ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 620 (2010) 397–400

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/nima

Praseodymium activation detector for measuring bursts of 14 MeV neutrons

Tim Meehan a,n, E.C. Hagen a, C.L. Ruiz b, G.W. Cooper b

a National Security Technologies, LLC, P.O. Box 98521, North Las Vegas, NV 89030, USAb Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185, USA

a r t i c l e i n f o

Article history:

Received 20 October 2009

Received in revised form

1 April 2010

Accepted 12 April 2010Available online 20 April 2010

Keywords:

Neutron detection

Activation detector141Pr

Dense plasma focus

Sum-peak

02/$ - see front matter & 2010 Elsevier B.V. A

016/j.nima.2010.04.037

esponding author.

ail address: meehanbt@nv.doe.gov (Tim Meeh

a b s t r a c t

A new, accurate, neutron activation detection scheme for measuring pulsed neutrons has been designed

and tested. The detection system is sensitive to neutrons with energies above 10 MeV; importantly, it is

insensitive to gamma radiation o10 MeV and to lower-energy (e.g., fission and thermal) neutrons. It is

based upon the use of 141Pr, an element that has a single, naturally occurring isotope, a significant n,2n

cross-section, and decays by positron emission that result in two coincident 511 keV photons. Neutron

fluences are thus inferred by relating measured reaction product decay activity to fluence. Specific

sample activity is measured using the sum-peak method to count gamma-ray coincidences from the

annihilation of the positron decay products. The system was tested using 14 and 2.45 MeV neutron

bursts produced by NSTec Dense Plasma Focus Laboratory fusion sources. Lead, copper, beryllium, and

silver activation detectors were compared. The detection method allows measurement of 14 MeV

neutron yield with a total error of E18%.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Activation detection schemes enable accurate measurement ofneutron fluences in high-background environments. The radiationto be quantified causes nuclear reactions that result in aradioactive product. The fluence information is inferred bymeasuring the decay of the radioactive reaction products; thedecay of these products can be measured separately either in ashielded environment, or in the case of pulsed sources after thesource has performed. Several neutron activation schemes [1–6]have been employed for use with fast neutrons. Some schemesincorporate the target element in the detector, while others countsamples that are transported from the radiation source to adetection system in a low-background environment. Both types ofdetection schemes have used yttrium [3], copper [1,4], arsenic [3],indium [1], lead [3,5] moderated rhodium [3], and moderatedsilver [3,6]. In general, when the detection system must accuratelyquantify 14 MeV fluence, a high reaction threshold is preferred.

In summary, parameters affecting choice of activation isotopeinclude reaction threshold, activation cross-section, and thelifetime of the reaction product (the combination of cross-sectionand lifetime determines the specific activity of the sample, andthe lifetime determines the practicality of transporting the sampleto a shielded counter); additional factors affecting countingaccuracy include the uniqueness of the decay radiation and the

ll rights reserved.

an).

threshold of the activation reaction. Finally, the half-life needs tobe long enough to allow accurate counting after detection systemrecovery from any initial burst of competing radiation. Table 1shows some properties of various activation detector materials.

For our 14 MeV neutron yield measurements, we employedactivation detectors with thresholds of at least 1 MeV. This wasdone to prevent scattered neutrons from collecting in the room orbackground neutron flux from artificially increasing the yieldestimate. The lack of sensitivity to low-energy neutrons andbackground neutrons simplified the calibration of these detectors.

When employed for detection of 14 MeV neutrons, 141Pr metthe above criteria well. 141Pr is the sole, naturally occurringisotope of praseodymium [7]. It is relatively inexpensive and doesnot compete with contaminant radiation from other elementalisotopes. The 141Pr(n,2n)140Pr reaction cross-section measure-ment reported in the literature varies by as much as 0.5 barn, andconstitute the majority of the error in measuring the 14 MeVneutron yield with this method (�18%)[9–11]; however, if thecross-section were known to greater precision, this woulddramatically improve the accuracy of the 141Pr activationdetector. As shown in Fig. 1, the activation product of thereaction is reached by two competing branches; the positronemission branch emits 511 keV gammas in coincidence. Thebranching ratio is also well known [8]. The half-life of3.3970.01 min [8] allows transfer from the site of radiation toa well-shielded coincidence counting system with sufficientsample residual activity for quantification. Therefore, it ispossible to infer the number of neutrons that have passedthrough a 141Pr sample by counting the resulting coincident

ARTICLE IN PRESS

Table 1Common 14.1 MeV neutron activation detectors.

