- Email: [email protected]
Toward a carbon nanotube anode gas-filled radiation detector
Nuclear Instruments and Methods in Physics Research A 652 (2011) 310–314
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Toward a carbon nanotube anode gas-filled radiation detector Timothy A. DeVol a,n, Landon Pruitt a, Jay Gallaird b, Lindsay Sexton c, Joseph Cordaro c, Apparao Rao b, Steven M. Serkiz c,d a
Environmental Engineering and Earth Sciences Department, Clemson University, Clemson, SC 29634-0919, USA Physics and Astronomy Department, Clemson University, Clemson, SC 29634-0978, USA c Savannah River National Laboratory, Aiken, SC 29802, USA d Material Science and Engineering Department, Clemson University, Clemson, SC 29634-0922, USA b
a r t i c l e in f o
a b s t r a c t
Available online 1 September 2010
A prototype gas-filled proportional counter (PC) based on micro-scale tungsten wire and carbon fiber, and nano-scale carbon nanotube (CNT) anodes was built and tested with a 90Sr source. Tungsten anodes of 500 mm down to 4 mm diameter were used to observe the gradual decrease in operating voltage for the proportional region with a decreasing anode diameter. The 40 nm diameter CNTs anodes ranged in length from 35 to 105 mm. The absolute detection efficiency was measured at 10 6%. An electrostatic computer model was used to predict the resulting electric field associated with a single CNT in the coaxial configuration. For a single anode coaxial design the model predicted that the electric field was insufficient for secondary ionizations which contributed to a low amplitude signal and that the small volume of the ionization region resulted in the low absolute detection efficiency. To overcome the problems of low absolute detection efficiency and operational issues with the single anode, CNT arrays were investigated. Electrostatic modeling of 100 nm 40 mm long CNTs in an array with a 50 mm pitch conducted for a parallel plate configuration indicated that each anode functioned independently. & 2010 Elsevier B.V. All rights reserved.
Keywords: Microdosimetry CNT array
1. Introduction Gas-filled detectors are commonly used as portable radiation detection devices because they are relatively easy to construct and operate. The three main gas-filled detectors are ionization chambers, proportional counters (PC), and Geiger Muller tubes. Compensated air-filled ionization chambers are widely used for measurement of radiation exposure. Geiger-Muller counters are being used as portable devices, but they are only good for knowing the presence of radiation without the possibility of identifying the radioisotope. PCs can detect a range of radiation types and are widely utilized to measure various radiation parameters, such as energy and dose equivalent using tissue equivalent gases [3]. A limiting factor of a common laboratory PC is transportability because of their reliance on a very high and stable voltage source. This dependence on a high voltage supply has been overcome by the use of step-up transformers to increase the voltage from a portable battery supply. Portable PCs manufactured and used today are based on this principle. As monitoring for illegal transport of radioactive materials at borders, seaports, and airports increases there is a need for detection devices that are easily portable, run on small portable
n
Corresponding author. E-mail address: [email protected] (T.A. DeVol).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.08.032
power supplies for long periods of time, and have high detection efficiencies. Also, as knowledge advances in the field of radiation oncology there is an increased need for micro or even nano-size, but efficient, in-vivo dosimeters. Because of the advancing knowledge of their electrical and structural properties, carbon nanotubes (CNT) could be used as nano-size anodes while retaining electrical properties. This idea of using CNTs for anodes in gas multiplication radiation detectors has been proposed by Kotani et al. [10]. Kotani et al. [11] suggested the use of singlewalled CNTs grown directly on the silicon wafers to be used as the anodes, but experimental results have yet to be published. A PC equipped with CNT anodes could have application to personnel dosimetry, micro in-vivo dosimetry for medical purposes, in-situ laboratory gas monitors, handheld radiation detectors, radiochemistry, and environmental monitoring. It is the objective of this work to investigate the design and construction of CNT anodes in gas-filled proportional detectors.
2. Experimental The operation of a gas-filled radiation detector works on the principle of primary and secondary ionization of the fill-gas. The ion saturation region is determined when the electric field is sufficient (44 104 V/m) such that the resultant signal is attributed to only primary ionizations and there is no recombination of ions.
