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A fast screening technique to evaluate detector response
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 579 (2007) 108–112 www.elsevier.com/locate/nima
A fast screening technique to evaluate detector response Y. Zhang, F. Gao, R. Devanathan, W.J. Weber Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA Available online 5 April 2007
Abstract A fast screening technique to evaluate detector response was demonstrated using a silicon detector. Pulse height was measured for H, He, Be, C, O, Mg, Si, Ni, Zr and Au ions over a wide energy range using a time-of-flight (TOF) telescope. Using a scattering or recoil process, a secondary beam with a continuous energy distribution but low intensity is generated to avoid direct beam exposure of the Si detector. Prior to impinging on the Si detector, the energy of individual ions is determined from the TOF and its tabulated isotopic mass. The pulse height–energy calibration for ions with a given atomic number can be described by a linear relationship with small systematic deviations. For particles that have the same velocity (500 keV/nucleon), a non-linear dependence on efficiency of electron–hole pair collection is observed as a function of electronic stopping power. The detector response is studied using He ions, and the measured energy resolution is given as function of deposition energies over a wide energy range. r 2007 Elsevier B.V. All rights reserved. PACS: 29.40.n; 29.40.Wk; 34.50.Bw; 07.77.Ka Keywords: Si detector calibration; Pulse height defect; Energy resolution
1. Introduction National security issues have recently prompted an urgent need for improved radiation detector materials. Existing materials do not meet the stringent requirements of nuclear non-proliferation and homeland security applications. Accelerated materials discovery efforts are needed to develop the next generation radiation detector materials with excellent energy resolution at room temperature. Fast screening techniques are, therefore, required to investigate numerous candidate materials to provide quick information about material properties relevant to detector performance. The response generated in detector materials by radiation energy deposition is the fundamental basis for understanding material functionality as a radiation detector. Energy resolution and detection efficiency can provide a critical basis for identification of possible candidate materials. The current work demonstrates a fast screening Corresponding author. Tel.: +1 509 376 3429; fax: +1 509 376 5106.
E-mail address: [email protected] (Y. Zhang). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.04.019
technique in obtaining relevant quantitative data on the response of a silicon detector, which is a benchmark material. The experimental approach used in this work can be applied to quickly evaluate the candidates of radiation detector materials. Silicon detectors are widely used for determining energies of neutral and charged particles in various ion beam analysis (IBA) techniques, radioactive ion beam applications, and satellite experiments. Despite their extremely widespread use for over four decades [1,2] and limited approaches to calibration [3–10], there exist no predictive calibrations for ions with a wide range of atomic number and energies. Contributions to pulse height defect (PHD), such as non-ionization energy loss and inefficient electron–hole (e–h) collection in the detector active volume, are not fully analyzed in a quantitative manner, and variation of the average energy required to produce an e–h pair remains controversial. The full analytical potential of IBA depends critically on establishing a reliable energy calibration of the Si detector used. For IBA applications in nanometer structures, where nanometer depth resolution is required, accurate energy calibration is of central impor-
ARTICLE IN PRESS Y. Zhang et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 108–112
2. Experimental Various ion beams were produced using either the forward scatter or recoil methods. Energetic particles of H, He, Be, C, O, Mg, Si, I and Au were produced by a NEC tandem accelerator. A bulk Au target was used to forward scatter the primary beams. Projectiles of 45 MeV 127 10+ I were used to create energetic target recoils of Ni and Zr from elemental nickel target and yttria stabilized zirconia (YSZ). Using the forward scatter and recoil method, 10 different ion species were produced over a continuous range of energies, from a few tens to hundreds of keV per nucleon. It should be stressed that the scattering or recoil process is only used to generate a secondary beam with a wide energy distribution but low intensity, so that direct beam exposure of the Si detector is avoided. The experimental setup is shown in Fig. 1. Target recoils and scattered particles were detected in a forward direction at 431 to the primary beam direction. A time-of-flight (TOF) telescope was utilized for measuring energy of an individual particle before impinging on the Si detector. The TOF telescope consists of two carbon foil timing detectors. The carbon foils were 8 mg cm2 thick, which corresponds to 34.6 nm assuming bulk graphite density. The flight length between the two carbon foil timing detectors is 437.5 mm. The Si detector was mounted after the TOF telescope with a collimator (10 mm in diameter) in front of it [11]. The entrance contact of the Si detector (ORTEC model u-013-150-300) is an extremely thin (500 A˚) boron implanted layer to maximize charge carriers collection. During the measurements, the output signals from the timing detectors were recorded in an inverse start–stop mode to minimize the dead time, and the TOF signal is delayed in order to coincide with the Si detector response.
