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Sample to moderator volume ratio effects in neutron yield from a PGNAA setup
Radiation Measurements 42 (2007) 241 – 244 www.elsevier.com/locate/radmeas
Sample to moderator volume ratio effects in neutron yield from a PGNAA setup A.A. Naqvi ∗ , Fazal-ur-Rehman, M.M. Nagadi, Khateeb-ur-Rehman Department of Physics, King Fahd University of Petroleum and Minerals, KFUPM Box 1815, Dhahran-31261, Saudi Arabia Received 3 June 2006; received in revised form 27 September 2006; accepted 30 October 2006
Abstract Performance of a prompt gamma ray neutron activation analysis (PGNAA) setup depends upon thermal neutron yield at the PGNAA sample location. For a moderator, which encloses a sample, thermal neutron intensity depends upon the effective moderator volume excluding the void volume due to sample volume. A rectangular moderator assembly has been designed for the King Fahd University of Petroleum and Minerals (KFUPM) PGNAA setup. The thermal and fast neutron yield has been measured inside the sample cavity as a function of its front moderator thickness using alpha particle tracks density and recoil proton track density inside the CR-39 nuclear track detectors (NTDs). The thermal/fast neutron yield ratio, obtained from the alpha particle tracks density to proton tracks density ratio in the NTDs, shows an inverse correlation with sample to moderator volume ratio. Comparison of the present results with the previously published results of smaller moderators of the KFUPM PGNAA setup confirms the observation. © 2006 Elsevier Ltd. All rights reserved. Keywords: PGNAA; Fast and thermal neutron yield measurement; 2.8 MeV neutrons; NTDs; Monte Carlo simulations
1. Introduction The performance of a Prompt Gamma ray Neutron Activation Analysis (PGNAA) setup depends upon thermal neutron flux available at the sample (Sowerby and Watt, 1994; Naqvi et al., 2003, 2004a, b, 2006; Olivera et al., 1993, 1997; Saleh and Livingston, 2000; Al-Jarallah et al., 2002; Collico Savio et al., 1995; Khelifi et al., 1999; Tickner, 2000; Lim et al., 2001). The thermal neutron flux of a PGNAA setup, which is produced by an external moderator, depends upon moderator volume available to slow down the fast neutrons to thermal energies. Therefore thermal/fast neutron intensity ratio in a PGNAA setup with adequate moderation volume, increases with moderator size. A rectangular moderator assembly has been designed for the PGNAA setup of King Fahd University of Petroleum and Minerals (KFUPM) to analyze bulk samples of cement and concrete for concrete corrosion study (Naqvi et al., 2004a, 2006). The design calculations of the rectangular
∗ Corresponding author. Tel.: +966 3860 4362; fax: +966 3860 4281.
E-mail address: [email protected] (A.A. Naqvi). 1350-4487/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2006.10.003
moderator have been verified experimentally through thermal neutron yield measurements as a function of its front moderator thickness using CR-39 nuclear track detectors (NTDs) (Naqvi et al., 2004a). The thermal neutron yield was measured inside the sample volume of the rectangular moderator by two NTDs fixed at back and front end of the sample cavity. Previously thermal neutron flux was measured using NTDs inside two different cylindrical moderators with 25.4 cm outer diameter and 14 cm length (Al-Jarallah et al., 2002; Naqvi et al., 2003). One moderator called ‘SSM’ was used with a sample having 7 cm radius while the other moderator called ‘LSM’ was used with a sample having a 8.5 cm radius (Al-Jarallah et al., 2002). It was observed that maximum yield of thermal neutron from the LSM was 22 ± 6% smaller than that from the SSM (Naqvi et al., 2003). This was expected because the volume of the LSM was about 17% smaller than that of the SSM. Now a large size rectangular moderator has been designed for the KFUPM PGNAA setup (Naqvi et al., 2004a, b). In this study, the thermal and fast neutron yield has been measured inside the moderator in the sample volume using NTDs. Results of this study are compared to those of the SSM and LSM data to study moderator volume dependence of the neutron yield.
