- Email: [email protected]
A new method for the determination of unknown neutron fluence for 14.0 MeV
ARTICLE IN PRESS
Physica B 385–386 (2006) 1318–1320 www.elsevier.com/locate/physb
A new method for the determination of unknown neutron fluence for 14.0 MeV Fariha Malika,,1, Ehsan U. Khanb, Imtinan Qureshia, Syed N. Husainia, Waqar Ahmada, Usman Rajputa, Qaiser Razac a
Physics Reasearch Division, PINSTECH, Nilore, Islamabad, Pakistan b Department of Physics, CIIT, Islamabad, Pakistan c Applied Physics Division, PINSTECH, Nilore, Islamabad, Pakistan
Abstract Measuring the correct neutron fluence in various energy intervals in and around the neutron sources is important for the purpose of personnel and environmental neutron dosimetry. In this paper, we present a new method for the measurement of the fluence of monoenergetic neutrons having the energy of 14.0 MeV. The samples exposed to neutrons from the 14.0 MeV neutron generator at PINSTECH with various fluence values ranging from 107 to 1010 n cm2 were etched for 10 min in 6 N NaOH at 70.071.0 1C and the transmittance of UV radiation was measured using a spectrophotometer. This procedure was repeated 20 times after etching the same sample each time for increasing time intervals till the stage when transmittance reached the constant minimum value. An exponential decay of the transmittance has been observed with respect to the increasing etching time interval in each of the samples exposed to various neutron fluence. Further, it has also been observed that there is a linear relationship between the transmittance decay constant and neutron fluence. Hence, the linear graph can be used as a calibration for measuring the unknown fluence of 14.0 MeV neutrons. r 2006 Elsevier B.V. All rights reserved. PACS: 28.20.Fc; 61.80.Hg; 29.40.–n; 79.60.Fr; 78.20.Ci Keywords: Neutron fluence; CR-39 detector; Chemical etching; UV transmittance
1. Introduction The measurement of neutron fluence presents unique problems as they are uncharged particles and have a wide range of neutron energies, from a few hundreds of eV to several hundreds of MeV. The irregular variation of neutron interaction cross-section with energy, particularly in the intermediate energy range where sharp resonance peaks are formed, therefore, require the development of techniques of neutron monitoring that can address such problems. Jamil et al. [1]. have shown that CR-39 can be used to monitor the beam profile of the thermal neutrons in a nuclear reactor that has the advantage to locate and Corresponding author. Tel.: +92 51 2207230; fax: +92 51 9290275.
E-mail address: [email protected] (F. Malik). F. Malik is a registered Ph. D scholar in NIE, Nanyang Tech. Univ., Singapore. 1
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.06.063
determine beam size of neutrons. However, the determination of neutron flux is important for many experiments in the fields of nuclear physics and neutron dosimetry. SSNTD-based neutron dosimeters have always been considered as one of the promising candidates, which may render solutions to various practical problems [2,3]. However, since the start of the application of various types of SSNTD’s as neutron dosimeters about four decades earlier, quite a rigorous activity has been going on in the track-detector laboratories throughout the globe [4]. The interest was further increased when CR-39 (a trade name for the polymer poly allyl diglycol carbonate, PADC) was used to detect neutrons via recoil protons generated in CR-39 material itself or in a converter material with high hydrogen content [5]. The recoil track generated in CR-39 material can be observed under the optical microscope after chemical/ electrochemical etching of the detector. The track density
ARTICLE IN PRESS F. Malik et al. / Physica B 385–386 (2006) 1318–1320
2. Experimental After cutting seven square pieces of 2 2 cm2 from a 0.1 cm-thick sheet of CR-39 (Pershore), the detectors were exposed at the Neutron Generator in Physics Research Division of PINSTECH that produces neutrons by D–T reaction. CR-39 detectors were exposed to mono-energetic neutrons at 14 MeV for different time intervals to obtain neutron fluence of 1.8 108, 3.6 108, 5.4 108, 7.2 108, 9.0 108 and 1.08 109 n cm2. These fluence values were calculated by a standard formula after incorporating the 1369 KeV g counts resulting from 24Na; a product emerging in the n–Al reaction. A high-purity germanium (HPGe) detector was used for the purpose. After carrying out the exposure, the detectors were etched in an aqueous solution of 6 M NaOH at (7071.0) 1C for 10 