1 L 2 EðkeVÞ ¼ M DE foil ¼ slope EðchÞ þ offset 2 TOF (1) where M is the ion mass, TOF is the time for the ion to pass through the flight length (L), and DEfoil is the energy loss in traversing the carbon foil in the second timing detector. The time calibration of the TOF telescope was done using 2500 Si detector response to carbon ions
2000 1500 1000 500 0 0
200
400 600 800 1000 1200 Detector Response (channel)
1400
Fig. 2. Time of flight versus detector response (linear scale). The unit of all axes is in channel number.
The basic detector configuration is based on a pn junction or surface barrier, where incident charged particles transfer their energy through collisions with target nuclei and electrons that induce excitation or ionization of atoms along a plasma column. Since the amount of ionization produced by a particle is converted to an TOF telescope
Ions
25000 20000 Energy (keV)
3. Results and discussion
Si detector
electrical signal (the measured pulse height) by collecting the charge carriers under a reversed bias voltage, it is of interest to study the detector response as a function of particle energy and determine the detection efficiency for the charge carriers as a function of stopping power. The detector response (pulse height) is simultaneously measured over a wide energy range, for which the energy is determined from the TOF and isotopic mass prior to impinging on the Si detector. The determination of the TOF-detector response for each element is demonstrated in Fig. 2 using carbon ions as an example, which is representative of a general behavior for other elements. The response (pulse height) versus energy (TOF information) plot for the C signals illustrates the continuous nature of the ion energies. The particle energies impinging the Si detector are determined using the corresponding exclusive TOF information, as indicated by the dotted and dashed arrows. For each ion, the energy is determined as
Time of Flight (channel)
tance for investigations of surfaces, interfaces, and multilayer structures. The emerging demands on better energy calibration of Si detectors make the current study an interesting topic.
109
15000 10000
Au Zr Ni Si
5000
Mg O C Be H
0 0
Flight length Collimator
Au Scatter
Fig. 1. Schematic diagram of the experimental setup.
1000
2000
3000
4000
Detector Response (channel number) Fig. 3. True energy of various ions versus the pulse channel number from the Si detector.
ARTICLE IN PRESS Y. Zhang et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 108–112
Table 1 Linear fitting parameters of Eq. (1) and the electronic stopping power at 500 keV/nucleon for various elements Element
H Be C O Mg Si Ni Zr Au
Atomic number
Slope (keV/ch)
Offset (keV)
Stopping at 500 keV/nucleon
1 4 6 8 12 14 28 40 79
5.18 5.22 5.23 5.25 5.30 5.36 5.58 7.48 10.30
139.16 165.36 176.14 207.83 290.27 306.53 829.88 1382.15 1918.00
0.54 3.24 5.13 7.17 11.49 13.95 31.39 48.50 94.17
0.8
H
Mg
Be
Si
C
Ni
O
Zr Au
0.7 0.6
0
20
40
60
80
100
2
Stopping Power (keV/ (µg/cm )) Fig. 5. Relative detection efficiency for e–h pairs as a function of electronic stopping power for a particle energy of 500 keV/nucleon.
9
a 3500 Detector response to He ions
3000 Energy (keV)
7
5 1500 Offset (keV)
0.9
0.5
11
Slope (keV/ch)
1.0 Relative Detection Efficiency
110
2500 2000 1500 1000 Variance at different energies
500
H Be C O Mg
1000 500
0
Si Ni Zr Au
1000 2000 3000 4000 Detector response (channel number)
b 4
0
20
40
60
80
Atomic Number Fig. 4. Atomic number dependence of the mean slope (top) and the mean intercept (bottom) from the linear fit of Eq. (1). The dashed lines are from polynomial curve fitting.