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A.A. Naqvi et al. / Radiation Measurements 42 (2007) 241 – 244
2. The KFUPM PGNAA setup with the rectangular moderator
regions of the moderator, being first a region located between the neutron source and the sample front end (called front moderator) and secondly in the moderator reflector collar surrounding the sample along its length. The fast neutrons that escape from the front moderator region, are reflected by the collar into the sample region. Fig. 1 shows the PGNAA setup with the rectangular moderator and the high density polyethylene plug with a sample with diameter less than the maximum diameter. The optimum value of the sample radius, front moderator thickness and the sample length were assumed to be the one which produced maximum yield of the prompt gamma rays at the detector. The optimum dimension of the sample to be used with the rectangular moderator is given in Table 1. For sake of comparison the similar data for the cylindrical moderator (Naqvi et al., 2003) is also listed in Table 1.
The rectangular moderator assembly of the KFUPM PGNAA setup has been described in detail elsewhere (Naqvi et al., 2004a, b) but for completion sake it will be described here briefly. The setup consists of a cylindrical sample placed inside a rectangular moderator. The moderator is made of paraffin wax and has a central cylindrical cavity, which can accommodate a sample with a maximum diameter of 25.4 cm. The rectangular moderator was designed for a 2.8 MeV neutron based PGNAA setup for the analysis of cement and concrete samples. The rectangular moderator is 19 cm thick and has a cross section of 49 cm × 49 cm (width x height). For a sample with an optimum diameter less than the maximum value, a high density polyethylene plug is used, which fills the gap between the sample and the walls of the cavity. Fast neutrons are thermalized in two
3. Fast and thermal neutron yield measurements
Fig. 1. Schematic representation of the PGNAA setup used to measure fast and thermal neutrons (Naqvi et al., 2004a). In thermal neutron measurements, the sample is replaced by a nuclear track detectors (NTDs) called middle detector placed in the middle of the front and back nuclear track detectors shown in the figure.
The thermal and fast neutron yield was measured inside the PGNAA moderator at the middle point of sample as a function of the thickness of the front moderator using NTDs. The cavity full volume was used without plugin to study the maximum effect of loss of moderation volume. One NTD was mounted in the middle of the cavity (mid detector) with reference to the incident neutron beam. The mid detector was placed at a distance of 7 cm from the front moderator and it was in the middle of the front and back detectors shown in Fig. 1 (Naqvi et al., 2004a). The mid detector was masked with boron converter to measure thermal neutron intensity via (n, ) reaction while a second NTD without boron converter, was placed next to the mid detector to measure fast neutron intensity at that location. The NTDs consist of 0.5 mm thick CR-39 NTD (Poly Allyle Diglycole Carbonate(PADC)–C12 H18 O7 ) with an area of 1.5 cm × 1.5 cm. The mid detector represents the average intensity of the thermal neutrons at the sample location. For sake of comparison with experimental data, thermal neutron yield of the middle detector was calculated using Monte Carlo code MCNP4B2 (Briesmeister, 1997) following the procedure described elsewhere (Naqvi et al., 2004a). The measurements were carried out using the PGNAA setup, built at the 45◦ beam line of the KFUPM 350 keV accelerator laboratory (Naqvi et al., 2004a, b, 2006). A 200 keV deuteron beam with 4.8 ns pulse width and a pulse repetition rate
Table 1 Dimensions of the rectangular and the cylindrical moderator assemblies of the KFUPM PGNAA setup Moderator parameter
Rectangular moderator
Cylindrical large sample moderator (LSM) (Naqvi et al., 2003)
Cylindrical small sample moderator (SSM) (Naqvi et al., 2003)
Moderator size (diameter) Sample radius (cm) Front moderator thickness (cm) Sample length (cm) Sample volume (Li) Moderator volume (Li) Sample/moderator volume ratio (%) Slope of therm/fast neutron yield
49 cm × 49 cm (width × height) 12–14 5–6 14 6.87 45.62 15.0 1.39 ± 0.1
25 cm diameter
25 cm diameter
8.47 3–4 14 2.704 9.33 28.9 0.63 ± 0.1
7 3–4 14 2.155 9.33 23.1 0.68 ± 0.1
Alpha Particles Track Density (Arbitrary Units)
Track Density Due to Neutrons (Arbitrary Units)
A.A. Naqvi et al. / Radiation Measurements 42 (2007) 241 – 244
60000 Rec-Middle-Det-Therm Rec-Middle-Fast
40000
20000
60000
Th-Exp-SSM Rec-Middle-Th-Exp Th-Exp-LSM-N
50000
40000
30000
20000
10000 0
0 1
3
5
7
9
Front Moderator Thickness (cm) Fig. 2. Experimentally measured alpha particle track density (due to thermal neutrons) and recoil proton track density (due to fast neutrons) for the mid detector plotted as a function of front moderator thickness. The calculated yield of thermal and fast neutrons through the Monte Carlo simulation for the mid detector is shown with the lines.