T.A. DeVol et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 310–314
311
The proportional region occurs when the electric field is sufficient (41 106 V/m) to result in secondary ionizations of the fill gas [8]. For a cylindrical anode at the center of a cylindrical cathode the electric field, E, can be found at any radius (r) away from the center of the detector by EðrÞ ¼
V0 r lnðrc =ra Þ
ð1Þ
where V0 is the bias voltage, r (4 ra) is some radius in question, and ra and rc are the radii of the anode and cathode, respectively [2]. This electric field intensity yields a multiplication factor that is often described by the Townsend coefficient (i.e. ionizations per unit length of the path of the electron) [4]. A point anode proportional counter, works on the same principal as the cylindrical geometry except it applies to an anode that is not nearly the same length of the entire detector volume bound by the cathode such that the primary and secondary electrons are collected at the tip of the anode. Mathieson and Sanford [12] showed the relationship between gas gain and anode diameter in a point anode gas-filled detector for quantification of 55 Fe (1963). The electric field associated with the tip of the anode is described by [2] EðrÞ ¼
V0 ðð1=ra Þð1=rc ÞÞr 2
ð2Þ
Although slightly different than the coaxial relationship, the electric field around the tip has a greater spatial dependence. This model was constructed by assuming that the electric field near the surface of the anode was that between two concentric spheres where ra is the anode radius of curvature and rc is the cathode radius of curvature [2]. The equation also assumes that the electric field will concentrate near the tip of the anode. CNTs have attracted considerable attention because of their unique physical, chemical, and most importantly, electrical transport properties [7]. They are metallic (single or multi-walled nanotubes) or semi-conducting (single-walled nanotubes), depending merely on their structure [7]. Various groups have utilized CNTs for such applications as gas sensing [13]. Gas ionization sensors operate at a voltage potential (high enough for the ionization of the gas inside the detector) across an anode wire. The Modi et al. [13] explored the potential of a sensor design consisting of a plate covered in vertically aligned array of multiwalled CNTs as a gas ionization sensor for quantification of the gas present. Modi et al. [13] suggest that the sharp tips ( o25 nm) are the reason for generating very high electric fields at relatively low voltages, lowering breakdown voltages several-fold in comparison to traditional electrodes, and thereby enabling compact design, low voltage battery run, and safe operation of their sensors. 2.1. Experimental set-up Experiments with different diameter anodes (W, carbon fibers and CNTs) were carried out to quantify changes in operating voltage and for a CNT the absolute detection efficiency. Tungsten wires were used for quantifying a relationship between the diameter and bias voltage on the micro-scale. A commercially manufactured 0.5 mm diameter W wire was etched down using an electrochemical process. W wires could not be etched down any smaller than a diameter of 50 mm thus smaller diameter (4, 7.5, and 25 mm) hard-temper W wires used for the anodes were purchased (Goodfellow Cambridge Limited). Carbon fibers (Alfa Aesar) were measured to be between 10 and 11 mm in diameter, by dark-field microscope images, and ranged up to 5 cm in length. Silver paint (conducting semi-adhesive) was used between the W wire and the carbon fiber. Single CNTs were
Fig. 1. A single CNT on the tip of a W tip.
produced at Clemson University using the thermal chemical vapor deposition process [1]. Fig. 1 is a scanning electron micrograph of an individual CNT on a W tip. The CNT was attached to a tungsten probe tip by applying a small dc bias ( 3 V) across the W tip and a bundle of CNTs to electrostatically extract a single CNT and hold it at the end of the W tip. This technique provides placement of CNTs with a lateral resolution of a micron and has been widely used in the literature [5,6]. CNTs of 40 nm diameter by 35, 40, and 105 mm long were used as the gas-filled detector anode at varying bias voltages. All detectors utilized P-10 as the fill gas. Charging the gas detector was done in the following manner: a vacuum of 6.58 10 4 atm (500 mTorr) was pulled on the chamber then back-filling with up to 4 atm of P-10. This process was repeated twice more to reduce the potential number of impurity atoms (10 ppt). A 3.85 108 Bq 90Sr/90Y source was used to characterize the detector. The 1 mm thick stainless steel cathode (1 cm inner radius) absorbed beta radiation less than 1.7 MeV from the source. Pulse height spectra were acquired using conventional NIM-style electronics connected to a 4k channel MCA (Aptec, Model 5004).
2.2. Electrostatic modeling For electrostatic modeling, Maxwell 3D (Ansoft Corporation) was used to simulate the system and model the electric fields inside the detector and guide the experiments. The chamber that was used for all the experiments was modeled to its closest possible specifications. The diameters of the experimental CNTs used in the program were modeled as 100 nm instead of 40 nm due to the limitations of the Maxwell 3D program. The single coaxial design with the CNT was modeled to evaluate the electric field surrounding the anode. The objective was to maximize the volume in which the electric field was greater than or equal to 4 104 V/m, in order to maximize the absolute detection efficiency. The model was also used to represent the parallel plate set-up with CNT array geometry. The pitch of the CNTs (100 nm diameter, 40 mm length) in the array was varied to maximize the electric field and assure that the CNTs function as individual anodes. An electrode spacing of 200 mm between the plates and arrays of 5 5 were used to model this system. From the model we were able to investigate anode arrays and suggest future detector changes in the geometry and materials to achieve lowered operating voltages and increased counting efficiencies.