min. After washing the samples thoroughly in the ultrasonic cleaning bath, the transmittance T, in percentage of the UV electromagnetic waves (l ¼ 800 nm) from a tungsten–iodine lamp, was measured in each sample. Transmittance is in fact determined by (SZ)/(RZ) which is the photometric output from the detector, namely reference signal R, sample signal S and zero signal Z, obtained in synchronization with rotation of the sector mirror for splitting the light beam are subjected to A/D conversion through the amplifier and the microcomputer. A Hitachi spectrophotometer (4100) was used for the purpose. The value of T was determined in each sample after etching all the detectors for various short time intervals of 10–60 min such that 20 values of T were obtained in each case. It was observed that T decays exponentially with
1E10
Neutron fluence (n-cm-2)
in the detector is directly proportional to the neutron fluence producing the recoil tracks [6], provided the track density is in the countable range. Hence, an alternate method of measuring the neutron fluence is needed for determining high neutron fluence. Although the use of spectrophotometer for measuring transmission of light through etched track detectors started as early as 1967–1970 [7,8] nevertheless, the only conclusion that was reached was that the transmission of light decayed exponentially with increasing etching time. However, the relationship of the transmission with neutron fluence was not established till recently when we developed a method of determining the unknown neutron fluence by determining the value of exponential decay constant that depends on the neutron fluence as well as neutron energy. We have already shown that the method is valid for the reactor neutrons having the usual spectrum of energy [9,10]. In order to resolve the neutron fluence with respect to energy, the method is being extended to mono-energetic neutrons. In the present study, we have presented a new method for the determination of neutron fluence from the neutron generator that produces neutron up to a maximum energy of 14.0 MeV.
1319
1E9
1E8
-0.0002
0.0000
0.0002
0.0004
0.0006
0.0008
Transmittance Decay Constant (min-1 ) Fig. 1. Calibration graph for 14.0 MeV neutrons. The straight line is a linear fit to the experimental data with regression coefficient r ¼ 0:98137 0:0602.
increasing etching time intervals in each case and the degree of the decay is governed by the neutron fluence to which the detectors were exposed. [9,10]. Since the transmittance is measured after the known etching time interval, the only unknown quantity, the neutron fluence, is thus retrieved by fitting the equation showing the exponential decay of transmittance T ¼ T 0 eðkÞt ,
(1)
where k has been defined to be the transmittance decay constant and t is the etching time. In order to determine the unknown neutron fluence, we, therefore, need to prepare a calibration graph as was done in one of our previous work [10] for reactor neutrons, having various energy values with a peak at 1 MeV and a long tail going upto 7 MeV. Hence six values of k obtained from six samples exposed to various known neutron fluence were plotted as shown in Fig. 1. The line in the figure represents a linear fit to the six experimental data points with the values of constants A and B shown in the following equation that can readily be used to determine any unknown value of fluence after incorporating the value of k measured from the transmittance–etching time graph. F ¼ 10ð9:190:05Þþð1839:6180:1Þk
(2)
It should be mentioned here that the slope of this calibration (14.0 MeV neutrons) is drastically different than the slope of the calibration for reactor neutrons [10]. 3. Determination of unknown fluence We had also exposed a CR-39 detector to neutrons having fluence of 8.0 108 n-cm2 previously. The values
ARTICLE IN PRESS F. Malik et al. / Physica B 385–386 (2006) 1318–1320
1320
4. Conclusion
100 90
We have produced a calibration for the 14.0 MeV neutrons, by plotting known values of fluence against the experimentally determined values of transmittance decay constants. It has been used to measure the unknown fluence by incorporating the experimentally determined transmittance decay constant through the exposed detector. The results obtained are within 10% of the actual neutron fluence. Hence, it is concluded that the method developed in this work for the determination of 14.0 MeV neutron fluence works with reasonable accuracy. It is also concluded that the slopes of the calibration line is strongly dependent on the energy of the neutrons.