of 31 kHz was used to produce 2.8 MeV neutron beam via D (d, n) reaction. The accelerator was operated with a typical beam current of 3.4 A. The pulsed beam allows to reduce -ray background in prompt -ray pulse height spectrum. The NTDs were mounted in the hollow cavity of the sample and their irradiation was carried out for a 8.3 mC charge measured at electrically isolated neutron producing target. The thermal and fast neutron yield was measured for the mid detector as a function of front moderator thickness over 1–10 cm range of the moderator thickness. Each run of the study continued for 40–45 min. The NTDs were etched chemically to reveal alpha (thermal neutron) tracks and proton (fast neutron) tracks for 3 and 4 h, respectively, with 30% KOH solution at 70◦ C. Finally, the tracks were counted manually under an optical microscope and the track density was calculated for the alpha and recoil proton tracks on the NTDs. Fig. 2 shows the alpha track density (which is proportional to thermal neutron yield) for the middle detector as a function of front moderator thickness. Data for the recoil proton track density (which is proportional to fast neutrons yield) measured by the bare NTD are also shown. The experimental uncertainty in the thermal neutron data ranges about 6–7%. 4. Results and discussion The yield of the thermal and fast neutrons measured at the location of the mid detector in the rectangular moderator is plotted as a function of the front moderator thickness. As shown in Fig. 2, the thermal neutron intensity increases with the front moderator thickness and then drops off. This is consistent with the trends observed previously (Naqvi et al., 2004a, b, 2003, 2006). Within the uncertainties, maximum yield of thermal neutrons for the front, the middle and the back detector is observed
243
2
4
6
8
10
Front Moderator Thickness(cm) Fig. 3. Experimentally measured alpha particle track density for the LSM (Khelifi et al., 1999), the SSM (Naqvi et al., 2003) and the rectangular moderator (mid detector) plotted as a function of front moderator thickness. The lines connecting the data points are results of Monte Carlo simulations.