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3. Results and discussion The primary goal of this work is to explore the influence of anode size and composition on gas-filled radiation detector performance with the application being a microdosimeter that is capable of operating at a low voltage. Two tasks were performed to meet this objective. The first task was to collect data with micro-scale anodes of different sizes (diameter and length) and composition as well as single CNT anodes of different sizes (diameter and length). The second task was to use quantitative computer modeling to simulate electrical fields inside detector and around the anodes to compare with experimental results and to help in the redesign of the detector.
indicates the maximum pulse height recorded both with and without the CNT attached to the W tip. The absolute detection efficiency was calculated from the net count rate obtained by subtraction of the count rate obtained with the CNT attached to the W tip from the count rate when there is no CNT. The absolute detection efficiency of the 40 mm anode is slightly larger than the 35 mm while that for the 105 mm CNT is roughly half that of the 40 mm anode efficiency, which is unexpected. The reason for
3.1. Single CNT anode experimental results Presented in Fig. 2 is pulse height versus bias voltage for tungsten anode diameters from 55 mm down to 4 mm and a 10 mm carbon fiber anode. The characteristic recombination, ionization, and proportional regions are apparent in all anodes. For the W anodes the bias voltage required to achieve a given pulse height decreases with decreasing anode diameter, which is consistent with the theory presented in Eq. (1) for a coaxial detector. The carbon fiber anode exhibits a lower voltage response (higher electric field for a given bias voltage) than the same diameter W anode, indicating an anode material dependence. The anode sizes were measured using a scanning electron microscope. The pulse height spectra recorded with the single CNT electrostatically held on the W tip were also recorded. The associated data are summarized in Table 1. Pulse height spectra were recorded with a CNT attached to the W tip then the high voltage was increased to ‘‘blow-off’’ the CNT followed by recording the pulse height spectra with just the W tip, which is considered to be the background signal. The recorded count rate were for a consistent lower level discriminator that was set just higher than the electronic noise. The maximum channel number
4µm 10µm carbon fiber 7.5µm 25µm 55µm
Pulse Height (V)
10
1
0.1 0
200
400
600 800 Bias Voltage (V)
1000
1200
Fig. 2. Pulse height vs. bias voltage for carbon fiber and several smaller W anodes; lines connect data points only to guide eye. Note that the 10 mm carbon fiber responds like a 6 mm W anode.
Fig. 3. (a) Electrostatic model simulation of the electric field distribution for a W tip and 100nm 105 mm CNT at bias voltage of 10 V; (b) same CNT at 200 V bias.
Table 1 Absolute detection efficiency for 40 nm diameter anodes with three different lengths, 24 h count time, bias voltage 10 V. CNT anode length (mm) 35 40 105
Counts with CNT 101 710 156 712.5 57 77.5
Counts without CNT 327 5.6 767 8.7 137 3.6
eabs 8
1.137 0.19 10 1.317 0.25 10 8 7.187 1.4 10 9
Max channel with CNT
Max channel without CNT
1910 835 930
685 772 650
T.A. DeVol et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 310–314
the discrepancy remains under investigation. The acquisition of this data was much more problematic than the W anodes because (1) the very low absolute detection efficiency of a single CNT and (2) the CNT was electrostatically held to the W tip. The low detection efficiency was overcome in part by utilizing a relatively high activity source with long data acquisition times (24 h). The presence of the CNT on the W tip was visually confirmed with a dark-field microscope before and after the pulse height acquisition. Setting the high voltage too high removed the CNT from the W-tip, thus the bias voltage had to be kept below 75 V. 3.2. Electrostatic modeling results The Maxwell 3D electrostatic modeling program was used to simulate the system and model the electric field inside the detector as well as guide the experiments. In Fig. 3a, the electric field around the W tip and the CNT (100 nm diameter, 105 mm long) for a 10 V bias is shown. The radius at which the electric field is 4 104 V/m is also present in Fig. 3a at approximately 43 mm. The maximum electric field of 4.68 106 V/m was calculated by the program and was located directly on the surface of the CNT anode. This electric field was high enough for gas multiplication, but the volume in which the electric field is at or
Maximum E field (V/m)
107 100 nm x 40µm 100 nm x 15µm 40 nm x 15µm
106
313