Transmittance (%)
80 70 60 50 40 30 20 10 0
0
200
400 600 800 1000 1200 1400 Etching time (min)
Fig. 2. Experimental transmittance vs. etching time plot fitted with the exponential decay of the first order (Eq. (1)).
of transmittance have been measured through this detector after various etching time intervals as shown in Fig. 2. The data have been fitted with Eq. (1) and a value of k ¼ 1:72 104 has been obtained. After incorporating the value of k in Eq. (2), we have obtained the value of neutron fluence Fn ¼ (7.4571.4) 108 n cm2. The dispersion (71.4) in the mean fluence value is the maximum deviation originating from the standard deviations in the values of calibration constants A and B.
References [1] K. Jamil, S. Ali, M. Ahmad, M.U. Rajput, W. Ahmad, H.A. Khan, The Nucleus 36 (1–2) (1999) 31. [2] H.A. Khan, Nucl. Instrum. Meth. 113 (1973) 55. [3] M.G. Balcazar, S.A. Durrani, Nucl. Instrum. Meth. 173 (1980) 131. [4] J.R. Harvey, R.J. Tanner, W.G. Alberts, D.T. Bartlett, E.K.A. Piesch, H. Schraube, Radiat. Prot. Dosim. (1997) 267. [5] W. Enge, Nucl. Tracks 4 (4) (1980) 283. [6] K. Jamil, S. Ali, I.E. Qureshi, F. Rehman, H.A. Khan, S. Manzoor, A. Waheed, R. Cherubini, Radiat. Measur. 28 (1–6) (1997) 495. [7] J.W.N. Tuyn, Nucl. Appl. 3 (1967). [8] J.W.N. Tuyn, Radiat. Eff. 5 (1970) 75. [9] F. Malik, E.U. Khan, I.E. Qureshi, A. Mahmood, S.N. Husaini, N. Al, The Nucleus 40 (1–4) (2003) 77. [10] E.U. Khan, F. Malik, I.E. Qureshi, S.N. Husaini, N. Ali, A. Mehmood, Radiat. Measur. 40 (2–6) (2005) 583.
Physica B 385–386 (2006) 1318–1320 www.elsevier.com/locate/physb
A new method for the determination of unknown neutron fluence for 14.0 MeV Fariha Malika,,1, Ehsan U. Khanb, Imtinan Qureshia, Syed N. Husainia, Waqar Ahmada, Usman Rajputa, Qaiser Razac a
Physics Reasearch Division, PINSTECH, Nilore, Islamabad, Pakistan b Department of Physics, CIIT, Islamabad, Pakistan c Applied Physics Division, PINSTECH, Nilore, Islamabad, Pakistan
Abstract Measuring the correct neutron fluence in various energy intervals in and around the neutron sources is important for the purpose of personnel and environmental neutron dosimetry. In this paper, we present a new method for the measurement of the fluence of monoenergetic neutrons having the energy of 14.0 MeV. The samples exposed to neutrons from the 14.0 MeV neutron generator at PINSTECH with various fluence values ranging from 107 to 1010 n cm2 were etched for 10 min in 6 N NaOH at 70.071.0 1C and the transmittance of UV radiation was measured using a spectrophotometer. This procedure was repeated 20 times after etching the same sample each time for increasing time intervals till the stage when transmittance reached the constant minimum value. An exponential decay of the transmittance has been observed with respect to the increasing etching time interval in each of the samples exposed to various neutron fluence. Further, it has also been observed that there is a linear relationship between the transmittance decay constant and neutron fluence. Hence, the linear graph can be used as a calibration for measuring the unknown fluence of 14.0 MeV neutrons. r 2006 Elsevier B.V. All rights reserved. PACS: 28.20.Fc; 61.80.Hg; 29.40.–n; 79.60.Fr; 78.20.Ci Keywords: Neutron fluence; CR-39 detector; Chemical etching; UV transmittance
1. Introduction The measurement of neutron fluence presents unique problems as they are uncharged particles and have a wide range of neutron energies, from a few hundreds of eV to several hundreds of MeV. The irregular variation of neutron interaction cross-section with energy, particularly in the intermediate energy range where sharp resonance peaks are formed, therefore, require the development of techniques of neutron monitoring that can address such problems. Jamil et al. [1]. have shown that CR-39 can be used to monitor the beam profile of the thermal neutrons in a nuclear reactor that has the advantage to locate and Corresponding author. Tel.: +92 51 2207230; fax: +92 51 9290275.