for 5–7 cm thick front moderator. Also plotted in the figure is yield of fast neutrons measured at the location of front detector. Since the moderating volume of the moderator is adequate to thermalize the neutrons, the fast and thermal neutron curves of the mid detectors have smooth trends. This is clearly shown in Fig. 3, where thermal neutron intensity is plotted from three different size of the moderators of the KFUPM PGNAA as a function of front moderator thickness. The data are shown for the cylindrical small sample moderator (SSM) (Al-Jarallah et al., 2002), cylindrical large sample moderator (LSM) (Naqvi et al., 2003) and the rectangular moderator (mid detector). Maximum yield of the thermal neutrons have been observed for the rectangular moderator, followed by that from the SSM. The lowest yield has been observed for the LSM assembly. The total moderator volume was constant for both the SSM and LSM setups but a larger volume of the sample cavity was used in the LSM setup, thereby reducing effective moderation volume. In the rectangular moderator the sample size can be increased to a maximum diameter of 25.4 cm and a length of 14 cm. In the present study, a 14 cm long cavity with 25.4 cm diameter was used in the rectangular moderator. The moderator volume effects are more pronounced in thermal to fast neutron yield ratio obtained at the sample location. The alpha track density /proton track density ratio (which is proportional to thermal/fast neutron yield ratio) from the three moderators is plotted in Fig. 4 as a function of front moderator thickness. The yields ratio data can be fitted with linear least square fit. Higher value of the slope of thermal/fast yield ratio has been observed for the rectangular moderator. For the SSM and LSM setup, the curves have almost same value of the slope because they have constant moderator volume. The values of slopes of the thermal/fast neutron yield curves for the three moderators are listed in Table 1 along with respective sample volume, moderator volume and sample/moderator volume ratio data. It is interesting to note the inverse correlation between
Alpha Particle Track Density/Proton Track Density
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A.A. Naqvi et al. / Radiation Measurements 42 (2007) 241 – 244
References
14 SSM-Th/Fast-Ratio
12
Rec-Mod-Th/Fast-Ratio
10
LSM-Th/Fast-Ratio
8 6 4 2 0 0
2
4
6
8
10
Front Moderator Thickness (cm) Fig. 4. Experimentally measured alpha particles track density to proton track density ratio for the LSM (Naqvi et al., 2003), the SSM (Naqvi et al., 2003) and the rectangular moderator (mid detector) plotted as a function of front moderator thickness.
the slope of the yields ratio and the corresponding sample/moderator volume ratio. The slope of the yield ratio for the rectangular moderator, is twice that of the LSM setups because LSM has two times larger sample to moderator volume ratio. This correlation is expected because thermal neutron intensity is directly proportional to moderator volume and inversely proportional to void volume (sample volume) in the moderator. 5. Conclusion A thermal and fast neutron yields have been measured at the sample location of the rectangular moderator assembly of the KFUPM PGNAA facility using CR-39 nuclear track detectors (NTDs). The thermal neutron yield measured in the PGNAA setup depends upon the moderation volume of the moderator. The thermal/fast neutron yield ratio is found to have inverse correlation with cavity/moderator volume ratio. The results of the present study for the rectangular moderator along with a similar study for a set of cylindrical moderators has provided a sound base for the useful application of NTDs in verification of design calculations of a PGNAA setup. Acknowledgments The authors wish to acknowledge the support of the Physics Department at King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia.