both the signals of the ions that are scattered from the Au target surface and the timing calibrator (ORTEC module 462), with the two independent results validating each other to achieve a reliable time calibration. The detector response (the measured pulse height) for different elements is shown in Fig. 3 as a function of particle energy. The results indicate that the pulse height–energy relation for each element can be described by a linear relationship (Eq. (1)), where the mean slope (keV/ channel) is the energy spanned by one channel and the mean offset (keV) is mainly the energy loss in the entrance window and dead layer of the detector. Although the data closely follow a linear behavior, small systematic deviations of the data are observed, even for light particles, which is currently under further investigation. The pulse height defect (PHD) is normally defined as the difference between the true energy of the heavy ion and its apparent energy, as
Resolution (%)
0 3
2
1
0 0
500
1000
1500 2000 Energy (keV)
2500
3000
Fig. 6. (a) Detector response for He ions over a wide energy range, and (b) the measured energy resolution as a function of energy deposited in the detector.
determined from an energy calibration of the detector using protons or alpha particles [12]. A significant PHD is observed for heavy elements, such as Zr and Au, as shown in Fig. 3. The fitting parameters from Eq. (1) are given in Table 1 and also shown in Fig. 4. A significant increase in the mean slope can be observed for heavy particles. The origin of this increase is mainly a consequence of energy loss in the entrance window and dead layer of the detector, nuclear energy loss that does not contribute to e–h pair production,
ARTICLE IN PRESS Y. Zhang et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 108–112
non-linear effects and possible plasma recombination [12]. The efficiency of e–h pair collection is markedly affected by the production, migration and recombination of e–h pairs in the dense plasma created along the ion track. Since the slope is proportional to the effective electronic energy deposition to excite an e–h pair divided by detection efficiency, the relative detection efficiency can be derived as a function of electronic stopping power for various particles, as shown in Fig. 5. The energy of 500 keV per nucleon is chosen to be the representative of the highenergy end of the heavy particles in the current study to minimize the influence of the nuclear stopping power. To demonstrate the TOF-ion approach as a fast screening technique, 3.0 MeV He ions forward scattered from the Au target were used to test the detector response. The electronic modules were optimized for processing the signals. During the measurement, more than 6 million He ions were detected by both the Si detector and TOF telescope in a coincident mode in less than 19 min. Fig. 6(a) shows the recorded detector response, for which the energy is determined by the TOF telescope. The signal broadening
Resolution (%)
4 3 2 1
111
results from the statistical fluctuations in the number of charge carriers, together with the energy straggling in the carbon foil in the second timing detector and the Si detector contact layer, the limited timing resolution, the uncertainties associated with the non-ionization part of the nuclear stopping, as well as the electronic noise. The measured energy resolution in the current study is defined as the full-width at half-maximum (FWHM) of the peak normalized to its position, and the results indicate a decrease with increasing particle energy, as shown in Fig. 6(b). The saturation level at 1.2% is the detector resolution for He ions including all sources of variances. The measured energy resolution for He ions is shown in Fig. 7 as a function of electronic [13] and nuclear energy deposition, as well as the ratio of nuclear energy deposition to the total ion energy. Similar behavior is observed from Figs. 6(b) and 7(a), since the majority of the e–h pairs are created from the electronic energy deposition. The broadening at lower energy ends, for both electronic and nuclear energy deposition, is attributed to statistical fluctuations due to relatively few charge carriers produced. Linear extrapolation of the results in Fig. 7(c) to zero should provide information about detector response when the radiation energy is deposited by electrons instead of ions. For fast screening purposes, the candidates of radiation detector materials can be simply investigated using readily available energetic ions, such as He ions. The measured energy resolution observed in the candidate materials can be compared to the results from a standard detector, such as the Si detector in the current study.
0 0
1000 2000 Total electronic energy deposition (keV)
3000
Resolution (%)
4 3 2 1 0 5.5
6.0 6.5 7.0 7.5 Total nuclear energy deposition (keV)
8.0
Resolution (%)
4 3 2
4. Conclusions It is demonstrated that TOF telescope radiation detector configuration can be used for fast screening of candidate radiation detector materials. Using a scattering or recoil process, ions with a continuous energy distribution can be used for depositing radiation energy. The detector response follows an approximately linear response for various elements. However, the detection efficiency of charge carrier collection decreases with increasing stopping power for heavy elements. Detector response of various candidate materials can be investigated using available ions, such as He ions. The measured energy resolution can provide intrinsic material properties relevant to detector performance that will assist and accelerate the discovery or identification of new detector materials. Acknowledgments
1 0 0.012 0.000 0.004 0.008 Nuclear energy deposition / Total energy deposition (%)
Fig. 7. Detector resolution as a function of (a) electronic energy deposition, (b) nuclear energy deposition, and (c) the ratio of nuclear energy deposition to electronic energy deposition.
