 0.3 V/lm) when no more gain in charge collection was achieved. To test the single crystal CVD diamond detector it was mounted to our single-stage accelerator mass spectrometer (SSAMS) [8], replacing the standard passivated implanted planar silicon (PIPS) detector that is used for routine measurements.
* Corresponding author. Tel.: +44 1355 270177; fax: +44 1355 229898. E-mail address: [email protected] (K.M. Wilcken). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.10.047
3. Results and discussion We re-measured a set of 14C samples with the diamond detector to show that electron hole pairs can be effectively and reliably collected across the crystal with the side electrodes. These samples had been previously measured with the PIPS detector allowing easy comparison of the detectors. The data collected with both detectors are plotted against each other in Fig. 2a and b.
852
K.M. Wilcken et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 851–853
Fig. 1. The first prototype CVD single crystal diamond detector mounted on a 600 CF. The contact electrodes are on the crystal sides to reduce ion energy losses.
There is good agreement with high level samples and blanks. Compared with the large area 150 mm2 PIPS detector the diamond
(a)
130 y = 1.0023x 120
Diamond [ pMC ]
110 100
is small and consequently the measurement statistics are poor. Also, the small area diamond detector can induce scatter to the data following from the sensitivity to beam energy. However, the goodness of agreement suggests good beam energy stability. The samples had been previously much sputtered too. Diamond detector spectra with 290 keV 14C ions from blank and Ox-II samples are presented in Fig. 3. The darkcount rate of the detector is also shown to visualise the contribution of electronic noise to the spectrum; only a small fraction of 14C events are lost in gating to dissociate the low level noise. As shown in Fig. 2b measurements of the organic blank samples were in the range of 0.09– 0.17% of modern carbon corresponding to 51–56 kyrs BP. This is comparable what is achieved using the standard PIPS detector. Average energies required to create a single electron–hole pair in silicon and in diamond, and an electron–ion pair in propane typically used in gas ionization chambers, are presented in Table 1. That it requires more energy to create an electron–hole pair in diamond than in silicon is advantageous for the charge separation due to the lower charge carrier density, assuming a big enough signal is produced. However, when comparing diamond to gas ionization chambers (using propane) only half of the energy is required to create an electron–hole pair. This raises an interesting possibility of a solid state diamond dE/dx detector, which would in principle produce twice the signal and separation with a given energy compared to a gas ionization chamber. In the earlier work Steier et al. [4,6] passed ions through a front contact layer, but observed energy losses even additional to the expected contact layer loss. These losses seem to correlate with the ionization density and are potentially because of low charge carrier collection efficiency at close proximity to the contact electrode. In our case ions enter the diamond crystal directly and therefore the situation is slightly different. However, additional losses compared
90 80
120 OXII Blank darkcount
70 100
50 50
60
70
80
90
100 110 120 130
PIPS [ pMC ]
(b) 0.28
y = 1.0023x
Intensity [ counts ]
60 80 60 40
0.24 20
Diamond [ pMC ]
0.2 0.16
0
50
150
200
250
300
Energy [ channel ]
0.12
Fig. 3. Comparison of darkcount, blank and Ox-II spectra with diamond detector. Low level noise is well separated from 14C events and is gated out in software.
0.08 0.04 0
100
Table 1 Average energies required to create an electron–hole pair in silicon and in diamond, and an electron–ion pair in propane.
0
0.04 0.08 0.12 0.16
0.2
0.24 0.28
PIPS [ pMC ] Fig. 2. Measurements of the same samples using diamond and PIPS detectors for (a) high and (b) low level 14C samples.
Average energy required for an ion–electron pair (eV) Silicon Diamond Propane
3.6 13 26
K.M. Wilcken et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 851–853
to the signal from the PIPS detector are observed and cannot be explained simply by the factor of four lower charge carrier production. 4. Conclusions It has been shown that CVD diamond detector can be used to measure low energy (290 keV) 14C ions. This is non-trivial because the range of these low energy ions is very short. Also, the signal is smaller than from silicon based detectors because more energy is required to create charge carries, and these are collected from a long distance to the side electrodes. The electrode design crucially both minimizes the ion energy losses and exploits the radiation hardness of the diamond. A very thin implanted contact layer would probably cause close to negligible energy losses but would most likely damage quite easily under a high fluence of heavy ions. Prospects for CVD diamond detectors in AMS look good. A possible application might be positive ion radiocarbon AMS, where both energy resolution and high count rate are required from the detector due to the large 14N interference. Alternatively, the side electrode could be segmented making a solid state dE/dx detector,
853
which would obviously do away with entrance window and gas all together. In principle this would allow very simple detector design and could be used, for example, in 36Cl detection. Less energy is required to create an electron–hole pair in diamond than is needed to create one in an ionization chamber using propane, and thus bigger signal and better separation should be achievable with the same given particle energy. References [1] E. Berdermann, K. Blasche, P. Moritz, H. Stelzer, B. Voss, Diam. Relat. Mater. 10 (2001) 1770. [2] R.J. Tapper, Rep. Prog. Phys. 63 (2000) 1273. [3] H. Pernegger, Phys. Stat. Sol. (a) 203 (2006) 3299. [4] P. Steier, R. Golser, W. Kutschera, V. Liechtenstein, A. Priller, C. Vockenhuber, S. Winkler, Nucl. Instrum. Meth. Phys. Res. B 223-224 (2004) 205. [5] V.Kh. Liechtenstein, N.V. Eremin, R. Golser, W. Kutschera, A.A. Paskhalov, A. Priller, P. Steier, C. Vockenhuber, S. Winkler, Nucl. Instrum. Meth. Phys. Res. A 521 (2004) 203. [6] P. Steier, V.Kh. Liechtenstein, D. Djokicˇ, R. Golser, A. Wallner, A.G. Alexeev, V.S. Khrunov, W. Kutschera, Nucl. Instrum. Meth. Phys. Res. A 590 (2008) 221. [7] P. Steier, et al., manuscript in preparation. [8] S.P.H.T. Freeman, A. Dougans, P. Naysmith K.M. Wilcken, S. Xu, Nucl. Instrum. Meth. Phys. Res. B, these proceedings. [9] Amptek Inc.,14 De Angelo Drive, Bedford, MA 01730, USA.