Al-Jarallah, M.I., Naqvi, A.A., Fazal-ur-Rehman, Abu-jarad, F., 2002. Fast and thermal neutron intensity measurements at the KFUPM PGNAA Setup. Nucl. Instrum. Methods B 195, 435–441. Briesmeister J.F. (Ed.), 1997. MCNP4B—A General Monte Carlo N-Particles Transport Code. Los Alamos National Laboratory Report, LA-12625. Version 4B, Los Alamos National Laboratory Report, LA-12625-M. Collico Savio, D.L., Mariscotti, M.A.J., Ribeiro Guevara, S., 1995. Elemental analysis of a concrete sample by capture gamma rays with a radioisotope neutron source. Nucl. Instrum. Methods Phys. Res. B 95, 379–388. Khelifi, R., Idiri, Z., Omari, L., Seghir, M., 1999. Prompt gamma neutron activation analysis of bulk concrete samples with an Am-Be neutron sources. Appl. Radiat. Isot. 51, 9. Lim, C.S., Tickner, J.R., Sowerby, B.D., Abernethy, D.A., McEwan, A.J., Rainey, S., Stevans, R., Manias, C., Retallack, D., 2001. An on-belt elemental analyzer for the cement industry. Appl. Radiat. Isot. 54, 11. Naqvi, A.A., Fazal-ur-Rehman, Al-Jarallah, M.I., Abujarad, F., Maslehuddin, M., 2003. M. Performance tests of external moderators of a PGNAA setup. Appl. Radiat. Isot. 58, 27–38. Naqvi, A.A., Fazal-ur-Rehman, Nagadi, M.M., Maslehuddin, M., Khateebur-Rehman, Kidwai, S., 2004a. Verification of design calculations of a PGNAA setup using nuclear track detectors. Radiat. Meas. 38/1, 37–41. Naqvi, A.A., Nagadi, M.M., Al-Amoudi, O.S.B., 2004b. Elemental analysis of concrete samples using an accelerator-based PGNAA setup. Nucl. Instrum. Methods in Phys. Res. B 225/3, 331–338. Naqvi, A.A., Nagadi, M.M., Al-Amoudi, O.S.B., 2006. Prompt gamma analysis of chlorine in concrete for corrosion study. Appli. Radiati. Isot. 64/2, 283–289. Olivera, C., Salgado, J., Goncalves, I.F., Carvalho, F.G., Leitao, F., 1993. Prompt gamma–ray neutron activation analysis of cement raw material. J. Nucl. Geophys. 7, 431–443. Olivera, C., Salgado, J., Carvalho, F.G., 1997. Optimization of PGNAA instrument design for cement raw materials using the MCNP code. J. Radioanal. Chem. 216, 191–198. Saleh, H.H., Livingston, R.A., 2000. Experimental evaluation of a portable neutron-based gamma spectroscopy system for chloride measurements in reinforced concrete. J. Radioanal. Nucl. Chem. 244, 367. Sowerby B.D., Watt J.S., 1994. Nuclear techniques for on-line analysis in the minerals and energy industries. In: Proceedings, Nineth Pacific Basin Nuclear Conference, Sydney, Australia, p. 379 Tickner, J., 2000. Determination of the spatial response of neutron based analyser using a Monte Carlo based method. Appl. Radiat. Isot. 53, 507.
Sample to moderator volume ratio effects in neutron yield from a PGNAA setup A.A. Naqvi ∗ , Fazal-ur-Rehman, M.M. Nagadi, Khateeb-ur-Rehman Department of Physics, King Fahd University of Petroleum and Minerals, KFUPM Box 1815, Dhahran-31261, Saudi Arabia Received 3 June 2006; received in revised form 27 September 2006; accepted 30 October 2006
Abstract Performance of a prompt gamma ray neutron activation analysis (PGNAA) setup depends upon thermal neutron yield at the PGNAA sample location. For a moderator, which encloses a sample, thermal neutron intensity depends upon the effective moderator volume excluding the void volume due to sample volume. A rectangular moderator assembly has been designed for the King Fahd University of Petroleum and Minerals (KFUPM) PGNAA setup. The thermal and fast neutron yield has been measured inside the sample cavity as a function of its front moderator thickness using alpha particle tracks density and recoil proton track density inside the CR-39 nuclear track detectors (NTDs). The thermal/fast neutron yield ratio, obtained from the alpha particle tracks density to proton tracks density ratio in the NTDs, shows an inverse correlation with sample to moderator volume ratio. Comparison of the present results with the previously published results of smaller moderators of the KFUPM PGNAA setup confirms the observation. © 2006 Elsevier Ltd. All rights reserved. Keywords: PGNAA; Fast and thermal neutron yield measurement; 2.8 MeV neutrons; NTDs; Monte Carlo simulations
1. Introduction The performance of a Prompt Gamma ray Neutron Activation Analysis (PGNAA) setup depends upon thermal neutron flux available at the sample (Sowerby and Watt, 1994; Naqvi et al., 2003, 2004a, b, 2006; Olivera et al., 1993, 1997; Saleh and Livingston, 2000; Al-Jarallah et al., 2002; Collico Savio et al., 1995; Khelifi et al., 1999; Tickner, 2000; Lim et al., 2001). The thermal neutron flux of a PGNAA setup, which is produced by an external moderator, depends upon moderator volume available to slow down the fast neutrons to thermal energies. Therefore thermal/fast neutron intensity ratio in a PGNAA setup with adequate moderation volume, increases with moderator size. A rectangular moderator assembly has been designed for the PGNAA setup of King Fahd University of Petroleum and Minerals (KFUPM) to analyze bulk samples of cement and concrete for concrete corrosion study (Naqvi et al., 2004a, 2006). The design calculations of the rectangular
∗ Corresponding author. Tel.: +966 3860 4362; fax: +966 3860 4281.