above 1 106 V/m was extremely small. As can be seen in Fig. 3a, the proportional region is concentrating near the tip of the CNT, but covers it somewhat radially and is only 50 nm thick at most. This region is much smaller than the theoretical distance of 17 mm for the mean free path for ionization [9]. These results suggest that the detector is actually operating in the ionization region. As indicated in Fig. 3b, if the bias voltage is raised to 200 V, there is a region ( 417 mm) in the detector where the electric field is greater than 106 V/m; hence the detector would function as a gas-filled proportional detector. As a means of increasing the overall detection efficiency of a microdosimeter, CNT arrays were investigated. Optimization of the CNT size and array pitch were modeled to assure that the electric field was sufficient to be in the proportional region and for the individual array elements to function independently. With the model the CNT array configuration was investigated to guide the experimental design of the geometry and materials to achieve lowered operating voltages and increased counting efficiencies relative to the single CNT. Fig. 4a displays some of the data from the optimization procedure when the spacing between the cathode and the anode base is maintained at 200 mm for a 5 5 array. The pitch of the CNTs (100 nm diameter 40 mm length, 100nm diameter 15 mm length, 40 nm diameter 15 mm length) in the array was varied from 5 to 150 mm to investigate the maximum electric field. The 100 nm 40 mm CNT array resulted in an electric field sufficient for the proportional region for a range of pitches. Fig. 4b illustrates the electric field for the 100 nm 40 mm CNT array with a 50 mm pitch when the anode is biased at 10 V. From the model, one would predict that this is the minimum configuration for operation in the proportional region. The increased efficiency comes about from the anodes functioning independently, thus the total detection efficiency will scale linearly with the number of anodes in the array. Further if the anodes in the array were individually connected to associated electronics then one would have a position sensitive detection system.
4. Conclusions
105 0
25
50
75
100
125
150
CNT spacing (µm)
E Field Legend
Fig. 4. (a) Electrostatic model simulation of the electric field as a function of pitch of CNT array on parallel plate; for three different size CNTs (legend: diameter length) (lines are to guide the eyes only); (b) wire frame drawing with electric field concentrations for the optimized pitch of CNT anodes (largest image is tilted downward slightly); CNTs of 100nm diameter, 40 mm length, with a pitch of 50 mm, plates 200 mm apart, biased at 10 V.
Single CNTs with a diameter of 40 nm and three different lengths were evaluated experimentally with a bias voltage of 10V. The highest absolute efficiency for beta radiation was at 1.3170.25 10 6% for the 40 nm 40 mm CNT anode. The electrostatic model supported the low detection efficiency and indicated that the proportional region volume to be less than a mean free path for secondary ionization. The model also showed that the bias voltage must be at least 200 V to have a high electric field volume large enough to allow for gas multiplication. A bias voltage of 200 V was not possible with our experimental set-up because the CNT anode would drop off the W tip. The model and the data suggest the detector is operating in the ionization region. The electrostatic model was then used to predict the electric field for a 100 nm by 40 mm CNT arrays in the parallel plate set-up (pitch of 50 mm and plate separation of 200 mm). For a 10 V bias, the electric field at least 4 104 V/m completely filled the space between the plates giving a large effective volume and the resultant proportional region volume (E4106 V/m) was large enough at the tip of every CNT to function in the proportional region. The CNT anode array will result in a significantly higher detection efficiency relative to a single CNT with the potential to function as a position sensitive detector. References [1] R. Andrews, D. Jacques, A.M. Rao, F. Derbyshire, D. Qian, X. Fan, E.C. Dickey, J. Chen, Chemical Physical Letters 303 (1999) 467.
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[2] J.E. Bateman, Physics Reports 375 (6) (2003) 411. [3] H. Date, K. Kondo, M. Shimosuma, H. Tagashira, Nuclear Instruments and Methods A 451 (2000) 588. [4] F. Fernandez, I. Ramos-Lerate, M. Kocsis, J.C. Martinez, D. Beltran, J. Bordas, J. Toledo, IEEE Transaction on Nuclear Science 53 (2) (2006) 544. [5] J.B. Gaillard, Harmonic detection of resonance in micro and nano-cantilevers, Ph.D. Dissertation, Clemson University, 2006. [6] E.D. Graugnard, The electronic properties of multiwalled carbon nanotubes, Ph. D. Dissertation, Purdue University, 2000. [7] S. Gupta, R.J. Ptel, N. Smith, R.E. Giedd, D. Hui, Diamond and Related Materials 16 (2007) 236.
