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Measurements of the 11B(d,nγ15.1)12C differential cross-section on thick and thin targets
Nuclear Instruments and Methods in Physics Research B 305 (2013) 45–50
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Measurements of the and thin targets
11
Bðd; nc15:1 Þ12 C differential cross-section on thick
Kevin W. Cooper ⇑, Thomas N. Massey, D.E. Carter, David C. Ingram Department of Physics and Astronomy, Institute of Nuclear and Particle Physics, Edwards Accelerator Laboratory, Ohio University, Athens, OH 45701, USA
a r t i c l e
i n f o
Article history: Received 18 September 2012 Received in revised form 25 March 2013 Available online 29 April 2013 Keywords: Bðd; cÞ Fðp; cÞ Thick target gamma BGO detector High energy c
a b s t r a c t The differential cross-section for the 15.1 MeV gamma ray produced by the 11 Bðd; ncÞ12 C reaction in a thick natural boron target has been measured for incident deuteron energies ranging from reaction threshold to 5 MeV. Measurements for a thin natural boron target have been carried out over a similar incident deuteron energy range. These results are compared to previous measurements made by Kavanagh (1958) and Kuan (1964). Measurements of the combined thick target yield for the 6.129, 6.917, and 7.116 MeV gamma rays from the 19 Fðp; acÞ16 O reaction have been carried out on a stopping thickness sulfur hexafluoride gas cell for effective incident proton energies ranging from 1 to 4 MeV as a consistency check on the procedure used for normalization of the detector response function. The results for the 11 Bðd; nc15:1 Þ12 C yield a significantly lower cross-section than that previously reported, while the measurements of the 19 Fðp; acÞ reaction are consistent with previous measurements made by Fessler (2000) and Micklich (2003). Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Relatively high energy gamma beams (P6 MeV) are of interest as possible probe beams for active interrogation of shielded fissile materials [1,2]. One possibility for the production of these beams comes from reactions with sufficiently positive Q values such that the incident particle beam energy is attainable by small accelerators. Two possible candidate reactions for the generation of these probe beams are the 11 Bðd; ncÞ12 C reaction (Q ¼ þ13:73 MeV), and the 19 Fðp; acÞ16 O reaction (Q ¼ þ8:115 MeV). The 11 Bðd; ncÞ12 C reaction is considered a favorable candidate for the source of a probe beam [3]. The reaction produces a 15.1 MeV gamma from an excited state of 12 C with an incident deuteron energy threshold of 1.63 MeV. This gamma energy corresponds to a value close to the photofission cross-section peak for 235 U and 239 Pu. Previous measurements of the values for the cross-section have been published by Kavanagh and Barnes [4] and Kuan et al. [5], but are reported with large uncertainties, 24% and 50%, respectively. In order for the reaction to be considered as a possible probe source and for inclusion in data libraries to improve the accuracy of simulations, the measurement of the cross-section should be carried out to a lower uncertainty. The thick target cross-section is of greater interest for application purposes due to the associated increased yield in comparison to the thin target. ⇑ Corresponding author. Tel.: +1 740 593 1980; fax: +1 740 593 1436. E-mail address: [email protected] (K.W. Cooper). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.04.048
The 19 Fðp; acÞ16 O reaction produces three gamma rays, (6.129, 6.917, and 7.116 MeV), at the lower range of interest for induced photofission detection of fissile materials. The advantage of this reaction as a probe beam is the lack of neutrons from 19 Fðp; acÞ16 O reaction itself. This would possibly allow a lower neutron background for measurement of photofission products from the shielded fissile material undergoing interrogation. Previous measurements of this reaction have been made by Fessler et al. [6] and Micklich et al. [7]. In the former work the measurements were made on a solid target (CaF2 ) with the determination of the yield given with 5–10% precision. In the latter work measurements were made on solid (CaF2 ; MgF2 ) targets and on a gas cell (SF6 ) with uncertainties in the yield again given as 5–10%. In both cases NaI detectors were utilized for the measurements. The reported measurements of this thick target yield serve as a consistency check on the methods used to normalize the detector response function for the 11 Bðd; nc15:1 Þ12 C cross-section. 2. Experimental details Measurements of the cross-sections were carried out using the 4.5 MV tandem Van de Graff style accelerator at Edwards Accelerator Lab located at Ohio University in Athens, Ohio. The gamma spectra from the reactions were obtained using a BGO detector from St. Gobain fitted with a Hamamatsu photomultiplier tube. The detector contained a 4 in. long and 4 in. diameter cylindrical bismuth germanate crystal which has the advantage of high gamma absorption efficiency (near 97%) at the energies of interest. The
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K.W. Cooper et al. / Nuclear Instruments and Methods in Physics Research B 305 (2013) 45–50
cylindrical natural boron stopping target used for the Bðd; ncÞ measurements was 3 mm thick and 25 mm in diameter. The thin boron target used for the measurements was supplied by Lebow with a specification of 1 l thickness with an uncertainty of ±10% as measured by a quartz crystal microbalance thickness monitor. Rutherford Backscattering measurements of a foil from the same manufacturing batch performed at Edwards Accelerator Lab yielded a thickness of approximately 910 nm based upon the stopping power of the boron. This thickness corresponds to an energy thickness of approximately 46 and 36 keV to deuterons with 2.0 and 3.0 MeV incident energy, respectively. The boron targets were contained in electrically isolated nylon holders with thin (0.5 mm) 99.998% pure Al end caps, as shown in Fig. 1. An 8 cm long electrically isolated gas cell with a 7 l thick Ni entrance window was operated at 14 psi of sulfur hexafluoride, ðSF6 Þ, to serve as a stopping target for the incident proton beam during the Fðp; acÞ measurements. In all cases an electron suppression ring with 300 V bias was prior to the target in order to reduce the effect of secondary electrons generated at the target on the beam current integration. Spectra were taken on the gas cell containing ultra-high purity He at 14 psi, and on the boron target holder under vacuum without the boron target to confirm the absence of signals in the energy ranges of the gamma rays to be measured.