E-mail address: [email protected] (F. Malik). F. Malik is a registered Ph. D scholar in NIE, Nanyang Tech. Univ., Singapore. 1
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.06.063
determine beam size of neutrons. However, the determination of neutron flux is important for many experiments in the fields of nuclear physics and neutron dosimetry. SSNTD-based neutron dosimeters have always been considered as one of the promising candidates, which may render solutions to various practical problems [2,3]. However, since the start of the application of various types of SSNTD’s as neutron dosimeters about four decades earlier, quite a rigorous activity has been going on in the track-detector laboratories throughout the globe [4]. The interest was further increased when CR-39 (a trade name for the polymer poly allyl diglycol carbonate, PADC) was used to detect neutrons via recoil protons generated in CR-39 material itself or in a converter material with high hydrogen content [5]. The recoil track generated in CR-39 material can be observed under the optical microscope after chemical/ electrochemical etching of the detector. The track density
ARTICLE IN PRESS F. Malik et al. / Physica B 385–386 (2006) 1318–1320
2. Experimental After cutting seven square pieces of 2 2 cm2 from a 0.1 cm-thick sheet of CR-39 (Pershore), the detectors were exposed at the Neutron Generator in Physics Research Division of PINSTECH that produces neutrons by D–T reaction. CR-39 detectors were exposed to mono-energetic neutrons at 14 MeV for different time intervals to obtain neutron fluence of 1.8 108, 3.6 108, 5.4 108, 7.2 108, 9.0 108 and 1.08 109 n cm2. These fluence values were calculated by a standard formula after incorporating the 1369 KeV g counts resulting from 24Na; a product emerging in the n–Al reaction. A high-purity germanium (HPGe) detector was used for the purpose. After carrying out the exposure, the detectors were etched in an aqueous solution of 6 M NaOH at (7071.0) 1C for 10 min. After washing the samples thoroughly in the ultrasonic cleaning bath, the transmittance T, in percentage of the UV electromagnetic waves (l ¼ 800 nm) from a tungsten–iodine lamp, was measured in each sample. Transmittance is in fact determined by (SZ)/(RZ) which is the photometric output from the detector, namely reference signal R, sample signal S and zero signal Z, obtained in synchronization with rotation of the sector mirror for splitting the light beam are subjected to A/D conversion through the amplifier and the microcomputer. A Hitachi spectrophotometer (4100) was used for the purpose. The value of T was determined in each sample after etching all the detectors for various short time intervals of 10–60 min such that 20 values of T were obtained in each case. It was observed that T decays exponentially with
1E10
Neutron fluence (n-cm-2)
in the detector is directly proportional to the neutron fluence producing the recoil tracks [6], provided the track density is in the countable range. Hence, an alternate method of measuring the neutron fluence is needed for determining high neutron fluence. Although the use of spectrophotometer for measuring transmission of light through etched track detectors started as early as 1967–1970 [7,8] nevertheless, the only conclusion that was reached was that the transmission of light decayed exponentially with increasing etching time. However, the relationship of the transmission with neutron fluence was not established till recently when we developed a method of determining the unknown neutron fluence by determining the value of exponential decay constant that depends on the neutron fluence as well as neutron energy. We have already shown that the method is valid for the reactor neutrons having the usual spectrum of energy [9,10]. In order to resolve the neutron fluence with respect to energy, the method is being extended to mono-energetic neutrons. In the present study, we have presented a new method for the determination of neutron fluence from the neutron generator that produces neutron up to a maximum energy of 14.0 MeV.