E-mail address: [email protected] (A.A. Naqvi). 1350-4487/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2006.10.003
moderator have been verified experimentally through thermal neutron yield measurements as a function of its front moderator thickness using CR-39 nuclear track detectors (NTDs) (Naqvi et al., 2004a). The thermal neutron yield was measured inside the sample volume of the rectangular moderator by two NTDs fixed at back and front end of the sample cavity. Previously thermal neutron flux was measured using NTDs inside two different cylindrical moderators with 25.4 cm outer diameter and 14 cm length (Al-Jarallah et al., 2002; Naqvi et al., 2003). One moderator called ‘SSM’ was used with a sample having 7 cm radius while the other moderator called ‘LSM’ was used with a sample having a 8.5 cm radius (Al-Jarallah et al., 2002). It was observed that maximum yield of thermal neutron from the LSM was 22 ± 6% smaller than that from the SSM (Naqvi et al., 2003). This was expected because the volume of the LSM was about 17% smaller than that of the SSM. Now a large size rectangular moderator has been designed for the KFUPM PGNAA setup (Naqvi et al., 2004a, b). In this study, the thermal and fast neutron yield has been measured inside the moderator in the sample volume using NTDs. Results of this study are compared to those of the SSM and LSM data to study moderator volume dependence of the neutron yield.
242
A.A. Naqvi et al. / Radiation Measurements 42 (2007) 241 – 244
2. The KFUPM PGNAA setup with the rectangular moderator
regions of the moderator, being first a region located between the neutron source and the sample front end (called front moderator) and secondly in the moderator reflector collar surrounding the sample along its length. The fast neutrons that escape from the front moderator region, are reflected by the collar into the sample region. Fig. 1 shows the PGNAA setup with the rectangular moderator and the high density polyethylene plug with a sample with diameter less than the maximum diameter. The optimum value of the sample radius, front moderator thickness and the sample length were assumed to be the one which produced maximum yield of the prompt gamma rays at the detector. The optimum dimension of the sample to be used with the rectangular moderator is given in Table 1. For sake of comparison the similar data for the cylindrical moderator (Naqvi et al., 2003) is also listed in Table 1.
The rectangular moderator assembly of the KFUPM PGNAA setup has been described in detail elsewhere (Naqvi et al., 2004a, b) but for completion sake it will be described here briefly. The setup consists of a cylindrical sample placed inside a rectangular moderator. The moderator is made of paraffin wax and has a central cylindrical cavity, which can accommodate a sample with a maximum diameter of 25.4 cm. The rectangular moderator was designed for a 2.8 MeV neutron based PGNAA setup for the analysis of cement and concrete samples. The rectangular moderator is 19 cm thick and has a cross section of 49 cm × 49 cm (width x height). For a sample with an optimum diameter less than the maximum value, a high density polyethylene plug is used, which fills the gap between the sample and the walls of the cavity. Fast neutrons are thermalized in two
3. Fast and thermal neutron yield measurements
Fig. 1. Schematic representation of the PGNAA setup used to measure fast and thermal neutrons (Naqvi et al., 2004a). In thermal neutron measurements, the sample is replaced by a nuclear track detectors (NTDs) called middle detector placed in the middle of the front and back nuclear track detectors shown in the figure.