[8] G.F. Knoll, in: Radiation Detection and Measurement, John Wiley and Sons, Inc., 2000. [9] S.A. Korff, in: Electron and Nuclear Counters, D. Van Nostrand Company, Inc., 1946. [10] T. Kotani, N. Kawai, S. Chiba, S. Kitamoto, Physica E—Low-Dimensional Systems & Nanostructures 29 (2005) 505. [11] T. Kotani, M. Ueno, N. Kawai, S. Kitamoto, Physica E—Low-Dimensional Systems & Nanostructures 40 (2007) 422. [12] E. Mathieson, P.W. Sanford, Journal of Scientific Instruments 40 (1963) 446. [13] A. Modi, N. Koratkar, E. Lass, B. Wei, P.M. Ajayan, Nature 424 (6945) (2003) 171.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Toward a carbon nanotube anode gas-filled radiation detector Timothy A. DeVol a,n, Landon Pruitt a, Jay Gallaird b, Lindsay Sexton c, Joseph Cordaro c, Apparao Rao b, Steven M. Serkiz c,d a
Environmental Engineering and Earth Sciences Department, Clemson University, Clemson, SC 29634-0919, USA Physics and Astronomy Department, Clemson University, Clemson, SC 29634-0978, USA c Savannah River National Laboratory, Aiken, SC 29802, USA d Material Science and Engineering Department, Clemson University, Clemson, SC 29634-0922, USA b
a r t i c l e in f o
a b s t r a c t
Available online 1 September 2010
A prototype gas-filled proportional counter (PC) based on micro-scale tungsten wire and carbon fiber, and nano-scale carbon nanotube (CNT) anodes was built and tested with a 90Sr source. Tungsten anodes of 500 mm down to 4 mm diameter were used to observe the gradual decrease in operating voltage for the proportional region with a decreasing anode diameter. The 40 nm diameter CNTs anodes ranged in length from 35 to 105 mm. The absolute detection efficiency was measured at 10 6%. An electrostatic computer model was used to predict the resulting electric field associated with a single CNT in the coaxial configuration. For a single anode coaxial design the model predicted that the electric field was insufficient for secondary ionizations which contributed to a low amplitude signal and that the small volume of the ionization region resulted in the low absolute detection efficiency. To overcome the problems of low absolute detection efficiency and operational issues with the single anode, CNT arrays were investigated. Electrostatic modeling of 100 nm 40 mm long CNTs in an array with a 50 mm pitch conducted for a parallel plate configuration indicated that each anode functioned independently. & 2010 Elsevier B.V. All rights reserved.
Keywords: Microdosimetry CNT array
1. Introduction Gas-filled detectors are commonly used as portable radiation detection devices because they are relatively easy to construct and operate. The three main gas-filled detectors are ionization chambers, proportional counters (PC), and Geiger Muller tubes. Compensated air-filled ionization chambers are widely used for measurement of radiation exposure. Geiger-Muller counters are being used as portable devices, but they are only good for knowing the presence of radiation without the possibility of identifying the radioisotope. PCs can detect a range of radiation types and are widely utilized to measure various radiation parameters, such as energy and dose equivalent using tissue equivalent gases [3]. A limiting factor of a common laboratory PC is transportability because of their reliance on a very high and stable voltage source. This dependence on a high voltage supply has been overcome by the use of step-up transformers to increase the voltage from a portable battery supply. Portable PCs manufactured and used today are based on this principle. As monitoring for illegal transport of radioactive materials at borders, seaports, and airports increases there is a need for detection devices that are easily portable, run on small portable
n
Corresponding author. E-mail address: [email protected] (T.A. DeVol).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.08.032
power supplies for long periods of time, and have high detection efficiencies. Also, as knowledge advances in the field of radiation oncology there is an increased need for micro or even nano-size, but efficient, in-vivo dosimeters. Because of the advancing knowledge of their electrical and structural properties, carbon nanotubes (CNT) could be used as nano-size anodes while retaining electrical properties. This idea of using CNTs for anodes in gas multiplication radiation detectors has been proposed by Kotani et al. [10]. Kotani et al. [11] suggested the use of singlewalled CNTs grown directly on the silicon wafers to be used as the anodes, but experimental results have yet to be published. A PC equipped with CNT anodes could have application to personnel dosimetry, micro in-vivo dosimetry for medical purposes, in-situ laboratory gas monitors, handheld radiation detectors, radiochemistry, and environmental monitoring. It is the objective of this work to investigate the design and construction of CNT anodes in gas-filled proportional detectors.
2. Experimental The operation of a gas-filled radiation detector works on the principle of primary and secondary ionization of the fill-gas. The ion saturation region is determined when the electric field is sufficient (44 104 V/m) such that the resultant signal is attributed to only primary ionizations and there is no recombination of ions.
