Measurement of photon mass attenuation coefficients of plutonium from 60 to 2615 keV

ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 581 (2007) 765–771 www.elsevier.com/locate/nima

Measurement of photon mass attenuation coefficients of plutonium from 60 to 2615 keV M. Rettschlag, R. Berndt, P. Mortreau European Commission, Joint Research Centre, Institute for the Protection and Security of the Citizen, Via Fermi, I-21020 Ispra (VA), Italy Received 10 January 2007; received in revised form 14 June 2007; accepted 9 August 2007 Available online 19 August 2007

Abstract Measurements have been made to determine plutonium photon mass attenuation coefficients by using a collimated-beam transmission method in the energy range from 60 to 2615 keV. These experimental results were compared with previous experimental and theoretical data. Good agreements are observed in the 240–800 keV energy range, whereas differences up to maximum 10% are observed out of these limits. r 2007 Elsevier B.V. All rights reserved. PACS: 29.40.n; 29.30.Kv; 29.87.+g Keywords: Mass attenuation coefficients; Plutonium; Photon; Germanium detector

1. Introduction As a part of the ongoing research and development program of special nuclear materials safeguarding, the mass attenuation coefficient m for plutonium has been measured in the energy range from 60 to 2615 keV. Its knowledge is required for applications such as plutonium mass determination using tomographic techniques. Table 1 [1] summarizes the different experimental work previously performed on this topic. Measurements in Table 1 cover the energy range from 25 to 2753 keV. However, above 250 keV, only six measurement points exist in Ref. [4] that gives mass attenuation coefficients. In addition to the measured data, two papers [7,8] refer to the theoretical estimation of the same coefficients. Ref. [7] considers the mass attenuation coefficient from 0.1 keV to 1 MeV for Z ¼ 1–94 whereas [8] based on the work of [9–11] tabulates the photon cross-section for Z ¼ 1–100 from 1 keV to 100 GeV.

Corresponding author. Tel.: +39 0332 785317; fax: +39 0332 785072.

E-mail address: [email protected] (R. Berndt). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.08.165

The experimental work available on the topic is a rather limited basis for nuclear data. It shows the need of additional accurate and consistent measurements of total attenuation coefficients. The narrow-beam total mass attenuation coefficient m in units of cm2/g is defined by I ¼ I 0 expðmxÞ;

counts=s

(1a)

or T ¼ expðmxÞ

(1b)

where I0 is the incident beam intensity, I is the transmitted beam intensity and x is the sample mass per unit area in g/ cm2. T is called the attenuation factor. The mentioned experiments of Connor et al. [4] were made using a NaI detector and radiation intensities I0 and I were determined with a single channel analysis. Our experiments differ from theirs in two important points: firstly, a Ge detector with very good energy resolution was used instead of a NaI detector; secondly, gamma radiation spectra were measured and net peak areas of full energy peaks were taken as a measure for the radiation intensities I instead of using a single channel analysis. These two

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differences are an asset especially when measuring Pu material as it always adds its own gamma radiation emission to the measured signal. In the present study [12], plutonium photon mass attenuation coefficients were determined at 11 different energies ranging from 60 to 2615 keV using radioactive isotopes as sources of gamma radiation (see Table 2). Table 1 Measurements of the photon mass attenuation coefficients m of Pu (in cm2/g) Authors

Ref.

Year

Energy range (in KeV)

Detector

Mc Crary et al. Roof Conner et al. Canada Chartier

[2] [3] [4] [5] [6]

1967 1959 1970 1977 1977

25.00–130.31 5.41–24.94 88.09–2752.70 96.73–197.95 39.98–220.31

NaI NaI NaI Ge(Li)

2. Experimental details 2.1. Samples The transmission measurements were made on certified discs of a plutonium gallium alloy produced by AEA technology. Two different isotopic mixtures of Pu were used. The PuGa alloys are characterized for plutonium isotopic compositions (in weight %) and impurities’ contents by high-accuracy destructive analysis. The atomic weight of the mixtures was 239.06 and 239.28, respectively, and we assign our results to the average value APu ¼ 239.17. For each Pu type, foils of 0.2 and 0.6 mm thickness were rolled, corresponding to (0.29970.002) and (0.92370.008) g/cm2, respectively. Circular discs with diameters from 2 to 36.5 mm were cut out and encapsulated between two 0.5 mm thick stainless steel plates. These discs are surrounded by a thin plastic foil.