The thermal and fast neutron yield was measured inside the PGNAA moderator at the middle point of sample as a function of the thickness of the front moderator using NTDs. The cavity full volume was used without plugin to study the maximum effect of loss of moderation volume. One NTD was mounted in the middle of the cavity (mid detector) with reference to the incident neutron beam. The mid detector was placed at a distance of 7 cm from the front moderator and it was in the middle of the front and back detectors shown in Fig. 1 (Naqvi et al., 2004a). The mid detector was masked with boron converter to measure thermal neutron intensity via (n, ) reaction while a second NTD without boron converter, was placed next to the mid detector to measure fast neutron intensity at that location. The NTDs consist of 0.5 mm thick CR-39 NTD (Poly Allyle Diglycole Carbonate(PADC)–C12 H18 O7 ) with an area of 1.5 cm × 1.5 cm. The mid detector represents the average intensity of the thermal neutrons at the sample location. For sake of comparison with experimental data, thermal neutron yield of the middle detector was calculated using Monte Carlo code MCNP4B2 (Briesmeister, 1997) following the procedure described elsewhere (Naqvi et al., 2004a). The measurements were carried out using the PGNAA setup, built at the 45◦ beam line of the KFUPM 350 keV accelerator laboratory (Naqvi et al., 2004a, b, 2006). A 200 keV deuteron beam with 4.8 ns pulse width and a pulse repetition rate
Table 1 Dimensions of the rectangular and the cylindrical moderator assemblies of the KFUPM PGNAA setup Moderator parameter
Rectangular moderator
Cylindrical large sample moderator (LSM) (Naqvi et al., 2003)
Cylindrical small sample moderator (SSM) (Naqvi et al., 2003)
Moderator size (diameter) Sample radius (cm) Front moderator thickness (cm) Sample length (cm) Sample volume (Li) Moderator volume (Li) Sample/moderator volume ratio (%) Slope of therm/fast neutron yield
49 cm × 49 cm (width × height) 12–14 5–6 14 6.87 45.62 15.0 1.39 ± 0.1
25 cm diameter
25 cm diameter
8.47 3–4 14 2.704 9.33 28.9 0.63 ± 0.1
7 3–4 14 2.155 9.33 23.1 0.68 ± 0.1
Alpha Particles Track Density (Arbitrary Units)
Track Density Due to Neutrons (Arbitrary Units)
A.A. Naqvi et al. / Radiation Measurements 42 (2007) 241 – 244
60000 Rec-Middle-Det-Therm Rec-Middle-Fast
40000
20000
60000
Th-Exp-SSM Rec-Middle-Th-Exp Th-Exp-LSM-N
50000
40000
30000
20000
10000 0
0 1
3
5
7
9
Front Moderator Thickness (cm) Fig. 2. Experimentally measured alpha particle track density (due to thermal neutrons) and recoil proton track density (due to fast neutrons) for the mid detector plotted as a function of front moderator thickness. The calculated yield of thermal and fast neutrons through the Monte Carlo simulation for the mid detector is shown with the lines.
of 31 kHz was used to produce 2.8 MeV neutron beam via D (d, n) reaction. The accelerator was operated with a typical beam current of 3.4 A. The pulsed beam allows to reduce -ray background in prompt -ray pulse height spectrum. The NTDs were mounted in the hollow cavity of the sample and their irradiation was carried out for a 8.3 mC charge measured at electrically isolated neutron producing target. The thermal and fast neutron yield was measured for the mid detector as a function of front moderator thickness over 1–10 cm range of the moderator thickness. Each run of the study continued for 40–45 min. The NTDs were etched chemically to reveal alpha (thermal neutron) tracks and proton (fast neutron) tracks for 3 and 4 h, respectively, with 30% KOH solution at 70◦ C. Finally, the tracks were counted manually under an optical microscope and the track density was calculated for the alpha and recoil proton tracks on the NTDs. Fig. 2 shows the alpha track density (which is proportional to thermal neutron yield) for the middle detector as a function of front moderator thickness. Data for the recoil proton track density (which is proportional to fast neutrons yield) measured by the bare NTD are also shown. The experimental uncertainty in the thermal neutron data ranges about 6–7%. 4. Results and discussion The yield of the thermal and fast neutrons measured at the location of the mid detector in the rectangular moderator is plotted as a function of the front moderator thickness. As shown in Fig. 2, the thermal neutron intensity increases with the front moderator thickness and then drops off. This is consistent with the trends observed previously (Naqvi et al., 2004a, b, 2003, 2006). Within the uncertainties, maximum yield of thermal neutrons for the front, the middle and the back detector is observed
243
2
4
6
8
10
Front Moderator Thickness(cm) Fig. 3. Experimentally measured alpha particle track density for the LSM (Khelifi et al., 1999), the SSM (Naqvi et al., 2003) and the rectangular moderator (mid detector) plotted as a function of front moderator thickness. The lines connecting the data points are results of Monte Carlo simulations.