1319
1E9
1E8
-0.0002
0.0000
0.0002
0.0004
0.0006
0.0008
Transmittance Decay Constant (min-1 ) Fig. 1. Calibration graph for 14.0 MeV neutrons. The straight line is a linear fit to the experimental data with regression coefficient r ¼ 0:98137 0:0602.
increasing etching time intervals in each case and the degree of the decay is governed by the neutron fluence to which the detectors were exposed. [9,10]. Since the transmittance is measured after the known etching time interval, the only unknown quantity, the neutron fluence, is thus retrieved by fitting the equation showing the exponential decay of transmittance T ¼ T 0 eðkÞt ,
(1)
where k has been defined to be the transmittance decay constant and t is the etching time. In order to determine the unknown neutron fluence, we, therefore, need to prepare a calibration graph as was done in one of our previous work [10] for reactor neutrons, having various energy values with a peak at 1 MeV and a long tail going upto 7 MeV. Hence six values of k obtained from six samples exposed to various known neutron fluence were plotted as shown in Fig. 1. The line in the figure represents a linear fit to the six experimental data points with the values of constants A and B shown in the following equation that can readily be used to determine any unknown value of fluence after incorporating the value of k measured from the transmittance–etching time graph. F ¼ 10ð9:190:05Þþð1839:6180:1Þk
(2)
It should be mentioned here that the slope of this calibration (14.0 MeV neutrons) is drastically different than the slope of the calibration for reactor neutrons [10]. 3. Determination of unknown fluence We had also exposed a CR-39 detector to neutrons having fluence of 8.0 108 n-cm2 previously. The values
ARTICLE IN PRESS F. Malik et al. / Physica B 385–386 (2006) 1318–1320
1320
4. Conclusion
100 90
We have produced a calibration for the 14.0 MeV neutrons, by plotting known values of fluence against the experimentally determined values of transmittance decay constants. It has been used to measure the unknown fluence by incorporating the experimentally determined transmittance decay constant through the exposed detector. The results obtained are within 10% of the actual neutron fluence. Hence, it is concluded that the method developed in this work for the determination of 14.0 MeV neutron fluence works with reasonable accuracy. It is also concluded that the slopes of the calibration line is strongly dependent on the energy of the neutrons.
Transmittance (%)
80 70 60 50 40 30 20 10 0
0
200
400 600 800 1000 1200 1400 Etching time (min)
Fig. 2. Experimental transmittance vs. etching time plot fitted with the exponential decay of the first order (Eq. (1)).
of transmittance have been measured through this detector after various etching time intervals as shown in Fig. 2. The data have been fitted with Eq. (1) and a value of k ¼ 1:72 104 has been obtained. After incorporating the value of k in Eq. (2), we have obtained the value of neutron fluence Fn ¼ (7.4571.4) 108 n cm2. The dispersion (71.4) in the mean fluence value is the maximum deviation originating from the standard deviations in the values of calibration constants A and B.
References [1] K. Jamil, S. Ali, M. Ahmad, M.U. Rajput, W. Ahmad, H.A. Khan, The Nucleus 36 (1–2) (1999) 31. [2] H.A. Khan, Nucl. Instrum. Meth. 113 (1973) 55. [3] M.G. Balcazar, S.A. Durrani, Nucl. Instrum. Meth. 173 (1980) 131. [4] J.R. Harvey, R.J. Tanner, W.G. Alberts, D.T. Bartlett, E.K.A. Piesch, H. Schraube, Radiat. Prot. Dosim. (1997) 267. [5] W. Enge, Nucl. Tracks 4 (4) (1980) 283. [6] K. Jamil, S. Ali, I.E. Qureshi, F. Rehman, H.A. Khan, S. Manzoor, A. Waheed, R. Cherubini, Radiat. Measur. 28 (1–6) (1997) 495. [7] J.W.N. Tuyn, Nucl. Appl. 3 (1967). [8] J.W.N. Tuyn, Radiat. Eff. 5 (1970) 75. [9] F. Malik, E.U. Khan, I.E. Qureshi, A. Mahmood, S.N. Husaini, N. Al, The Nucleus 40 (1–4) (2003) 77. [10] E.U. Khan, F. Malik, I.E. Qureshi, S.N. Husaini, N. Ali, A. Mehmood, Radiat. Measur. 40 (2–6) (2005) 583.