Table 2 Radiation sources, measurements and determination of the attenuation factor T Source

Energy (in KeV)

Sample thickness (in mm)* sample thickness

Number n of replicated measurements

Number m of parameters and their identification

152 EU Point Source

244 778 964

0.6 ¼ 1*0.6 1.2 ¼ 2*0.6 0.6 ¼ 1*0.6

5 5 5

2 I0 I

133 Ba Point source

1112 1409 81 302 355

1.2 ¼ 2*0.6 2.4 ¼ 4*0.6 0.2 0.6

5 4 12 12

2614

3 ¼ 5*0.6

5

232

Th Volume

2 I0 I 4 Ia

Souce

Ib

87 mm

Id

Diameter

Ic

241 Am Pu disc Source

59.6 0.2

16

3 Ie If Ig

137

Cs Point Source

661.6

0.6 1.2 1.8

5 5 5

3 Ie If

2.4

5

Ig

3 3.6

5 5

Corresponding measurements

Expression of Ti

Eu source Eu source + Pu absorber sample

Ti ¼ I/I0

Ba source Ba source + Pu absorber sample

Ti ¼ I/I0

Th source + Th background of the lab. Th source + Pu absorber (with its Th) + Th background of the lab. Pu absorber (with its Th) + Th background of the lab.

Ti ¼ (IbIc)/ (IaId)

Am souce Am + Pu (with its 60 keV line of Am) Pu (with its 60 keV line of Am)

Ti ¼ (IfIg)/Ie

Cs source Cs + Pu (with 662 keV line of Am) Pu (with 662 keV line of Am)

Ti ¼ (IfIg)/Ie

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ments at the energy 59.6 keV of the 241Am source. The energy resolution was 1.24 keV at 661.6 keV. The detector was coupled to a Mini MCA-166 of GBS Rossendorf, Germany.

The sample was placed into the transmission beam at 22 cm distance from the detector crystal. The thickness of the sample (or the number of samples) was chosen as a function of the source radiation energy. For a given experiment time, the measurement error becomes smaller if large attenuation coefficients (at low energy) are measured with thin absorbers and if small ones (at high energy) are measured with thick absorbers.

2.3. Radiation sources The gamma radiation sources were either point sources or volume sources. They are listed in Table 2.

2.2. Detector 2.4. Measurement geometry The detector was a 1000 mm2 and 19 mm thick planar germanium detector. A 0.25 mm thick cadmium filter covered the end cap of the detector and absorbed a part of the low energy radiation emitted by the plutonium samples. This filter was not used in the case of measure-

Figs. 1a–c show the experimental arrangement. The point sources, 133Ba, 137Cs and 152Eu (Fig. 1a) were located 33 cm from the detector. A 1 cm diameter, 5 cm length tungsten collimator was placed in front of the detector. Cd filter W collimator detector crystal

Pu sample radiation source

50 nm

detector housing

220 mm 330 mm Pb collimator Pu sample W collimator Cd filter Pb collimator

W collimator Pb collimator

detector crystal

Th source

50 mm

50 mm

50 mm 60 mm

50 mm

detector housing

140 mm

330 mm

Pb collimator

Pu sample

Pb collimator

radiation source

W collimator detector crystal

50 mm

50 mm

50 mm 220 mm

detector housing

600 mm Fig. 1. (a) Experimental arrangement for the point sources 133Ba, (c) Experimental arrangement for the 59.6 keV measurements.

137

Cs and

152

Eu, Fig. (b) Experimental arrangement for the Thorium volume source,

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In this arrangement, the maximum scattering angle for Compton scattering in the transmission sample was about 101. For the 232Th volume source (Fig. 1b), a 5 cm long tungsten collimator, following an 11 cm long lead collimator, both 2 cm diameter, were used to shield the large volume of the thorium container and to form a beam in direction towards the detector. On the detector side, there were two collimators of 1 cm diameter, the first 5 cm away from the detector being made of tungsten, followed by 5 cm of lead. The maximum possible Compton scattering angle in the plutonium sample was lower than 51. Transmission measurements at 59.6 keV are disturbed by photons of the same energy emitted by the 241Am contained in the Pu sample itself. Consequently, the radiation source should have a higher 241Am activity than the transmission sample. For this reason, one of the 0.6 mm thick Pu samples (named ‘‘Pu210’’) from the Pu type with the higher 241Am content was chosen as radiation source. It has a higher activity than the available typical commercial 241Am sources. Transmission samples were only 0.2 mm thick. As Fig. 1c shows, the radiation source ‘‘Pu210’’ was placed at a distance of 60 cm from the detector. Both the incident and transmitted beam were collimated with thick tungsten and lead collimators to reduce the angular distribution of the beam. The maximum possible Compton scattering angle in the plutonium sample was lower than 4.51. In all three cases the prolongation of the photon path in the absorber due to the non-parallelism of the beam is negligible.