T.A. DeVol et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 310–314
311
The proportional region occurs when the electric field is sufficient (41 106 V/m) to result in secondary ionizations of the fill gas [8]. For a cylindrical anode at the center of a cylindrical cathode the electric field, E, can be found at any radius (r) away from the center of the detector by EðrÞ ¼
V0 r lnðrc =ra Þ
ð1Þ
where V0 is the bias voltage, r (4 ra) is some radius in question, and ra and rc are the radii of the anode and cathode, respectively [2]. This electric field intensity yields a multiplication factor that is often described by the Townsend coefficient (i.e. ionizations per unit length of the path of the electron) [4]. A point anode proportional counter, works on the same principal as the cylindrical geometry except it applies to an anode that is not nearly the same length of the entire detector volume bound by the cathode such that the primary and secondary electrons are collected at the tip of the anode. Mathieson and Sanford [12] showed the relationship between gas gain and anode diameter in a point anode gas-filled detector for quantification of 55 Fe (1963). The electric field associated with the tip of the anode is described by [2] EðrÞ ¼
V0 ðð1=ra Þð1=rc ÞÞr 2
ð2Þ
Although slightly different than the coaxial relationship, the electric field around the tip has a greater spatial dependence. This model was constructed by assuming that the electric field near the surface of the anode was that between two concentric spheres where ra is the anode radius of curvature and rc is the cathode radius of curvature [2]. The equation also assumes that the electric field will concentrate near the tip of the anode. CNTs have attracted considerable attention because of their unique physical, chemical, and most importantly, electrical transport properties [7]. They are metallic (single or multi-walled nanotubes) or semi-conducting (single-walled nanotubes), depending merely on their structure [7]. Various groups have utilized CNTs for such applications as gas sensing [13]. Gas ionization sensors operate at a voltage potential (high enough for the ionization of the gas inside the detector) across an anode wire. The Modi et al. [13] explored the potential of a sensor design consisting of a plate covered in vertically aligned array of multiwalled CNTs as a gas ionization sensor for quantification of the gas present. Modi et al. [13] suggest that the sharp tips ( o25 nm) are the reason for generating very high electric fields at relatively low voltages, lowering breakdown voltages several-fold in comparison to traditional electrodes, and thereby enabling compact design, low voltage battery run, and safe operation of their sensors. 2.1. Experimental set-up Experiments with different diameter anodes (W, carbon fibers and CNTs) were carried out to quantify changes in operating voltage and for a CNT the absolute detection efficiency. Tungsten wires were used for quantifying a relationship between the diameter and bias voltage on the micro-scale. A commercially manufactured 0.5 mm diameter W wire was etched down using an electrochemical process. W wires could not be etched down any smaller than a diameter of 50 mm thus smaller diameter (4, 7.5, and 25 mm) hard-temper W wires used for the anodes were purchased (Goodfellow Cambridge Limited). Carbon fibers (Alfa Aesar) were measured to be between 10 and 11 mm in diameter, by dark-field microscope images, and ranged up to 5 cm in length. Silver paint (conducting semi-adhesive) was used between the W wire and the carbon fiber. Single CNTs were
Fig. 1. A single CNT on the tip of a W tip.
produced at Clemson University using the thermal chemical vapor deposition process [1]. Fig. 1 is a scanning electron micrograph of an individual CNT on a W tip. The CNT was attached to a tungsten probe tip by applying a small dc bias ( 3 V) across the W tip and a bundle of CNTs to electrostatically extract a single CNT and hold it at the end of the W tip. This technique provides placement of CNTs with a lateral resolution of a micron and has been widely used in the literature [5,6]. CNTs of 40 nm diameter by 35, 40, and 105 mm long were used as the gas-filled detector anode at varying bias voltages. All detectors utilized P-10 as the fill gas. Charging the gas detector was done in the following manner: a vacuum of 6.58 10 4 atm (500 mTorr) was pulled on the chamber then back-filling with up to 4 atm of P-10. This process was repeated twice more to reduce the potential number of impurity atoms (10 ppt). A 3.85 108 Bq 90Sr/90Y source was used to characterize the detector. The 1 mm thick stainless steel cathode (1 cm inner radius) absorbed beta radiation less than 1.7 MeV from the source. Pulse height spectra were acquired using conventional NIM-style electronics connected to a 4k channel MCA (Aptec, Model 5004).