The BGO detector was placed at 0° with respect to the beam and at a distance of 7.4 m during the measurements of the thick target Bðd; ncÞ reaction. The large distance for detector placement was chosen in order to reduce the count rate and lower dead time in the data acquisition system resulting from the large yield from this reaction and was used for the majority of the thick target measurements. The detector was moved to a distance of 3 m during measurements carried out at incident beam energy near the reaction threshold. Measurements of the thin target Bðd; ncÞ reaction were carried out with the detector at a distance of 1.1 m from the target. During the measurements of the Fðp; acÞ reaction the detector was placed at 3.2 m from the target. In all measurements the detector was 1.78 m from the concrete floor of the experimental area and a greater distance from other objects of significant mass including walls and ceiling. Energy calibration for the detection system was performed by reference to known peaks within the pulse height spectra from the reactions as well as a reference Pu(Be) source. Deuteron and proton beams were pulsed and bunched such that a time-of-flight spectrum was obtained during the runs. The standoff distance of the detector during the measurements allows for accurate separation of the gamma peak from signals associated with the beam on upstream collimation. The TAC signal from the time-of-flight spectrum was used via a single channel analyzer to produce an external gate signal for the ADC assigned to the pulse height spectrum. This allowed the pulse height spectrum to be gated on the gamma peak in the time-of-flight spectrum in order to reduce background in the measurements. 3. Results and discussion
Fig. 1. Sketch of electrically isolated nylon target holder used for the boron targets. The 3 mm thick and 25 mm cylindrical boron target was set inside the Al end cap and flush against the nylon target holder that mounted to a snout on the accelerator vacuum beam line. The thin boron target was similarly mounted via the 17 mm outer diameter support ring. The Al cap overlapped an o-ring on the nylon piece providing for the vacuum seal. An Al plate with 300 V bias served as electron suppression for the target assembly.
The use of time-of-flight gating allowed for a dramatic reduction in background counts during the 11 Bðd; ncÞ12 C measurements. An example of the ungated pulse height spectrum compared to a spectrum with a time-of-flight window gate placed on the gamma peak is given in Fig. 2. This method is not only useful in the reduction of the background counts in the spectrum, but also has the benefit of reducing system dead time through the rejection of signals associated with those background counts in the ADC system. At the highest beam energies in the thick target Bðd; ncÞ measurements this allowed the count rate in the detection system to be reduced from 35 kHz to 6.5 kHz. The reduction of background in
Fig. 2. Pulse height spectrum comparison between an ungated and time-of-flight gated measurement for the 11 Bðd; ncÞ12 C reaction with 3.5 MeV incident deuteron energy on a thick boron target using a BGO detector. The gating window was set on the gamma peak in the time-of-flight spectrum. This illustrates the reduction in background counts due to the tof gating method.