This research was supported by the Radiation Detection Materials Discovery (RDMD) initiative. RDMD is a Laboratory Directed Research and Development Program at the Pacific Northwest National Laboratory, a multiprogram national laboratory operated by Battelle for the US Department of Energy under Contract DE-AC0576RL01830.
ARTICLE IN PRESS 112
Y. Zhang et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 108–112
References [1] A. Owens, A. Peacock, Nucl. Instr. and Meth. A 531 (2004) 18. [2] R. Devanathan, L.R. Corrales, F. Gao, W.J. Weber, Nucl. Instr. and Meth. A 565 (2006) 637. [3] E.C. Finch, M. Asghar, M. Forte, Nucl. Instr. and Meth. 163 (1979) 467. [4] Y. Zhang, G. Possnert, W.J. Weber, Appl. Phys. Lett. 80 (2002) 4662. [5] S.B. Kaufman, E.P. Stenberg, B.D. Wilkins, J. Unik, A.J. Gorski, M. Fluss, Nucl. Instr. and Meth. 115 (1974) 47. [6] W.N. Lennard, K.B. Winterbon, Nucl. Instr. and Meth. B 24/25 (1987) 1035.
[7] Y. Zhang, H.J. Whitlow, Nucl. Instr. and Meth. B 190 (2002) 383. [8] A. Lo Giudice, F. Fizzott, C. Manfredotti, E. Vittone, F. Nava, Appl. Phys. Lett. 87 (2005) 222105. [9] W.N. Lennart, H. Geissel, K.B. Winterbon, D. Phillips, T.K. Alexander, J.S. Forster, Nucl. Instr. and Meth. A 248 (1986) 454. [10] D. Comedi, J. Davies, Nucl. Instr. and Meth. B 67 (1992) 93. [11] Y. Zhang, Nucl. Instr. and Meth. B 196 (2002) 1. [12] G.F. Knoll, Radiation Detection and Measurement, third ed, Wiley, 2000 (Chapter 11). [13] Y. Zhang, W.J. Weber, H.J. Whitlow, Nucl. Instr. and Meth. B 215 (2004) 48.
Nuclear Instruments and Methods in Physics Research A 579 (2007) 108–112 www.elsevier.com/locate/nima
A fast screening technique to evaluate detector response Y. Zhang, F. Gao, R. Devanathan, W.J. Weber Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA Available online 5 April 2007
Abstract A fast screening technique to evaluate detector response was demonstrated using a silicon detector. Pulse height was measured for H, He, Be, C, O, Mg, Si, Ni, Zr and Au ions over a wide energy range using a time-of-flight (TOF) telescope. Using a scattering or recoil process, a secondary beam with a continuous energy distribution but low intensity is generated to avoid direct beam exposure of the Si detector. Prior to impinging on the Si detector, the energy of individual ions is determined from the TOF and its tabulated isotopic mass. The pulse height–energy calibration for ions with a given atomic number can be described by a linear relationship with small systematic deviations. For particles that have the same velocity (500 keV/nucleon), a non-linear dependence on efficiency of electron–hole pair collection is observed as a function of electronic stopping power. The detector response is studied using He ions, and the measured energy resolution is given as function of deposition energies over a wide energy range. r 2007 Elsevier B.V. All rights reserved. PACS: 29.40.n; 29.40.Wk; 34.50.Bw; 07.77.Ka Keywords: Si detector calibration; Pulse height defect; Energy resolution
1. Introduction National security issues have recently prompted an urgent need for improved radiation detector materials. Existing materials do not meet the stringent requirements of nuclear non-proliferation and homeland security applications. Accelerated materials discovery efforts are needed to develop the next generation radiation detector materials with excellent energy resolution at room temperature. Fast screening techniques are, therefore, required to investigate numerous candidate materials to provide quick information about material properties relevant to detector performance. The response generated in detector materials by radiation energy deposition is the fundamental basis for understanding material functionality as a radiation detector. Energy resolution and detection efficiency can provide a critical basis for identification of possible candidate materials. The current work demonstrates a fast screening Corresponding author. Tel.: +1 509 376 3429; fax: +1 509 376 5106.