2.2. Electrostatic modeling For electrostatic modeling, Maxwell 3D (Ansoft Corporation) was used to simulate the system and model the electric fields inside the detector and guide the experiments. The chamber that was used for all the experiments was modeled to its closest possible specifications. The diameters of the experimental CNTs used in the program were modeled as 100 nm instead of 40 nm due to the limitations of the Maxwell 3D program. The single coaxial design with the CNT was modeled to evaluate the electric field surrounding the anode. The objective was to maximize the volume in which the electric field was greater than or equal to 4 104 V/m, in order to maximize the absolute detection efficiency. The model was also used to represent the parallel plate set-up with CNT array geometry. The pitch of the CNTs (100 nm diameter, 40 mm length) in the array was varied to maximize the electric field and assure that the CNTs function as individual anodes. An electrode spacing of 200 mm between the plates and arrays of 5 5 were used to model this system. From the model we were able to investigate anode arrays and suggest future detector changes in the geometry and materials to achieve lowered operating voltages and increased counting efficiencies.
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T.A. DeVol et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 310–314
3. Results and discussion The primary goal of this work is to explore the influence of anode size and composition on gas-filled radiation detector performance with the application being a microdosimeter that is capable of operating at a low voltage. Two tasks were performed to meet this objective. The first task was to collect data with micro-scale anodes of different sizes (diameter and length) and composition as well as single CNT anodes of different sizes (diameter and length). The second task was to use quantitative computer modeling to simulate electrical fields inside detector and around the anodes to compare with experimental results and to help in the redesign of the detector.
indicates the maximum pulse height recorded both with and without the CNT attached to the W tip. The absolute detection efficiency was calculated from the net count rate obtained by subtraction of the count rate obtained with the CNT attached to the W tip from the count rate when there is no CNT. The absolute detection efficiency of the 40 mm anode is slightly larger than the 35 mm while that for the 105 mm CNT is roughly half that of the 40 mm anode efficiency, which is unexpected. The reason for
3.1. Single CNT anode experimental results Presented in Fig. 2 is pulse height versus bias voltage for tungsten anode diameters from 55 mm down to 4 mm and a 10 mm carbon fiber anode. The characteristic recombination, ionization, and proportional regions are apparent in all anodes. For the W anodes the bias voltage required to achieve a given pulse height decreases with decreasing anode diameter, which is consistent with the theory presented in Eq. (1) for a coaxial detector. The carbon fiber anode exhibits a lower voltage response (higher electric field for a given bias voltage) than the same diameter W anode, indicating an anode material dependence. The anode sizes were measured using a scanning electron microscope. The pulse height spectra recorded with the single CNT electrostatically held on the W tip were also recorded. The associated data are summarized in Table 1. Pulse height spectra were recorded with a CNT attached to the W tip then the high voltage was increased to ‘‘blow-off’’ the CNT followed by recording the pulse height spectra with just the W tip, which is considered to be the background signal. The recorded count rate were for a consistent lower level discriminator that was set just higher than the electronic noise. The maximum channel number
4µm 10µm carbon fiber 7.5µm 25µm 55µm
Pulse Height (V)
10
1
0.1 0
200
400
600 800 Bias Voltage (V)
1000
1200
Fig. 2. Pulse height vs. bias voltage for carbon fiber and several smaller W anodes; lines connect data points only to guide eye. Note that the 10 mm carbon fiber responds like a 6 mm W anode.
Fig. 3. (a) Electrostatic model simulation of the electric field distribution for a W tip and 100nm 105 mm CNT at bias voltage of 10 V; (b) same CNT at 200 V bias.
Table 1 Absolute detection efficiency for 40 nm diameter anodes with three different lengths, 24 h count time, bias voltage 10 V. CNT anode length (mm) 35 40 105
Counts with CNT 101 710 156 712.5 57 77.5
Counts without CNT 327 5.6 767 8.7 137 3.6
eabs 8
1.137 0.19 10 1.317 0.25 10 8 7.187 1.4 10 9
Max channel with CNT
Max channel without CNT
1910 835 930
685 772 650
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the discrepancy remains under investigation. The acquisition of this data was much more problematic than the W anodes because (1) the very low absolute detection efficiency of a single CNT and (2) the CNT was electrostatically held to the W tip. The low detection efficiency was overcome in part by utilizing a relatively high activity source with long data acquisition times (24 h). The presence of the CNT on the W tip was visually confirmed with a dark-field microscope before and after the pulse height acquisition. Setting the high voltage too high removed the CNT from the W-tip, thus the bias voltage had to be kept below 75 V. 3.2. Electrostatic modeling results The Maxwell 3D electrostatic modeling program was used to simulate the system and model the electric field inside the detector as well as guide the experiments. In Fig. 3a, the electric field around the W tip and the CNT (100 nm diameter, 105 mm long) for a 10 V bias is shown. The radius at which the electric field is 4 104 V/m is also present in Fig. 3a at approximately 43 mm. The maximum electric field of 4.68 106 V/m was calculated by the program and was located directly on the surface of the CNT anode. This electric field was high enough for gas multiplication, but the volume in which the electric field is at or