K.W. Cooper et al. / Nuclear Instruments and Methods in Physics Research B 305 (2013) 45–50
the Fðp; acÞ pulse height spectrum using the same method was far less significant to the point that its effect is mainly negligible. System live-time was nominally 95% for most measurements, with higher values during measurements on the thin boron target. The system live-time decreased at higher incident beam energies for both the Bðd; ncÞ and Fðp; acÞ measurements due to the increasing reaction yields at the associated values. In the case of the thick target Bðd; ncÞ measurements the system live-time fell to 75% at the highest incident beam energy. The highest incident beam energy measurement in the Fðp; acÞ had a system live-time of 86%. The detector response function for all sets of measurements was modeled using the MCNP5 code [8] by examining the results of the F8 tally with the Gaussian Energy Broadening (GEB) tally modifier. The simulation was ran in full electron transport mode (mode p e), and not by employing the thick target Bremsstrahlung approximation for the electron transportp(mode p). The MCNP applied enffiffiffi ergy broadening follows a DE / E relation, and corresponds to a DE = 0.43 MeV and DE = 0.46 MeV at 15.1 MeV for the thick and thin boron target measurements respectively. The energy broadening applied to the Fðp; acÞ measurements corresponded to a DE ¼ 0:25 MeV at 6.5 MeV. The input into the MCNP model included the BGO crystal, acrylic light guide, Al casing, lead brick housing/ holder, and the air path from the source to the detector. This model is likely to underestimate the Compton scattering from other materials in the measurement area, thus there could be a systematic error in that the measured pulse height spectrum could contain counts from scattering not present in the model. It should be noted that this error would be minimized by the large distance separating the detector from other objects of significant mass coupled with the time-of-flight gating method used in measuring the spectra. The normalization of the pulse height spectra to detector response was calculated by comparing the integrated counts in an energy region of the measured pulse height spectrum to that of the MCNP modeled pulse height spectrum. The energy region chosen for the thick target Bðd; ncÞ spectrum was from 13.5 to 16 MeV. This higher energy region was chosen in consideration of reducing the effect of background counts in the measured spectrum. The fractional sum of the MCNP5 modeled pulse height spectrum for the 13.5–16 MeV energy region was calculated to be 0.593 for the detector at its most distant position, and 0.567 for the detector at its closest position. A computer run time of nearly 67 h on a machine with a 2.2 GHz Celeron processor was used to simulate
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1:6 1010 particles from the source. A comparison of the measured pulse height spectrum to the detector modeled detector response for the 15.1 MeV gamma is given in Fig. 3. The same method was applied to the normalization of the detector response for the thin boron target measurements. The energy region of 14–15.8 MeV in the thin boron target pulse height spectrum was found to have a fractional sum of 0.396 of the total modeled pulse height spectrum. A comparison of the measured to modeled detector response for the thin boron target 15.1 MeV gamma is given in Fig. 4. The energy region chosen for the Fðp; acÞ reaction was 5–8 MeV. The efficiency of the detector to each of the three gamma rays in this region was found to be constant within our modeling parameters, and thus a summing of the pulse height in this region is independent of the relative proportion of the three gamma ray constituents. This region was also chosen due to consideration of background count reduction, although it is far less problematic for this reaction compared to the prolific Bðd; ncÞ reaction. The 5– 8 MeV region was found to contain a fractional sum of 0:758 of the total modeled pulse height spectrum. An example of the pulse height spectrum from the Fðp; acÞ reaction is given in Fig. 5. Measurements of the thick target 11 Bðd; nc15:1 Þ reaction differential cross-section were carried out at incident deuteron beam energies ranging from near reaction threshold to 5 MeV. The beam energy was taken in steps of 10 keV through the threshold value and near the peak in the differential cross-section at 3.08 incident deuteron energy. These measurements confirmed both the previously reported threshold and peak in the excitation function as well as serving as a consistency check for systematic error in incident beam energy. Larger energy steps were taken between points away from the threshold or peak feature in the excitation function. In the calculation of the differential cross-section for the 15.1 MeV gamma generated in the boron reaction the target areal density used was that required to cause the incident deuteron sufficient energy loss to fall below the threshold of the reaction (1.63 MeV). Energy loss calculations were made using the well known SRIM [9] package. Uncertainties in the values of the differential cross sections are approximately 5% for most measurements ranging up to 10% with higher values near the threshold. Uncertainties in the measurement are mainly attributed to uncertainties in target stopping power, beam current integration, and determination of detector solid angle, while those measurements near the threshold are dominated by counting statistics.
Fig. 3. Comparison between the measured pulse height spectrum and the MCNP5 modeled pulse height spectrum for the 15.1 MeV gamma incident upon the BGO detector. The measured pulse height spectrum was taken from the thick target Bðd; ncÞ reaction with 2.5 MeV incident deuteron beam energy. The indicated energy region is used to determine the fractional sum of the detector response for use in normalization of the measured 15.1 MeV gamma yield.
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K.W. Cooper et al. / Nuclear Instruments and Methods in Physics Research B 305 (2013) 45–50
Fig. 4. Comparison between the measured pulse height spectrum and the MCNP5 modeled pulse height spectrum for the 15.1 MeV gamma incident upon the BGO detector. The measured pulse height spectrum was taken from the thin target Bðd; ncÞ reaction with 2.2 MeV incident deuteron beam energy. The indicated energy region is used to determine the fractional sum of the detector response for use in normalization of the measured 15.1 MeV gamma yield.
Fig. 5. A sample pulse height spectrum from the Fðp; acÞ reaction measurements with incident effective proton energy of 2.5 MeV. The vertical lines indicate the summing region 5–8 MeV for normalization with the MCNP5 detector response function model. Two of the gamma peaks (7.116 and 6.917 MeV) are not separately resolved while the 6.129 MeV peak is distinguishable. Also present is the unresolved single escape peak from the two higher energy gamma rays and the single escape peak from the 6.129 MeV gamma.