energy from the attenuation object, the Pu plate. This situation made necessary three measurements instead of two:

2.5. Measurement time



The measurement time was chosen in a way that the contribution of counting statistics to the total error became clearly smaller than the contribution from other sources of errors. This led to long measurement times and consequently the long time stability of the equipment was supervised. The longest measurement times were required for the 59.6 keV peak of 241Am and many repeated measurements were made in this case. Very small errors from counting statistics were needed because the 241Am activity of the Pu absorber itself caused a high count rate which had to be subtracted from the sum signal coming from transmission source and attenuation object Pu. 2.6. Measurements For the determination of T according to (1b) we performed repeated measurements i. The attenuation factor Ti was a simple ratio T i ¼ ðI i =I 0;i Þ only in case of the measurements with the transmission sources 152Eu (five energies) and 133Ba (three energies). In case of the 59.6 keV peak of the 241Am transmission source, there is also an emission of photons of the same

  

firstly, a measurement with the transmission source and the attenuation object in place, the net peak area is called If, secondly, a measurement without transmission source but with the attenuation object in place, the net peak area is called Ig, and thirdly, a measurement with the transmission source but without attenuation object, the net peak area is called Ie.

The ratio Ti is then T i ¼ ðI f;i  I g;i Þ=I e;i . The same problem appeared with the 661.6 keV peak of 137 Cs which is overlapping the 662.4 keV line of 241Am, present in the attenuation objects. The 2614 keV peak of the 232Th transmission source is also present in the background radiation of the laboratory. For this reason, even four measurements were required for the determination of the ratio Ti:







a first measurement with the transmission source and the attenuation object in place plus the contribution of the laboratory background, the net peak area is called Ib, a second measurement without the transmission source but with the attenuation object in place plus the contribution of the laboratory background, the net peak area is called Ic, -a third measurement with the transmission source plus the contribution of the laboratory background and without the attenuation object, the net peak area is called Ia, a fourth measurement of the laboratory background, the net peak area is called Id.

The ratio Ti is then T i ¼ ðI b;i  I c;i Þ=ðI a;i  I d;i Þ . The measurements required to determine the Ti are given in columns 5 and 6 of Table 2. Normally, measurements were made for different absorber thicknesses, i.e. using one or more samples in the radiation beam. The above-mentioned repetitions of the determinations of T were made with different samples or with various combinations of samples. Each repetition consisted of the repetition of the two, three or four measurements which belong together. For the 662 keV line, ratios Ti were determined on six thickness values ranging from 0.6 to 3.6 mm. Also here, different combinations of attenuation objects were chosen, when possible. The determined attenuation coefficients mi have shown a good agreement within the individual measurements’ errors. This confirms that the error propagation modelling as applied below (see Section 4) corresponds to the significant real sources of measurement variations.

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3. Data analysis and results



For the analysis of the attenuation measurements on the multi-layer situation of our experiment with one or more samples of different composite materials, the attenuation law (1b) is expressed as ! n X T ¼ exp  mi xi . (2)



i¼1

Formula (2) contains the mass attenuation coefficients mi (in cm2/g) and the masses per unit area xi (in g/cm2) of all n chemical elements i which occur in the sample or the samples. Pu is one of them. By separating mPu, we obtain ! n1 X mj xj =xPu (3) mPu ¼  ln T  j¼1

where j stands for all chemical elements except Pu. In the practical evaluation the values mPu were determined individually for all the repetitions, and afterwards the average was calculated: m 1X mPu ¼ m . m i¼1 Pu;i The measurements, as listed in Table 2 for the respective photon energies (in keV), led to the mass attenuation coefficients in Table 3 and Fig. 3. 4. Uncertainties Several sources of bias and variation have influence on the determined values of the attenuation coefficients mPu and on their uncertainty DmPu. The discussion below will consider in more detail the following factors:

 

scattering effects, instability of equipment,

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uncertainty of nuclear data mi and experimental data xi used in (3), counting statistics variation.