Maximum E field (V/m)
107 100 nm x 40µm 100 nm x 15µm 40 nm x 15µm
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above 1 106 V/m was extremely small. As can be seen in Fig. 3a, the proportional region is concentrating near the tip of the CNT, but covers it somewhat radially and is only 50 nm thick at most. This region is much smaller than the theoretical distance of 17 mm for the mean free path for ionization [9]. These results suggest that the detector is actually operating in the ionization region. As indicated in Fig. 3b, if the bias voltage is raised to 200 V, there is a region ( 417 mm) in the detector where the electric field is greater than 106 V/m; hence the detector would function as a gas-filled proportional detector. As a means of increasing the overall detection efficiency of a microdosimeter, CNT arrays were investigated. Optimization of the CNT size and array pitch were modeled to assure that the electric field was sufficient to be in the proportional region and for the individual array elements to function independently. With the model the CNT array configuration was investigated to guide the experimental design of the geometry and materials to achieve lowered operating voltages and increased counting efficiencies relative to the single CNT. Fig. 4a displays some of the data from the optimization procedure when the spacing between the cathode and the anode base is maintained at 200 mm for a 5 5 array. The pitch of the CNTs (100 nm diameter 40 mm length, 100nm diameter 15 mm length, 40 nm diameter 15 mm length) in the array was varied from 5 to 150 mm to investigate the maximum electric field. The 100 nm 40 mm CNT array resulted in an electric field sufficient for the proportional region for a range of pitches. Fig. 4b illustrates the electric field for the 100 nm 40 mm CNT array with a 50 mm pitch when the anode is biased at 10 V. From the model, one would predict that this is the minimum configuration for operation in the proportional region. The increased efficiency comes about from the anodes functioning independently, thus the total detection efficiency will scale linearly with the number of anodes in the array. Further if the anodes in the array were individually connected to associated electronics then one would have a position sensitive detection system.
4. Conclusions
105 0
25
50
75
100
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CNT spacing (µm)
E Field Legend
Fig. 4. (a) Electrostatic model simulation of the electric field as a function of pitch of CNT array on parallel plate; for three different size CNTs (legend: diameter length) (lines are to guide the eyes only); (b) wire frame drawing with electric field concentrations for the optimized pitch of CNT anodes (largest image is tilted downward slightly); CNTs of 100nm diameter, 40 mm length, with a pitch of 50 mm, plates 200 mm apart, biased at 10 V.
Single CNTs with a diameter of 40 nm and three different lengths were evaluated experimentally with a bias voltage of 10V. The highest absolute efficiency for beta radiation was at 1.3170.25 10 6% for the 40 nm 40 mm CNT anode. The electrostatic model supported the low detection efficiency and indicated that the proportional region volume to be less than a mean free path for secondary ionization. The model also showed that the bias voltage must be at least 200 V to have a high electric field volume large enough to allow for gas multiplication. A bias voltage of 200 V was not possible with our experimental set-up because the CNT anode would drop off the W tip. The model and the data suggest the detector is operating in the ionization region. The electrostatic model was then used to predict the electric field for a 100 nm by 40 mm CNT arrays in the parallel plate set-up (pitch of 50 mm and plate separation of 200 mm). For a 10 V bias, the electric field at least 4 104 V/m completely filled the space between the plates giving a large effective volume and the resultant proportional region volume (E4106 V/m) was large enough at the tip of every CNT to function in the proportional region. The CNT anode array will result in a significantly higher detection efficiency relative to a single CNT with the potential to function as a position sensitive detector. References [1] R. Andrews, D. Jacques, A.M. Rao, F. Derbyshire, D. Qian, X. Fan, E.C. Dickey, J. Chen, Chemical Physical Letters 303 (1999) 467.
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[8] G.F. Knoll, in: Radiation Detection and Measurement, John Wiley and Sons, Inc., 2000. [9] S.A. Korff, in: Electron and Nuclear Counters, D. Van Nostrand Company, Inc., 1946. [10] T. Kotani, N. Kawai, S. Chiba, S. Kitamoto, Physica E—Low-Dimensional Systems & Nanostructures 29 (2005) 505. [11] T. Kotani, M. Ueno, N. Kawai, S. Kitamoto, Physica E—Low-Dimensional Systems & Nanostructures 40 (2007) 422. [12] E. Mathieson, P.W. Sanford, Journal of Scientific Instruments 40 (1963) 446. [13] A. Modi, N. Koratkar, E. Lass, B. Wei, P.M. Ajayan, Nature 424 (6945) (2003) 171.