The measurements of the thin target 11 Bðd; nc15:1 Þ reaction differential cross-section were carried out over a similar beam energy range. The lowest incident beam energy being 1.8 MeV and the highest at 4.5 MeV. The results of thick and thin target measurements are compared to those carried out by Kuan et al. [5] and given in Fig. 6. Uncertainties in the values are nominally 9–10% with a systematic uncertainty in the target thickness being the largest contribution to the estimate, followed by counting statistics for measurements at incident beam energies approaching threshold and at or above 4.0 MeV. The values found by this group tend to be significantly lower than those reported by Kuan and higher value measurements, not reported here, previously given by Kavanagh and Barnes [4]. Two facts concerning the comparison of the sets of measurements are of consideration. First, the measurements reported by Kuan were given with uncertainties of ±50% and ±24% for Kavanagh. Secondly, it has been suggested [10] that the methods used by
Kavanagh and presumably followed by Kuan to determine the NaI detector response function used in their work could have led to an overestimation of the 15.1 MeV gamma yield that was measured. The method that was used by Kavanagh consisted of taking the difference between a pulse height spectrum taken with incident deuteron energy above reaction threshold with that of a spectrum with incident deuteron energy below reaction threshold. This result was then used to approximate the low energy tailing of the detector response function as a constant Compton plateau that was extrapolated to zero pulse height. The current state of computer simulations allow for a more accurate modeling of the detector response function. Our modeling of the detector response function with the setup reported by Kavanagh (and presumably Kuan) using MCNP5 shows an overestimation of the integrated counts by a factor of 1.7. This would bring the differential cross-section measurements reported by Kuan into agreement with our findings within experimental uncertainties.
K.W. Cooper et al. / Nuclear Instruments and Methods in Physics Research B 305 (2013) 45–50
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Fig. 6. Measured thick and thin target differential cross-sections for the 11 Bðd; nc15:1 Þ reaction are compared to the thin target measurements by Kuan et al. [5]. Thick target areal density for the purpose of our calculations was calculated as the thickness required to cause sufficient energy loss in the incident deuteron beam to reach the reaction threshold energy of 1.63 MeV.
Fig. 7. Measurements of the Fðp; acÞ thick target yield compared to those made previously by Micklich et al. [7]. Included is a fit to the excitation function provided in the work by Micklich. The uncertainties in the measurements by the current work range from 4% to 7% at the lowest beam energies. Included for comparison are measurements made by Anttila [11] and Kiss [12].
Measurements for the thick target yield of the sum of the 6.129, 6.917, and 7.116 MeV gamma rays from the 19 Fðp; acÞ16 O reaction at 0° with respect to the beam are shown in Fig. 7. The data show a marked increase in the gamma yield starting near an effective incident proton beam energy of 2 MeV followed by a plateau near 3.5 MeV. The effective incident energy of the proton beam is determined by reducing the energy of the beam by the loss, as determined using SRIM [9] calculations, through the 7 l thick Ni entrance window to the SF6 gas cell. Uncertainties in the values range from 4% to 7%. The measurements made in the current work are in relative agreement with those determined previously by Fessler [6] and Micklich et al. [7]. The change in relative yields of the three gamma rays as a function of incident beam energy was not determined. The thick target yield for the three gamma rays from the 19 Fðp; acÞ16 O reaction was found to be isotropic to within roughly 15% for the three angles measured, namely 0°, 55°, and 90° with respect to the beam. The results of the measurements as a function of
effective beam energy are given in Fig. 7. This result is also in agreement with the previously cited measurements by Fessler and Micklich.
4. Conclusions Measurements of the differential cross-sections for the Bðd; nc15:1 Þ12 C reaction on thick and thin targets have generated results that are significantly lower than those previously measured. The measurement of the thick target yield of the 6.129, 6.917, and 7.116 MeV gamma rays from the 19 Fðp; acÞ16 O reaction have been consistent with those previously measured. An accurate determination of the differential cross sections is interesting from a basic research standpoint, and useful in the inclusion of the values in data libraries used for simulation purposes. The thick target 11 Bðd; nc15:1 Þ12 C measurements being of greater interest for application as an active interrogation probe beam source. 11
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K.W. Cooper et al. / Nuclear Instruments and Methods in Physics Research B 305 (2013) 45–50
The 11 Bðd; nc15:1 Þ12 C reaction has two advantages in regards to its use as the source of a probe beam for active interrogation. The 15.1 MeV gamma is a very advantageous energy in the photofission cross-section for 235 U and 239 Pu, and the reaction is quite prolific. It has the possible disadvantage in that there is associated generation of neutrons that could interfere with the discrimination of the neutrons generated during photofission. The 19 Fðp; acÞ16 O reaction at the measured beam energies has the advantage for the use as an active interrogation probe of a relatively clean spectrum such that the byproducts of an induced photofission are clearly distinguishable from the probe. It has the disadvantage in the gamma rays generated are at the edge of the favorable energy range of induced photofission in the fissile materials of interest. Acknowledgments The authors would like to gratefully acknowledge the Defense Threat Reduction Agency for funding contributing to the completion of this work, and many conversations with Terry Taddeucci of Los Alamos National Laboratory.