4.1. Scattering effects Contributions of scattering effects to the measurement uncertainty DmPu are considered as follows: Compton scattering in forward direction in the attenuating Pu layer can not be avoided in non-ideal conditions. It always increases the radiation intensity I and consequently leads to systematically too small mass attenuation coefficients m. In this situation, at least for energies above 1000 keV, the very good energy resolution of the Ge detector helps. Photons which are scattered by only two degrees, lose so much energy that they do not fall anymore into the narrow full energy peak. They increase only the low energy foot of the peak. The latter is even welcome: the linear function, which describes the background below a peak, starts on this elevated level on the low energy side, low energy scattering events in the region of the peak will fall below this background line and do not contribute to the net peak area. Experiments with NaI detector bring much more difficulties in this respect due to their large peak width. The above consideration does not hold for energies far below 1000 keV. Here, special additional scattering experiments helped to quantify the contribution of scattered photons in the attenuation experiments. A setup was made, where a long thin lead absorber was placed on the axis between a point source and the detector, suppressing a direct irradiation of the detector (Fig. 2). The radiation intensity, i.e. the net peak area, was measured with and without the absorber. Then, in the further two setups, a tube and a plate, respectively, were arranged in vicinity of the absorber around the axis (see Fig. 2), to provoke the measurement of both elastic and inelastic scattered radiation. These experiments were made for photons from 121 to 1408 keV. They have shown negligible effects or even no effect at all.

Table 3 Measured values of photon mass attenuation coefficients m of Pu (in cm2/g) Energy (in KeV)

m (in cm2/g) Present work [9] with 1s uncertainty

m (in cm2/g) Previous work

Difference (in %)

Ref.

59.5 81.0 244.7 244.7 302.7 355.9 661.6 778.9 964.0 1112.1 1407.9 2614.5

8.0470.1180 3.7070.0330 0.82570.0050 0.82570.0050 0.52670.0030 0.37970.0030 0.13770.0008 0.11270.0020 0.090070.0019 0.076470.0018 0.064870.0013 0.048970.0016

7.57 3.33 0.884 0.837 0.529 0.382 0.134 1.107 0.0836 0.0723 0.0594 0.0464

6.0 9.9 7.2 0.5 1.6 1.2 2.1 3.7 6.2 4.1 7.0 6.9

[2] [2] [5] [4] [4] [4] [4] [4] [4] [4] [4] [4]

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lead tube

absorber cylinder

radiation source

detector crystal

detector housing

radiation source

absorber cylinder

lead plate detector crystal

detector housing Fig. 2. Two scattering situations considered: (a) scattering inside a tube of lead, surrounding the axis source-detector, (b) scattering in a 1 mm lead plate perpendicular to the axis source—detector.

repeated monitoring measurements were made for the whole time period of the experiments. They gave no hint on instability. Moreover, there is good agreement amongst the repetition measurements. Consequently, contributions to the uncertainty DmPu due to the experimental factor instability could be neglected.

µ (cm2/g)

10

1 100

1000 E / keV

0.1

present work Mac Crary, reference 2 Conner, reference 4 Canada, reference 5

Fig. 3. Pu mass attenuation coefficients in cm2/g obtained in this work and compared to other experimental data.

These above reflections concerning scattered radiation, together with the scattering experiments, allow concluding that the contribution of forward scattered photons to the net peak areas I is negligible. As a result, in our experiments the determination of mPu is not affected by a coherent or incoherent small angle scattering in the sample, i.e. contributions from the experimental parameter scattering to the uncertainty DmPu were neglected. 4.2. Instability of equipment Long term instability of the spectrometer electronics may contribute to the uncertainty DmPu. For this reason

4.3. Nuclear and experimental data and counting statistics The remaining factors—uncertainty of nuclear data mi, experimental data xi and counting statistics variation which influence T—are all independent from each other. For this reason, the error propagation law could be applied on (3) for the estimation of DmPu "     n1  n1  X qmPu 2 2 X qmPu 2 2 qmPu 2 2 DmPu ¼ Dmi þ Dxi þ DxPu qmi qxi qxPu j¼1 j¼1 #1=2   qmPu 2 2 þ DT . ð4Þ qT Dmi, Dxi and DTi are the uncertainties, expressed as standard deviations 1s. The contributions ðqmPu =qmi ÞDmi , ðqmPu =qxi ÞDxi and ðqmPu =qTÞDT were calculated for the attenuation coefficients mi and for the masses per unit area xi of all chemical elements in the transmission beam, for the Pu mass per unit area xPu and for the ratios T, which depend on the net peak areas I. The uncertainty computation for the net peak areas started from Poissonian assumptions and independence of energy channels. It included taking into account of the linear background subtraction (Linear background subtraction instead of

ARTICLE IN PRESS M. Rettschlag et al. / Nuclear Instruments and Methods in Physics Research A 581 (2007) 765–771 Table 4 Observed differences diff ¼ (m[4]m[9])/m[4] for Pu, Cu and Al for different experiments E (in KeV)

Diff Pu (%)

Diff Cu (%)

Diff Al (%)

244 788 964 1112 1408

7 4 6 4 7

4.0 1.0 1.0 0.3 1.9

0.6 3.0 1.2 1.7 1.9

fitting the peak shape is sufficiently good for the isolated peaks to be evaluated.). The net peak areas and the associated uncertainties were calculated as described in Ref. [13]. The resulting uncertainties DmPu are represented in Table 3 column 2, they are given as standard deviations (1s).