E-mail address: [email protected] (Y. Zhang). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.04.019
technique in obtaining relevant quantitative data on the response of a silicon detector, which is a benchmark material. The experimental approach used in this work can be applied to quickly evaluate the candidates of radiation detector materials. Silicon detectors are widely used for determining energies of neutral and charged particles in various ion beam analysis (IBA) techniques, radioactive ion beam applications, and satellite experiments. Despite their extremely widespread use for over four decades [1,2] and limited approaches to calibration [3–10], there exist no predictive calibrations for ions with a wide range of atomic number and energies. Contributions to pulse height defect (PHD), such as non-ionization energy loss and inefficient electron–hole (e–h) collection in the detector active volume, are not fully analyzed in a quantitative manner, and variation of the average energy required to produce an e–h pair remains controversial. The full analytical potential of IBA depends critically on establishing a reliable energy calibration of the Si detector used. For IBA applications in nanometer structures, where nanometer depth resolution is required, accurate energy calibration is of central impor-
ARTICLE IN PRESS Y. Zhang et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 108–112
2. Experimental Various ion beams were produced using either the forward scatter or recoil methods. Energetic particles of H, He, Be, C, O, Mg, Si, I and Au were produced by a NEC tandem accelerator. A bulk Au target was used to forward scatter the primary beams. Projectiles of 45 MeV 127 10+ I were used to create energetic target recoils of Ni and Zr from elemental nickel target and yttria stabilized zirconia (YSZ). Using the forward scatter and recoil method, 10 different ion species were produced over a continuous range of energies, from a few tens to hundreds of keV per nucleon. It should be stressed that the scattering or recoil process is only used to generate a secondary beam with a wide energy distribution but low intensity, so that direct beam exposure of the Si detector is avoided. The experimental setup is shown in Fig. 1. Target recoils and scattered particles were detected in a forward direction at 431 to the primary beam direction. A time-of-flight (TOF) telescope was utilized for measuring energy of an individual particle before impinging on the Si detector. The TOF telescope consists of two carbon foil timing detectors. The carbon foils were 8 mg cm2 thick, which corresponds to 34.6 nm assuming bulk graphite density. The flight length between the two carbon foil timing detectors is 437.5 mm. The Si detector was mounted after the TOF telescope with a collimator (10 mm in diameter) in front of it [11]. The entrance contact of the Si detector (ORTEC model u-013-150-300) is an extremely thin (500 A˚) boron implanted layer to maximize charge carriers collection. During the measurements, the output signals from the timing detectors were recorded in an inverse start–stop mode to minimize the dead time, and the TOF signal is delayed in order to coincide with the Si detector response.
1 L 2 EðkeVÞ ¼ M DE foil ¼ slope EðchÞ þ offset 2 TOF (1) where M is the ion mass, TOF is the time for the ion to pass through the flight length (L), and DEfoil is the energy loss in traversing the carbon foil in the second timing detector. The time calibration of the TOF telescope was done using 2500 Si detector response to carbon ions
2000 1500 1000 500 0 0
200
400 600 800 1000 1200 Detector Response (channel)
1400
Fig. 2. Time of flight versus detector response (linear scale). The unit of all axes is in channel number.
The basic detector configuration is based on a pn junction or surface barrier, where incident charged particles transfer their energy through collisions with target nuclei and electrons that induce excitation or ionization of atoms along a plasma column. Since the amount of ionization produced by a particle is converted to an TOF telescope
Ions
25000 20000 Energy (keV)
3. Results and discussion
Si detector
electrical signal (the measured pulse height) by collecting the charge carriers under a reversed bias voltage, it is of interest to study the detector response as a function of particle energy and determine the detection efficiency for the charge carriers as a function of stopping power. The detector response (pulse height) is simultaneously measured over a wide energy range, for which the energy is determined from the TOF and isotopic mass prior to impinging on the Si detector. The determination of the TOF-detector response for each element is demonstrated in Fig. 2 using carbon ions as an example, which is representative of a general behavior for other elements. The response (pulse height) versus energy (TOF information) plot for the C signals illustrates the continuous nature of the ion energies. The particle energies impinging the Si detector are determined using the corresponding exclusive TOF information, as indicated by the dotted and dashed arrows. For each ion, the energy is determined as
Time of Flight (channel)
tance for investigations of surfaces, interfaces, and multilayer structures. The emerging demands on better energy calibration of Si detectors make the current study an interesting topic.
109
15000 10000
Au Zr Ni Si
5000
Mg O C Be H
0 0
Flight length Collimator
Au Scatter
Fig. 1. Schematic diagram of the experimental setup.