References [1] B.J. Micklich, D.L. Smith, Nucl. Instr. Meth. B 241 (2005) 782. [2] A. Proctor, T.A. Gabriel, A.W. Hunt, J. Manges, T. Handler, Nucl. Instr. Meth. A 662 (2012) 71. [3] M.B. Goldberg et al., IAEA, International Topical Meeting on Research Applications and Utilization of Accelerators, 2009 (URL:). [4] R.W. Kavanagh, C.A. Barnes, Phys. Rev. 112 (1958) 503. [5] H.-M. Kuan, P.R. Almond, G.U. Din, T.W. Bonner, Nucl. Phys. 60 (1964) 509. [6] A. Fessler, T.N. Massey, B.J. Micklich, D.L. Smith, Nucl. Instr. Meth. A 450 (2000) 353. [7] B.J. Micklich, D.L. Smith, T.N. Massey, C.L. Fink, D. Ingram, Nucl. Instr. Meth. A 505 (2003) 1. [8] X-5 Monte Carlo Team, MCNP – A General N-Particle Transport Code, Version 5 LA-UR-03-1987 (2003, rev. 2005). [9] J.F. Ziegler, M.D. Ziegler, J.P. Biersack et al., SRIM Code, 2008, URL: . [10] T.N. Taddeucci, personal communication. [11] A. Anttila, R. Hanninen, J. Raisanen, J. Radioanal. Nucl. Chem. 62 (1981) 293. [12] A.Z. Kiss, E. Koltay, B. Nyako, E. Somorjai, A. Anttila, J. Raisanen, J. Radioanal. Nucl. Chem. 89 (1985) 123.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Measurements of the and thin targets
11
Bðd; nc15:1 Þ12 C differential cross-section on thick
Kevin W. Cooper ⇑, Thomas N. Massey, D.E. Carter, David C. Ingram Department of Physics and Astronomy, Institute of Nuclear and Particle Physics, Edwards Accelerator Laboratory, Ohio University, Athens, OH 45701, USA
a r t i c l e
i n f o
Article history: Received 18 September 2012 Received in revised form 25 March 2013 Available online 29 April 2013 Keywords: Bðd; cÞ Fðp; cÞ Thick target gamma BGO detector High energy c
a b s t r a c t The differential cross-section for the 15.1 MeV gamma ray produced by the 11 Bðd; ncÞ12 C reaction in a thick natural boron target has been measured for incident deuteron energies ranging from reaction threshold to 5 MeV. Measurements for a thin natural boron target have been carried out over a similar incident deuteron energy range. These results are compared to previous measurements made by Kavanagh (1958) and Kuan (1964). Measurements of the combined thick target yield for the 6.129, 6.917, and 7.116 MeV gamma rays from the 19 Fðp; acÞ16 O reaction have been carried out on a stopping thickness sulfur hexafluoride gas cell for effective incident proton energies ranging from 1 to 4 MeV as a consistency check on the procedure used for normalization of the detector response function. The results for the 11 Bðd; nc15:1 Þ12 C yield a significantly lower cross-section than that previously reported, while the measurements of the 19 Fðp; acÞ reaction are consistent with previous measurements made by Fessler (2000) and Micklich (2003). Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Relatively high energy gamma beams (P6 MeV) are of interest as possible probe beams for active interrogation of shielded fissile materials [1,2]. One possibility for the production of these beams comes from reactions with sufficiently positive Q values such that the incident particle beam energy is attainable by small accelerators. Two possible candidate reactions for the generation of these probe beams are the 11 Bðd; ncÞ12 C reaction (Q ¼ þ13:73 MeV), and the 19 Fðp; acÞ16 O reaction (Q ¼ þ8:115 MeV). The 11 Bðd; ncÞ12 C reaction is considered a favorable candidate for the source of a probe beam [3]. The reaction produces a 15.1 MeV gamma from an excited state of 12 C with an incident deuteron energy threshold of 1.63 MeV. This gamma energy corresponds to a value close to the photofission cross-section peak for 235 U and 239 Pu. Previous measurements of the values for the cross-section have been published by Kavanagh and Barnes [4] and Kuan et al. [5], but are reported with large uncertainties, 24% and 50%, respectively. In order for the reaction to be considered as a possible probe source and for inclusion in data libraries to improve the accuracy of simulations, the measurement of the cross-section should be carried out to a lower uncertainty. The thick target cross-section is of greater interest for application purposes due to the associated increased yield in comparison to the thin target. ⇑ Corresponding author. Tel.: +1 740 593 1980; fax: +1 740 593 1436. E-mail address: [email protected] (K.W. Cooper). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.04.048
The 19 Fðp; acÞ16 O reaction produces three gamma rays, (6.129, 6.917, and 7.116 MeV), at the lower range of interest for induced photofission detection of fissile materials. The advantage of this reaction as a probe beam is the lack of neutrons from 19 Fðp; acÞ16 O reaction itself. This would possibly allow a lower neutron background for measurement of photofission products from the shielded fissile material undergoing interrogation. Previous measurements of this reaction have been made by Fessler et al. [6] and Micklich et al. [7]. In the former work the measurements were made on a solid target (CaF2 ) with the determination of the yield given with 5–10% precision. In the latter work measurements were made on solid (CaF2 ; MgF2 ) targets and on a gas cell (SF6 ) with uncertainties in the yield again given as 5–10%. In both cases NaI detectors were utilized for the measurements. The reported measurements of this thick target yield serve as a consistency check on the methods used to normalize the detector response function for the 11 Bðd; nc15:1 Þ12 C cross-section. 2. Experimental details Measurements of the cross-sections were carried out using the 4.5 MV tandem Van de Graff style accelerator at Edwards Accelerator Lab located at Ohio University in Athens, Ohio. The gamma spectra from the reactions were obtained using a BGO detector from St. Gobain fitted with a Hamamatsu photomultiplier tube. The detector contained a 4 in. long and 4 in. diameter cylindrical bismuth germanate crystal which has the advantage of high gamma absorption efficiency (near 97%) at the energies of interest. The
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K.W. Cooper et al. / Nuclear Instruments and Methods in Physics Research B 305 (2013) 45–50
cylindrical natural boron stopping target used for the Bðd; ncÞ measurements was 3 mm thick and 25 mm in diameter. The thin boron target used for the measurements was supplied by Lebow with a specification of 1 l thickness with an uncertainty of ±10% as measured by a quartz crystal microbalance thickness monitor. Rutherford Backscattering measurements of a foil from the same manufacturing batch performed at Edwards Accelerator Lab yielded a thickness of approximately 910 nm based upon the stopping power of the boron. This thickness corresponds to an energy thickness of approximately 46 and 36 keV to deuterons with 2.0 and 3.0 MeV incident energy, respectively. The boron targets were contained in electrically isolated nylon holders with thin (0.5 mm) 99.998% pure Al end caps, as shown in Fig. 1. An 8 cm long electrically isolated gas cell with a 7 l thick Ni entrance window was operated at 14 psi of sulfur hexafluoride, ðSF6 Þ, to serve as a stopping target for the incident proton beam during the Fðp; acÞ measurements. In all cases an electron suppression ring with 300 V bias was prior to the target in order to reduce the effect of secondary electrons generated at the target on the beam current integration. Spectra were taken on the gas cell containing ultra-high purity He at 14 psi, and on the boron target holder under vacuum without the boron target to confirm the absence of signals in the energy ranges of the gamma rays to be measured.