for 5–7 cm thick front moderator. Also plotted in the figure is yield of fast neutrons measured at the location of front detector. Since the moderating volume of the moderator is adequate to thermalize the neutrons, the fast and thermal neutron curves of the mid detectors have smooth trends. This is clearly shown in Fig. 3, where thermal neutron intensity is plotted from three different size of the moderators of the KFUPM PGNAA as a function of front moderator thickness. The data are shown for the cylindrical small sample moderator (SSM) (Al-Jarallah et al., 2002), cylindrical large sample moderator (LSM) (Naqvi et al., 2003) and the rectangular moderator (mid detector). Maximum yield of the thermal neutrons have been observed for the rectangular moderator, followed by that from the SSM. The lowest yield has been observed for the LSM assembly. The total moderator volume was constant for both the SSM and LSM setups but a larger volume of the sample cavity was used in the LSM setup, thereby reducing effective moderation volume. In the rectangular moderator the sample size can be increased to a maximum diameter of 25.4 cm and a length of 14 cm. In the present study, a 14 cm long cavity with 25.4 cm diameter was used in the rectangular moderator. The moderator volume effects are more pronounced in thermal to fast neutron yield ratio obtained at the sample location. The alpha track density /proton track density ratio (which is proportional to thermal/fast neutron yield ratio) from the three moderators is plotted in Fig. 4 as a function of front moderator thickness. The yields ratio data can be fitted with linear least square fit. Higher value of the slope of thermal/fast yield ratio has been observed for the rectangular moderator. For the SSM and LSM setup, the curves have almost same value of the slope because they have constant moderator volume. The values of slopes of the thermal/fast neutron yield curves for the three moderators are listed in Table 1 along with respective sample volume, moderator volume and sample/moderator volume ratio data. It is interesting to note the inverse correlation between
Alpha Particle Track Density/Proton Track Density
244
A.A. Naqvi et al. / Radiation Measurements 42 (2007) 241 – 244
References
14 SSM-Th/Fast-Ratio
12
Rec-Mod-Th/Fast-Ratio
10
LSM-Th/Fast-Ratio
8 6 4 2 0 0
2
4
6
8
10
Front Moderator Thickness (cm) Fig. 4. Experimentally measured alpha particles track density to proton track density ratio for the LSM (Naqvi et al., 2003), the SSM (Naqvi et al., 2003) and the rectangular moderator (mid detector) plotted as a function of front moderator thickness.
the slope of the yields ratio and the corresponding sample/moderator volume ratio. The slope of the yield ratio for the rectangular moderator, is twice that of the LSM setups because LSM has two times larger sample to moderator volume ratio. This correlation is expected because thermal neutron intensity is directly proportional to moderator volume and inversely proportional to void volume (sample volume) in the moderator. 5. Conclusion A thermal and fast neutron yields have been measured at the sample location of the rectangular moderator assembly of the KFUPM PGNAA facility using CR-39 nuclear track detectors (NTDs). The thermal neutron yield measured in the PGNAA setup depends upon the moderation volume of the moderator. The thermal/fast neutron yield ratio is found to have inverse correlation with cavity/moderator volume ratio. The results of the present study for the rectangular moderator along with a similar study for a set of cylindrical moderators has provided a sound base for the useful application of NTDs in verification of design calculations of a PGNAA setup. Acknowledgments The authors wish to acknowledge the support of the Physics Department at King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia.
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