1000
2000
3000
4000
Detector Response (channel number) Fig. 3. True energy of various ions versus the pulse channel number from the Si detector.
ARTICLE IN PRESS Y. Zhang et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 108–112
Table 1 Linear fitting parameters of Eq. (1) and the electronic stopping power at 500 keV/nucleon for various elements Element
H Be C O Mg Si Ni Zr Au
Atomic number
Slope (keV/ch)
Offset (keV)
Stopping at 500 keV/nucleon
1 4 6 8 12 14 28 40 79
5.18 5.22 5.23 5.25 5.30 5.36 5.58 7.48 10.30
139.16 165.36 176.14 207.83 290.27 306.53 829.88 1382.15 1918.00
0.54 3.24 5.13 7.17 11.49 13.95 31.39 48.50 94.17
0.8
H
Mg
Be
Si
C
Ni
O
Zr Au
0.7 0.6
0
20
40
60
80
100
2
Stopping Power (keV/ (µg/cm )) Fig. 5. Relative detection efficiency for e–h pairs as a function of electronic stopping power for a particle energy of 500 keV/nucleon.
9
a 3500 Detector response to He ions
3000 Energy (keV)
7
5 1500 Offset (keV)
0.9
0.5
11
Slope (keV/ch)
1.0 Relative Detection Efficiency
110
2500 2000 1500 1000 Variance at different energies
500
H Be C O Mg
1000 500
0
Si Ni Zr Au
1000 2000 3000 4000 Detector response (channel number)
b 4
0
20
40
60
80
Atomic Number Fig. 4. Atomic number dependence of the mean slope (top) and the mean intercept (bottom) from the linear fit of Eq. (1). The dashed lines are from polynomial curve fitting.
both the signals of the ions that are scattered from the Au target surface and the timing calibrator (ORTEC module 462), with the two independent results validating each other to achieve a reliable time calibration. The detector response (the measured pulse height) for different elements is shown in Fig. 3 as a function of particle energy. The results indicate that the pulse height–energy relation for each element can be described by a linear relationship (Eq. (1)), where the mean slope (keV/ channel) is the energy spanned by one channel and the mean offset (keV) is mainly the energy loss in the entrance window and dead layer of the detector. Although the data closely follow a linear behavior, small systematic deviations of the data are observed, even for light particles, which is currently under further investigation. The pulse height defect (PHD) is normally defined as the difference between the true energy of the heavy ion and its apparent energy, as
Resolution (%)
0 3
2
1
0 0
500
1000
1500 2000 Energy (keV)
2500
3000
Fig. 6. (a) Detector response for He ions over a wide energy range, and (b) the measured energy resolution as a function of energy deposited in the detector.
determined from an energy calibration of the detector using protons or alpha particles [12]. A significant PHD is observed for heavy elements, such as Zr and Au, as shown in Fig. 3. The fitting parameters from Eq. (1) are given in Table 1 and also shown in Fig. 4. A significant increase in the mean slope can be observed for heavy particles. The origin of this increase is mainly a consequence of energy loss in the entrance window and dead layer of the detector, nuclear energy loss that does not contribute to e–h pair production,
ARTICLE IN PRESS Y. Zhang et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 108–112
non-linear effects and possible plasma recombination [12]. The efficiency of e–h pair collection is markedly affected by the production, migration and recombination of e–h pairs in the dense plasma created along the ion track. Since the slope is proportional to the effective electronic energy deposition to excite an e–h pair divided by detection efficiency, the relative detection efficiency can be derived as a function of electronic stopping power for various particles, as shown in Fig. 5. The energy of 500 keV per nucleon is chosen to be the representative of the highenergy end of the heavy particles in the current study to minimize the influence of the nuclear stopping power. To demonstrate the TOF-ion approach as a fast screening technique, 3.0 MeV He ions forward scattered from the Au target were used to test the detector response. The electronic modules were optimized for processing the signals. During the measurement, more than 6 million He ions were detected by both the Si detector and TOF telescope in a coincident mode in less than 19 min. Fig. 6(a) shows the recorded detector response, for which the energy is determined by the TOF telescope. The signal broadening
Resolution (%)
4 3 2 1
111
results from the statistical fluctuations in the number of charge carriers, together with the energy straggling in the carbon foil in the second timing detector and the Si detector contact layer, the limited timing resolution, the uncertainties associated with the non-ionization part of the nuclear stopping, as well as the electronic noise. The measured energy resolution in the current study is defined as the full-width at half-maximum (FWHM) of the peak normalized to its position, and the results indicate a decrease with increasing particle energy, as shown in Fig. 6(b). The saturation level at 1.2% is the detector resolution for He ions including all sources of variances. The measured energy resolution for He ions is shown in Fig. 7 as a function of electronic [13] and nuclear energy deposition, as well as the ratio of nuclear energy deposition to the total ion energy. Similar behavior is observed from Figs. 6(b) and 7(a), since the majority of the e–h pairs are created from the electronic energy deposition. The broadening at lower energy ends, for both electronic and nuclear energy deposition, is attributed to statistical fluctuations due to relatively few charge carriers produced. Linear extrapolation of the results in Fig. 7(c) to zero should provide information about detector response when the radiation energy is deposited by electrons instead of ions. For fast screening purposes, the candidates of radiation detector materials can be simply investigated using readily available energetic ions, such as He ions. The measured energy resolution observed in the candidate materials can be compared to the results from a standard detector, such as the Si detector in the current study.
0 0
1000 2000 Total electronic energy deposition (keV)
3000
Resolution (%)
4 3 2 1 0 5.5
6.0 6.5 7.0 7.5 Total nuclear energy deposition (keV)
8.0
Resolution (%)
4 3 2
4. Conclusions It is demonstrated that TOF telescope radiation detector configuration can be used for fast screening of candidate radiation detector materials. Using a scattering or recoil process, ions with a continuous energy distribution can be used for depositing radiation energy. The detector response follows an approximately linear response for various elements. However, the detection efficiency of charge carrier collection decreases with increasing stopping power for heavy elements. Detector response of various candidate materials can be investigated using available ions, such as He ions. The measured energy resolution can provide intrinsic material properties relevant to detector performance that will assist and accelerate the discovery or identification of new detector materials. Acknowledgments
1 0 0.012 0.000 0.004 0.008 Nuclear energy deposition / Total energy deposition (%)
Fig. 7. Detector resolution as a function of (a) electronic energy deposition, (b) nuclear energy deposition, and (c) the ratio of nuclear energy deposition to electronic energy deposition.
This research was supported by the Radiation Detection Materials Discovery (RDMD) initiative. RDMD is a Laboratory Directed Research and Development Program at the Pacific Northwest National Laboratory, a multiprogram national laboratory operated by Battelle for the US Department of Energy under Contract DE-AC0576RL01830.
ARTICLE IN PRESS 112
Y. Zhang et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 108–112
References [1] A. Owens, A. Peacock, Nucl. Instr. and Meth. A 531 (2004) 18. [2] R. Devanathan, L.R. Corrales, F. Gao, W.J. Weber, Nucl. Instr. and Meth. A 565 (2006) 637. [3] E.C. Finch, M. Asghar, M. Forte, Nucl. Instr. and Meth. 163 (1979) 467. [4] Y. Zhang, G. Possnert, W.J. Weber, Appl. Phys. Lett. 80 (2002) 4662. [5] S.B. Kaufman, E.P. Stenberg, B.D. Wilkins, J. Unik, A.J. Gorski, M. Fluss, Nucl. Instr. and Meth. 115 (1974) 47. [6] W.N. Lennard, K.B. Winterbon, Nucl. Instr. and Meth. B 24/25 (1987) 1035.
[7] Y. Zhang, H.J. Whitlow, Nucl. Instr. and Meth. B 190 (2002) 383. [8] A. Lo Giudice, F. Fizzott, C. Manfredotti, E. Vittone, F. Nava, Appl. Phys. Lett. 87 (2005) 222105. [9] W.N. Lennart, H. Geissel, K.B. Winterbon, D. Phillips, T.K. Alexander, J.S. Forster, Nucl. Instr. and Meth. A 248 (1986) 454. [10] D. Comedi, J. Davies, Nucl. Instr. and Meth. B 67 (1992) 93. [11] Y. Zhang, Nucl. Instr. and Meth. B 196 (2002) 1. [12] G.F. Knoll, Radiation Detection and Measurement, third ed, Wiley, 2000 (Chapter 11). [13] Y. Zhang, W.J. Weber, H.J. Whitlow, Nucl. Instr. and Meth. B 215 (2004) 48.