The BGO detector was placed at 0° with respect to the beam and at a distance of 7.4 m during the measurements of the thick target Bðd; ncÞ reaction. The large distance for detector placement was chosen in order to reduce the count rate and lower dead time in the data acquisition system resulting from the large yield from this reaction and was used for the majority of the thick target measurements. The detector was moved to a distance of 3 m during measurements carried out at incident beam energy near the reaction threshold. Measurements of the thin target Bðd; ncÞ reaction were carried out with the detector at a distance of 1.1 m from the target. During the measurements of the Fðp; acÞ reaction the detector was placed at 3.2 m from the target. In all measurements the detector was 1.78 m from the concrete floor of the experimental area and a greater distance from other objects of significant mass including walls and ceiling. Energy calibration for the detection system was performed by reference to known peaks within the pulse height spectra from the reactions as well as a reference Pu(Be) source. Deuteron and proton beams were pulsed and bunched such that a time-of-flight spectrum was obtained during the runs. The standoff distance of the detector during the measurements allows for accurate separation of the gamma peak from signals associated with the beam on upstream collimation. The TAC signal from the time-of-flight spectrum was used via a single channel analyzer to produce an external gate signal for the ADC assigned to the pulse height spectrum. This allowed the pulse height spectrum to be gated on the gamma peak in the time-of-flight spectrum in order to reduce background in the measurements. 3. Results and discussion
Fig. 1. Sketch of electrically isolated nylon target holder used for the boron targets. The 3 mm thick and 25 mm cylindrical boron target was set inside the Al end cap and flush against the nylon target holder that mounted to a snout on the accelerator vacuum beam line. The thin boron target was similarly mounted via the 17 mm outer diameter support ring. The Al cap overlapped an o-ring on the nylon piece providing for the vacuum seal. An Al plate with 300 V bias served as electron suppression for the target assembly.
The use of time-of-flight gating allowed for a dramatic reduction in background counts during the 11 Bðd; ncÞ12 C measurements. An example of the ungated pulse height spectrum compared to a spectrum with a time-of-flight window gate placed on the gamma peak is given in Fig. 2. This method is not only useful in the reduction of the background counts in the spectrum, but also has the benefit of reducing system dead time through the rejection of signals associated with those background counts in the ADC system. At the highest beam energies in the thick target Bðd; ncÞ measurements this allowed the count rate in the detection system to be reduced from 35 kHz to 6.5 kHz. The reduction of background in
Fig. 2. Pulse height spectrum comparison between an ungated and time-of-flight gated measurement for the 11 Bðd; ncÞ12 C reaction with 3.5 MeV incident deuteron energy on a thick boron target using a BGO detector. The gating window was set on the gamma peak in the time-of-flight spectrum. This illustrates the reduction in background counts due to the tof gating method.