Isotopic abundance [7] Threshold (MeV) [8] Half-life (s) [8] Reaction Cross-section at 14.1 MeV (barns) [9]

1.0000 0.693 0.01762 75As(n,n0)75mAs 0.76

0.5240 7.404 0.806 208Pb(n,2n)207mPb 2.22

0.9571 9.120 71.9 115In(n,2n)114mIn 1.47

1.0000 9.464 203.4 141Pr(n,2n)140Pr 1.44

0.6917 11.02 580.38 63Cu(n,2n)62Cu 0.45

0.3083 10.06 45,723.6 65Cu(n,2n)64Cu 0.91

Fig. 1. 140Pr e/b+ decay [12]

Fig. 2. Sum-peak counting setup.

T. Meehan et al. / Nuclear Instruments and Methods in Physics Research A 620 (2010) 397–400398

511 keV photons from the decay of the reaction product. Adisadvantage of 141Pr is that it readily oxidizes in air, forming acrumbly greenish oxide, thereby slowly degrading an unprotectedsample. This oxidation can be mitigated by storing the samples ina container filled with inert gas; samples so stored have lasted forseveral years with only moderate degradation.

2. Experimental setup

The sources of pulsed 14 MeV neutrons for this experimentwere supplied by two dense plasma focus (DPF) fusion neutronsources, one emitting approximately 109 neutrons per pulse, theother emitting 41012 14 MeV neutrons per pulse into 4p. Both ofthese DPFs were of the Mather type [13–17] with the neutron-emitting plasma a line source approximately 1 cm in axial lengthand 1 mm in diameter. The activation samples viewed the sourceside-on at a range of 3 m. These sources used a 50% by numbermix of deuterium and tritium for fuel. The smaller of the twosources stored 1.5 kJ of energy, the larger 133 kJ. Pulse widths atthe samples were measured to be less than 50 ns. Two sources for2.45 MeV neutrons were employed: the 133 kJ source and a larger(41 MJ) bank both having the DPF tube charged with puredeuterium.

Two methods of quantifying the number of coincidental511 keV photons were tested. The first was a standard slow/fastcoincidence counting setup [18] the second system (see Fig. 2)utilized a sum-peak coincidence counting system [19]. Foraccuracy of measurements, the slow–fast system requirescareful measurement of detector efficiency as a function ofposition, and very accurate positioning of the activation samplebetween the two detectors during counting. Further, backscattercoincidences have to be minimized. These factors force loweringthe solid angle of the detectors relative to the well countersutilized in a sum-peak setup. This article focuses on a neutronquantification system based on the 141Pr(n,2n)140Pr reaction,using the sum-peak system to measure the specific activity of thedecaying sample. This system is intrinsically more accurate than

the standard slow–fast coincidence system, has higher countingefficiency, and can be rapidly set up.

2.1. Sum-peak system

A sum-peak method was used to determine the absoluteactivity of the sample. Two large (8 in. diameter, 8 in. tall) NaI(Tl)well detectors were used. When counting they were buttedtogether, the sample enclosed in the geometric center of thearrangement. The detectors were enclosed in a coaxial shieldingassembly formed of two 3/16 in. thick steel cylinders with leadshot between the steel cylinders. The outer steel cylinder had anouter diameter of 18 in. and an inner diameter of 8 in. Thecounting system was located remotely from the experimentalhall; the samples were transported to the counting system afterexposure to the neutron burst. When acquiring data for neutronyield determination, three sets of signals were digitized: multi-channel scaler (MCS) data from one of the NaI(Tl) crystals, pulse-height spectra from each of the spectroscopy amplifiers, and asummed pulse-height spectrum from the summing amplifier. Theacquisition time for the summed signal was 1017 s (five 140Prlifetimes).

In the sum-peak configuration, the NaI detectors wereexperimentally determined to have a total efficiency for the511 keV gammas of �90%. The sum-peak detectors wereexperimentally determined to have a photopeak efficiency forthe 511 keV gammas of �40%. As an additional benefit, the needsfor precise efficiency calibration and sample placement werereduced [19]. This method delivered accuracies higher than thetraditional coincidence counting measurements and higher coin-cidence rates. The sum-peak method is only moderately sensitiveto the location of the radioactive sample in the wells of the

ARTICLE IN PRESS

T. Meehan et al. / Nuclear Instruments and Methods in Physics Research A 620 (2010) 397–400 399

enclosing NaI(Tl) crystals [20,21]. This insensitivity to samplepositioning in the counting system contributes to this method’spracticality as a means of determining neutron yield. Ourmeasurements of the activity of a NIST-traceable 22Na standardwere within 2% of the given value for that sample’s activity.

Assuming that the total detection efficiency is near 100%, thesum-peak system gives an accurate measure of the absolute

activity of the sample being counted; it is in essence self-calibrating. A �280 Bq 22Na calibration source was used tocalibrate this system and to ensure that it was functioningproperly prior to data collection.

Fig. 4. 141Pr(n,2n)140Pr cross-section compared with 141Pr(g,n)140Pr to illustrate

insensitivity to low-energy gamma radiation.

3. Results

3.1. Accuracy

The accuracy of the 141Pr counting system was primarilylimited by the reported accuracy of the 141Pr(n,2n)140Pr cross-section, in fact, this uncertainty overwhelms other error in thisactivation detector, and is �18%[11]; in low-fluence environ-ments, as in all counting experiments, counting statistics canbecome a significant factor. Additional factors that limit theaccuracy of the neutron fluence measurement include the solidangle subtended by the activation sample, the self-absorption ofthe 511 keV gamma-ray in the sample, and the amount of energystraggling caused by the neutron burst passing through matter. Inthis case, a DPF capable of producing �1012 14.1 MeV neutronsper pulse, 141Pr samples, 3.81 cm in radius and 0.159 cm thick,were situated 3 m from the neutron source. The shape of thepraseodymium samples was modeled as they would be placed foractivation, as well as how they would be situated in the detectorassembly, in MCNPX prior to them being ordered to ensure thatthe self-absorption would be negligible. The sample positioningvariance was less than 1 cm. Thus, the solid angle and self-absorption were small. Energy straggling, caused by the passageof the neutrons through the DPF vacuum vessel wall and 3 m ofair, was also minimal.

Fig. 3 shows the reported cross-section data and the area ofconfidence as reported in the IRDF cross-section database. Onlythe IRDF database had error cloud information, and the amountsby which the various databases differ on the reported valuejustifies an interest in measuring this quantity with morecertainty. Also of note in Fig. 3, is that the threshold of the141Pr(n,2n)140Pr reaction is above both the 2.45 MeV deuteriumfusion neutron, as well as the neutron energy spectrum from

Fig. 3. 141Pr(n,2n)140Pr cross-section data with error cloud illustrating the

insensitivity to fission neutrons.

uranium [9]. This fact coupled with praseodymium’s relativeinsensitivity to gamma radiation below 10 MeV (Fig. 4) makes itideal for detecting pulses of deuterium–tritium fusion neutrons[9].

3.2. Error

During our experiments, we used an activation sample diskthat was 1/16 in. thick and 3 in. in diameter. For neutron yieldslower than 5�1010, the background noise was the dominantcontribution to the yield measurement. For neutron yields greaterthan 5�1010, the dominant contribution to the error in the yieldwas the error in the measurement of the cross-section for thepraseodymium activation, approximately 18%[11]. By our monte-carlo simulations and projections of experimental results, neutronyields above 1�1013 would start to have significant dead timewhen counting the sample (43%). At these large yields, the deadtime issue could easily be mitigated using a smaller activationsample. At much lower count rates the counting error wouldbecome equal to or greater than the cross-section error.

3.3. Linearity

The plot in Fig. 5 shows the relationship between a moderatedsilver detector and the 141Pr system. The silver system was

Fig. 5. Correlation of 141Pr yield with 109Ag activation yield.

ARTICLE IN PRESS

T. Meehan et al. / Nuclear Instruments and Methods in Physics Research A 620 (2010) 397–400400

calibrated at 2.45 MeV. The detection efficiency ratio between2.45 and 14.1 MeV was simulated using MCNPX. The graphillustrates the linearity of the 141Pr system. The 141Pr system relieson counting of beta-plus decays. Pileup and dead time are twofactors that limit the count rate and could contribute tononlinearity. At a neutron yield of 1�1012, using a 1/16 in.thick, 3 in. wide disk 3 m from the Z-pinch, the average dead timeduring post-shot counting was about 1% (depends on yield).Radioactive sources of various strengths were used to measurethe effect of pileup and dead time.

4. Summary

A method for accurately measuring bursts of 14 MeV neutronshas been demonstrated and tested. It has measured neutronfluences of 1�105–1�107 n/cm2. The dynamic range of measur-able fluences can be extended by choosing activation sample sizeand position. The sum-peak method’s insensitivity to low-energyneutron background and relative insensitivity to gamma radiationmake it particularly attractive for quantifying fusion output fromdevices that simultaneously emit large bursts of competingradiation.

Acknowledgments

This work was supported by the NSTec Site-Directed Researchand Development (SDRD) Program, under DOE/NNSA contractnumber DE-AC52-06NA25946. We would like to thank co-authorsCarlos Ruiz and Gary Cooper from Sandia National Laboratoriesfor their assistance with this project. We would also like to thankSteve Molar, Larry Robbins, and Ron Swegle of the NSTec DensePlasma Focus Lab for their expertise, Daniel Lowe and Robert

O’Brien for the monte-carlo modeling of the detector, Jim Pigg forhis assistance with the early portion of this research, and MicheleVochosky for her editing assistance.

References

[1] G.W. Cooper, C.L. Ruiz, Rev. Sci. Instrum. 72 (2001) 814.[2] R.E. Chrien, Rev. Sci. Instrum. 62 (1991) 1489.[3] E.L. Jacobs, S.D. Bonaparte, P.D. Thacher, Nucl. Instr. and Meth. 213 (1983)

387.[4] J.W. McKlveen, IEEE Trans. Nucl. Sci. 28 (1981) 1632.[5] C.E. Spencer, E.L. Jacobs, IEEE Trans. Nucl. Sci. 12 (1965) 407.[6] R.J. Lanter, D.E. Bannerman, Rev. Sci. Instrum. 39 (1968) 1588.[7] P. De Bievre, P.D.P. Taylor, Int J. Mass Spectrom. Ion Phys. 123 (1993) 149.[8] Nuclear Data Sheets 108 (2007) 1287.[9] Brookhaven National Laboratory, Evaluated Nuclear Data File (ENDF)

Retrieval & Plotting, http://www.nndc.bnl.gov/sigma/, (accessed March 10,2009).

[10] International Atomic Energy Agency, Experimental Nuclear Reaction Data(EXFOR), http://www-nds.iaea.org/exfor/exfor.htm, (accessed February 20,2009).

[11] R.E. Little, et al.Low-Fidelity Covariance Project, Nuclear Data Sheets 109(2008) 2828.

[12] S.Y.F. Chu, L.P. Ekstrom and R.B. Firestone, www. Table of RadioactiveIsotopes, http://nucleardata.nuclear.lu.se/NuclearData/toi/nucSearch.asp,(accessed July 22, 2009).

[13] N.V. Filippov, T.I. Filippova, V.P. Vinogradov, Nucl. Fusion Suppl. 2 (1967) 557.[14] J.W. Mather, P.J. Bottoms, J.P. Carpenter, A.H. Williams, K.D. Ware, Phys.

Fluids 12 (1969) 2343.[15] M. Scholz, R. Miklaszewski, M. Paduch, M. Sadowski, A. Szydlowski, K.

Tomaszewski, IEEE Trans. Plasma Sci. 30 (2002) 476.[16] H. Herold, A. Jerzykiewicz, M. Sadowski, H. Schmidt, Nucl. Fusion 29 (1989)

1255.[17] A. Bernard, P. Cloth, H. Conrads, A. Coudeville, G. Gourlan, A. Jolas, Ch.

Maisonnier, J.P. Rager, Nucl. Instr. and Meth. 145 (1977) 191.[18] E.C. Hagen, L.H. Ziegler, NTS SDRD Report (2002) 21.[19] J.M.R. Hutchinson, W.B. Mann, P.A. Mullen, Nucl. Instr. and Meth. 112 (1973)

187.[20] J.M.R. Hutchinson, et al., IEEE Trans. Nucl. Sci. NS-19 (1) (1972) 117.[21] G.A. Brinkman, A.H.W. Aten Jr., Int. J. Appl. Radiat. Isot. 14 (1963) 503.