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6. Conclusion The present results of photon mass attenuation coefficients of plutonium complete the existing set of data measured in the past [2–6]. These measurements were the first ones made with a germanium detector in the energy range from 60 to 2615 keV. Good agreements are observed in the energy range from 240–800 keV, whereas differences up to maximum 10% are observed out of this energy range. Complementary measurements made on Cu and Al showed that observed discrepancies could not be explained by scattering. Further investigation of the differences compared to previous results is not possible because of the lack of published information about how exactly the earlier measurements were made. Acknowledgment The authors would like to thank Mr. Michael Franklin for helpful discussions.

5. Comparison with previous experimental work

References

The comparison with previous experiments [4,5] shows a good agreement for attenuation coefficients in the middle energy range from 244 to 778 keV. Both for low energies 59.6 and 81 keV and for energies above 1000 keV, higher values were obtained; the difference with previous results is in the order of 4–9% whereas the estimated uncertainties are 1–3% (see Table 3). The differences with Connor et al. [4] were considered to be large enough to justify additional comparison measurements. We wanted to see if the differences observed with Pu appeared also with other elements. For this purpose, attenuation coefficients of Cu and Al were determined with the same set-up and the same method, but reduced effort (only 152Eu was used as transmission source, less repetitions were made). On purpose, low and medium atomic weights were chosen, to reveal a potential influence of scattering in our experiments: the ratio of the cross sections sinelastic/stotal is much larger for Cu and even bigger for Al in comparison to the same ratio for Pu. Results were compared to the data of [4]. The differences (in %) between experimental results of [4] and this study [12] are presented in Table 4. The differences with Cu and Al do not exceed the order of magnitude of the errors of the experiment. This result is a support for our experimental work on Pu.

[1] J.H. Hubbell, H.M. Gerstenberg, and E.B. Saloman, Bibliography of photon total cross section (attenuation coefficient) measurements 10 eV to 13.5 GeV NBSIR 86-3461, July 1986. [2] J.H. Mc Crary, E.H. Plassmann, J.M. Puckett, A.L. Conner, G.W. Zimermann, Phys.Rev. 153 (1967) 307. [3] R.B. Roof Jr, Phys. Rev. 113 (1959) 820. [4] A.L. Conner, H.F. Atwater, E.H. Plassman, J.H. McCrary, Phys. Rev. A 1 (3) (1970) 539. [5] T.R. Canada, R.C. Bearse, J.W. Tape, Nucl. Instr. And Meth 142 (1977) 609. [6] J.L. Chartier, Methode de mesure de la section efficace photo electrique. Application au cas d’elements de Z eleve entre 40 et 220 keV.Mesure de l’energie de la discontinuite d’absorption K de Au, Th, U, Pu, Thesis, Universite´ Pierre et Marie Curie (1977) Access to Ref. [6] was not available. [7] W.M.J. Veigele, Atomic Data tables 5 (1973) 51. [8] M.J. Berger, J.H. Hubbell, Photon cross Sections on a personal computer NBSIR 87-3597, 1987, /http://physics.nist.gov/PhysRefData/ photonics/html/attencoef.htmlS. [9] J.H. Scofield, Theoretical Photoionization cross sections from 1 to 1500 keV, UCRL-51326, 1973. [10] J.H. Hubbell, W.J. Veigele, E.A. Briggs, R.T. Brown, D.T. Cromer, R.J. Howerton, J. Phys. Chem. Ref. Data 4 (1975) 471. [11] J.H. Hubbell, I. Overbo, J. Phys. Chem. Ref. Data 8 (1979) 69. [12] M. Rettschlag, Ermittlung der Photonenmassenschwachungskoefizienten von Pu im Energiebereich von 60–2600 keV, Diplomarbeit, Hochschule Zittau /Gorlitz (FH), 2001. [13] P. Mortreau, R. Berndt, Handbook of Gamma Spectrometry Methods for Non-destructive Assay of Nuclear materials, /http:// npns.jrc.it/nda/handbook3-2006.pdfS, EUR19822 EN. 3rd ed., 2006.