K.W. Cooper et al. / Nuclear Instruments and Methods in Physics Research B 305 (2013) 45–50
the Fðp; acÞ pulse height spectrum using the same method was far less significant to the point that its effect is mainly negligible. System live-time was nominally 95% for most measurements, with higher values during measurements on the thin boron target. The system live-time decreased at higher incident beam energies for both the Bðd; ncÞ and Fðp; acÞ measurements due to the increasing reaction yields at the associated values. In the case of the thick target Bðd; ncÞ measurements the system live-time fell to 75% at the highest incident beam energy. The highest incident beam energy measurement in the Fðp; acÞ had a system live-time of 86%. The detector response function for all sets of measurements was modeled using the MCNP5 code [8] by examining the results of the F8 tally with the Gaussian Energy Broadening (GEB) tally modifier. The simulation was ran in full electron transport mode (mode p e), and not by employing the thick target Bremsstrahlung approximation for the electron transportp(mode p). The MCNP applied enffiffiffi ergy broadening follows a DE / E relation, and corresponds to a DE = 0.43 MeV and DE = 0.46 MeV at 15.1 MeV for the thick and thin boron target measurements respectively. The energy broadening applied to the Fðp; acÞ measurements corresponded to a DE ¼ 0:25 MeV at 6.5 MeV. The input into the MCNP model included the BGO crystal, acrylic light guide, Al casing, lead brick housing/ holder, and the air path from the source to the detector. This model is likely to underestimate the Compton scattering from other materials in the measurement area, thus there could be a systematic error in that the measured pulse height spectrum could contain counts from scattering not present in the model. It should be noted that this error would be minimized by the large distance separating the detector from other objects of significant mass coupled with the time-of-flight gating method used in measuring the spectra. The normalization of the pulse height spectra to detector response was calculated by comparing the integrated counts in an energy region of the measured pulse height spectrum to that of the MCNP modeled pulse height spectrum. The energy region chosen for the thick target Bðd; ncÞ spectrum was from 13.5 to 16 MeV. This higher energy region was chosen in consideration of reducing the effect of background counts in the measured spectrum. The fractional sum of the MCNP5 modeled pulse height spectrum for the 13.5–16 MeV energy region was calculated to be 0.593 for the detector at its most distant position, and 0.567 for the detector at its closest position. A computer run time of nearly 67 h on a machine with a 2.2 GHz Celeron processor was used to simulate
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1:6 1010 particles from the source. A comparison of the measured pulse height spectrum to the detector modeled detector response for the 15.1 MeV gamma is given in Fig. 3. The same method was applied to the normalization of the detector response for the thin boron target measurements. The energy region of 14–15.8 MeV in the thin boron target pulse height spectrum was found to have a fractional sum of 0.396 of the total modeled pulse height spectrum. A comparison of the measured to modeled detector response for the thin boron target 15.1 MeV gamma is given in Fig. 4. The energy region chosen for the Fðp; acÞ reaction was 5–8 MeV. The efficiency of the detector to each of the three gamma rays in this region was found to be constant within our modeling parameters, and thus a summing of the pulse height in this region is independent of the relative proportion of the three gamma ray constituents. This region was also chosen due to consideration of background count reduction, although it is far less problematic for this reaction compared to the prolific Bðd; ncÞ reaction. The 5– 8 MeV region was found to contain a fractional sum of 0:758 of the total modeled pulse height spectrum. An example of the pulse height spectrum from the Fðp; acÞ reaction is given in Fig. 5. Measurements of the thick target 11 Bðd; nc15:1 Þ reaction differential cross-section were carried out at incident deuteron beam energies ranging from near reaction threshold to 5 MeV. The beam energy was taken in steps of 10 keV through the threshold value and near the peak in the differential cross-section at 3.08 incident deuteron energy. These measurements confirmed both the previously reported threshold and peak in the excitation function as well as serving as a consistency check for systematic error in incident beam energy. Larger energy steps were taken between points away from the threshold or peak feature in the excitation function. In the calculation of the differential cross-section for the 15.1 MeV gamma generated in the boron reaction the target areal density used was that required to cause the incident deuteron sufficient energy loss to fall below the threshold of the reaction (1.63 MeV). Energy loss calculations were made using the well known SRIM [9] package. Uncertainties in the values of the differential cross sections are approximately 5% for most measurements ranging up to 10% with higher values near the threshold. Uncertainties in the measurement are mainly attributed to uncertainties in target stopping power, beam current integration, and determination of detector solid angle, while those measurements near the threshold are dominated by counting statistics.
Fig. 3. Comparison between the measured pulse height spectrum and the MCNP5 modeled pulse height spectrum for the 15.1 MeV gamma incident upon the BGO detector. The measured pulse height spectrum was taken from the thick target Bðd; ncÞ reaction with 2.5 MeV incident deuteron beam energy. The indicated energy region is used to determine the fractional sum of the detector response for use in normalization of the measured 15.1 MeV gamma yield.
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Fig. 4. Comparison between the measured pulse height spectrum and the MCNP5 modeled pulse height spectrum for the 15.1 MeV gamma incident upon the BGO detector. The measured pulse height spectrum was taken from the thin target Bðd; ncÞ reaction with 2.2 MeV incident deuteron beam energy. The indicated energy region is used to determine the fractional sum of the detector response for use in normalization of the measured 15.1 MeV gamma yield.
Fig. 5. A sample pulse height spectrum from the Fðp; acÞ reaction measurements with incident effective proton energy of 2.5 MeV. The vertical lines indicate the summing region 5–8 MeV for normalization with the MCNP5 detector response function model. Two of the gamma peaks (7.116 and 6.917 MeV) are not separately resolved while the 6.129 MeV peak is distinguishable. Also present is the unresolved single escape peak from the two higher energy gamma rays and the single escape peak from the 6.129 MeV gamma.
The measurements of the thin target 11 Bðd; nc15:1 Þ reaction differential cross-section were carried out over a similar beam energy range. The lowest incident beam energy being 1.8 MeV and the highest at 4.5 MeV. The results of thick and thin target measurements are compared to those carried out by Kuan et al. [5] and given in Fig. 6. Uncertainties in the values are nominally 9–10% with a systematic uncertainty in the target thickness being the largest contribution to the estimate, followed by counting statistics for measurements at incident beam energies approaching threshold and at or above 4.0 MeV. The values found by this group tend to be significantly lower than those reported by Kuan and higher value measurements, not reported here, previously given by Kavanagh and Barnes [4]. Two facts concerning the comparison of the sets of measurements are of consideration. First, the measurements reported by Kuan were given with uncertainties of ±50% and ±24% for Kavanagh. Secondly, it has been suggested [10] that the methods used by
Kavanagh and presumably followed by Kuan to determine the NaI detector response function used in their work could have led to an overestimation of the 15.1 MeV gamma yield that was measured. The method that was used by Kavanagh consisted of taking the difference between a pulse height spectrum taken with incident deuteron energy above reaction threshold with that of a spectrum with incident deuteron energy below reaction threshold. This result was then used to approximate the low energy tailing of the detector response function as a constant Compton plateau that was extrapolated to zero pulse height. The current state of computer simulations allow for a more accurate modeling of the detector response function. Our modeling of the detector response function with the setup reported by Kavanagh (and presumably Kuan) using MCNP5 shows an overestimation of the integrated counts by a factor of 1.7. This would bring the differential cross-section measurements reported by Kuan into agreement with our findings within experimental uncertainties.
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Fig. 6. Measured thick and thin target differential cross-sections for the 11 Bðd; nc15:1 Þ reaction are compared to the thin target measurements by Kuan et al. [5]. Thick target areal density for the purpose of our calculations was calculated as the thickness required to cause sufficient energy loss in the incident deuteron beam to reach the reaction threshold energy of 1.63 MeV.
Fig. 7. Measurements of the Fðp; acÞ thick target yield compared to those made previously by Micklich et al. [7]. Included is a fit to the excitation function provided in the work by Micklich. The uncertainties in the measurements by the current work range from 4% to 7% at the lowest beam energies. Included for comparison are measurements made by Anttila [11] and Kiss [12].
Measurements for the thick target yield of the sum of the 6.129, 6.917, and 7.116 MeV gamma rays from the 19 Fðp; acÞ16 O reaction at 0° with respect to the beam are shown in Fig. 7. The data show a marked increase in the gamma yield starting near an effective incident proton beam energy of 2 MeV followed by a plateau near 3.5 MeV. The effective incident energy of the proton beam is determined by reducing the energy of the beam by the loss, as determined using SRIM [9] calculations, through the 7 l thick Ni entrance window to the SF6 gas cell. Uncertainties in the values range from 4% to 7%. The measurements made in the current work are in relative agreement with those determined previously by Fessler [6] and Micklich et al. [7]. The change in relative yields of the three gamma rays as a function of incident beam energy was not determined. The thick target yield for the three gamma rays from the 19 Fðp; acÞ16 O reaction was found to be isotropic to within roughly 15% for the three angles measured, namely 0°, 55°, and 90° with respect to the beam. The results of the measurements as a function of
effective beam energy are given in Fig. 7. This result is also in agreement with the previously cited measurements by Fessler and Micklich.
4. Conclusions Measurements of the differential cross-sections for the Bðd; nc15:1 Þ12 C reaction on thick and thin targets have generated results that are significantly lower than those previously measured. The measurement of the thick target yield of the 6.129, 6.917, and 7.116 MeV gamma rays from the 19 Fðp; acÞ16 O reaction have been consistent with those previously measured. An accurate determination of the differential cross sections is interesting from a basic research standpoint, and useful in the inclusion of the values in data libraries used for simulation purposes. The thick target 11 Bðd; nc15:1 Þ12 C measurements being of greater interest for application as an active interrogation probe beam source. 11
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The 11 Bðd; nc15:1 Þ12 C reaction has two advantages in regards to its use as the source of a probe beam for active interrogation. The 15.1 MeV gamma is a very advantageous energy in the photofission cross-section for 235 U and 239 Pu, and the reaction is quite prolific. It has the possible disadvantage in that there is associated generation of neutrons that could interfere with the discrimination of the neutrons generated during photofission. The 19 Fðp; acÞ16 O reaction at the measured beam energies has the advantage for the use as an active interrogation probe of a relatively clean spectrum such that the byproducts of an induced photofission are clearly distinguishable from the probe. It has the disadvantage in the gamma rays generated are at the edge of the favorable energy range of induced photofission in the fissile materials of interest. Acknowledgments The authors would like to gratefully acknowledge the Defense Threat Reduction Agency for funding contributing to the completion of this work, and many conversations with Terry Taddeucci of Los Alamos National Laboratory.
References [1] B.J. Micklich, D.L. Smith, Nucl. Instr. Meth. B 241 (2005) 782. [2] A. Proctor, T.A. Gabriel, A.W. Hunt, J. Manges, T. Handler, Nucl. Instr. Meth. A 662 (2012) 71. [3] M.B. Goldberg et al., IAEA, International Topical Meeting on Research Applications and Utilization of Accelerators, 2009 (URL: