Scholarly article on topic 'Measurement of absolute γ-ray emission probabilities in the decay of  235 U'

Measurement of absolute γ-ray emission probabilities in the decay of 235 U Academic research paper on "Physical sciences"

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{"γ-ray spectrometry" / " 235U" / " 231Th" / "Decay data" / "γ-ray emission probabilities / NORM"}

Abstract of research paper on Physical sciences, author of scientific article — M. Marouli, G. Lutter, S. Pommé, R. Van Ammel, M. Hult, et al.

Abstract Accurate measurements were performed of the photon emission probabilities following the α decay of 235U to 231Th. Sources of highly enriched 235U were characterised in terms of isotopic composition by mass spectrometry and their activities were standardised by means of alpha-particle counting at a low defined solid angle. The standardised sources were subsequently measured by high-resolution γ-ray spectrometry with calibrated high-purity germanium detectors to determine the photon emission probabilities. Four laboratories participated in this work and reported emission probabilities for 33 γ-ray lines. Most of them agree with previously published evaluated data. In addition, new values are proposed for γ-lines which have been measured only once in the past.

Academic research paper on topic "Measurement of absolute γ-ray emission probabilities in the decay of 235 U"

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Measurement of absolute y-ray probabilities in the decay of 235U


M. Marouli, G. Lutter, S. Pommé, R. Van Ammel, M. Huit, S. Richter, R. Eykens, V. Peyrés, E. Garcia-Torano, P. Dryak, M. Mazanova, P. Carconi

PII: S0969-8043(17)31111-9


Reference: ARI8145

To appear in: Applied Radiation and Isotopes

Received date: 20 September 2017 Revised date : 27 October 2017 Accepted date: 27 October 2017

Cite this article as: M. Marouli, G. Lutter, S. Pommé, R. Van Ammel, M. Huit, S. Richter, R. Eykens, V. Peyrés, E. Garcia-Torano, P. Dryak, M. Mazanova and P. Carconi, Measurement of absolute y-ray emission probabilities in the decay of 235U , Applied Radiation and Isotopes,

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Measurement of absolute y-ray emission probabilities in the decay of 235U

M. Marouli1, G. Lutter1, S. Pommé1, R. Van Ammel1, M. Hult1, S. Richter1, R.

12 2 3 3 4

Eykens , V. Peyrés , E. García-Toraño , P. Dryák , M. Mazánová , P. Carconi

European Commission, Joint Research Centre, JRC, Directorate for Nuclear Safety and

Security, Retieseweg 111, B-2440 Geel, Belgium

Laboratorio de Metrología de Radiaciones Ionizantes, CIEMAT, Avenida Complutense 40,

28040 Madrid, Spain

3Cesky Metrologicky Institut, CMI, Radiová 1, 102 00 Praha 10, Czech Republic

4ENEA, Instituto Nazionale di Metrologia delle Radiazioni Ionizzanti, C. R. Casaccia, P.O.

Box 2400, I-00100 Rome A.D., Italy


Accurate measurements were performed of the photon emission probabilities following the a

decay of 235U to Th. Sources of highly enriched U were characterised in terms of isotopic composition by mass spectrometry and their activities were standardised by means of alpha-particle counting at a low defined solid angle. The standardised sources were subsequently measured by highresolution y-ray spectrometry with calibrated high-purity germanium detectors to determine the photon emission probabilities. Four laboratories participated in this work and reported emission probabilities for 33 y-ray lines. Most of them agree with previously published evaluated data. In addition, new values are proposed for y-lines which have been measured only once in the past.

Keywords: y-ray spectrometry; 235U; 231Th; decay data; y-ray emission probabilities, NORM

1 Introduction

235 238

Owing to their long half-lives, U and U and their decay products are still ubiquitously present in nature and therefore are often subject of radiological measurements. This is of particular importance in the NORM (naturally occurring radioactive material) industry where their activity concentration is significantly enhanced in products, by-products and waste derived from natural resources. Legislation, such as the Council Directive 2013/59/Euratom of 5 December 2013 (2014) in Europe, sets activity concentration levels for exemption of practices and clearance of materials from authorised practises. Since y-ray spectrometry is the most commonly used method for quantifying activity concentrations, there is an obvious need for accurate y-ray emission probabilities to convert count rates into activity values traceable to the SI-derived unit becquerel. In particular, the main U y-ray at 185.72 keV is interfered by the Ra y-ray at 186.21 keV, and therefore distinction between the U and U decay series (Hult et al., 2012) critically depends on the correct assignment of the corresponding emission probabilities.

The long-lived 235U (Тш = 704 (1) 106 a) is an alpha emitter decaying to the short-lived 231Th (25.522 (10) h) (Fig. 1), which de-excites through several y-transitions (DDEP, 2017). The y-ray emission probabilities from this decay have been studied several times in the past, from 1966 till 1996. Measured values of the emission probability of the 185.72 keV y-ray have been reported in five works, the latest one 25 years ago. A new accurate determination of Py(185.72 keV) would solidify old results, if not improve them. Furthermore, the emission intensities of some of the other y-rays have been measured only once and in some cases no uncertainties were assigned.

In this paper the y-ray emission probabilities PY of the daughter nucleus Th were measured by four laboratories. Their results are presented and compared with previously evaluated data. A fifth laboratory participated as well in this measurement campaign and has already published its results (Lepy et al., 2017).

2 Standardised U source

2.1 S ource preparation

sources were prepared from a highly enriched U in 1.5M HNO3 solution containing 20 mg U per ml. Mass spectrometric measurements were performed of the initial

solution to identify the isotopic ratios of the nuclides present using the Triton® TIMS (thermal ionisation mass spectrometer) at the Joint Research Centre (JRC) in Geel (Table 1). Sources of 15-100 ^g were prepared from the initial solution by drop deposition onto glass plates pre-treated with wetting and seeding agent, as described by Van Ammel et al. (2011). The diameters by which the activity was spread were 20 and 34 mm, on 34 mm and 55 mm glass discs respectively.

2.2 Activity standardisation

The activity of the sources was accurately determined by means of a primary standardisation method (Pommé, 2007) based on the counting of emitted alpha particles under a well-defined low solid angle or DSA (Bambynek, 1967; Pommé, 2015), using two DSA set-ups of the JRC (Pommé and Sibbens, 2008) and one at CIEMAT (Garcla-Torano et al., 2008). Passivated implanted planar silicon detectors were used with sensitive areas of 5000 mm and 3000 mm . The radial activity distribution of the sources was determined by means of autoradiographs (Sibbens et al., 2003) in order to optimise the accuracy of the calculated solid angle (Pommé et al., 2003). The sources were placed at approximately 5 cm distance from the detector, setting a geometrical efficiency of 5.5%-6.5% (=solid angle/4n). Baffles were placed inside the counting chamber to reduce the scattering of alpha-particles off inner walls of the counter chamber. The sources were measured both uncovered and covered with VYNS (vinyl chloride vinyl acetate copolymer) foils of 35^g/cm thickness. No effect of the covering on the measured activity was observed. The lower energy threshold selected for the counting was such as to include all the alpha decays of U and its decay products and impurities present above 1.6 MeV, and the missing tailing fraction of typically 0.1 % was estimated by extrapolating the tailing of the energy distribution down to zero energy. The calculated activities of the sources are shown in Table 2.

3 Gamma-ray measurements and analysis

All laboratories performed y-ray spectrometric measurements using high purity germanium (HPGe) detectors. An overview of the experimental setups and analysis methods is provided in Table 3.

3.1 JRC

source was placed at 120 mm from the endcap of a so-called Broad Energy Germanium (BEGe) detector installed in the underground laboratory HADES (Andreotti et

al., 2011) and measured for 41 days. The relative efficiency of the detector is 50% and the energy resolution is 1.7 keV at 1332 keV. The experimental full energy peak efficiency (FEP) was determined from eight calibration sources of various nuclides. Correction factors to take

into account the geometry and matrix of the U source have been calculated by Monte Carlo simulations using EGSnrc (Kawrakow et al, 2015) and included in the full energy peak efficiency (FEP) calculations. Differences between experimental and computational FEP of around 4% below and 3% above 100 keV, in accordance with the EUROMET Action 428, were included as an uncertainty component in the efficiencies (Lepy et al., 2001).

By using the Monte Carlo method with the DDEP decay data (Lutter, 2017), the coincidence summing corrections were estimated to be around 1% with a relative uncertainty of approximately 7%. As the coincidence summing effect depends on the radionuclide decay scheme which is not well established, no coincidence summing corrections were applied. Instead, an uncertainty of 1% was assigned to each y-ray emission probability to take into account the possible coincidence summing effect.

Peaks were identified manually and fitted by a Gaussian function using the Canberra Genie 2000 Interactive Fit Peak function. Overriding the automatic peak fitting is of particular importance for the treatment of doublet peaks. X-rays, and y-rays with energies less than 30 keV were not considered in the analysis of the spectrum.

3.2 CIEMAT The U

source was measured by an extended-range coaxial HPGe at a source-to-detector distance of 15 cm. Gamma-ray peak areas were determined using the IAEA software GRILS, a non-linear fitting code that provides accurate uncertainty estimations and is a part of the GANAAS suite (IAEA, 1991). The detector was originally calibrated by Monte Carlo simulation in an energy range from 14 to 1800 keV (Peyres and Garcla-Torano, 2007). To obtain the FEP efficiencies for the gamma-ray lines of U the Monte Carlo code PENELOPE 2014 was used (Salvat, 2015). To simulate the extended source, the emission point was randomly sampled from a numerical model based on data taken from the autoradiography of the source. Point sources from mono-energetic and multi-gamma emitters were measured at 15 cm from the detector window and provided a set of 26 experimental values to which the results of the simulations were compared. Discrepancies between simulated and experimental values were, in most cases, below 1%.

Coincidence summing corrections were done by a combination of simulations made with the PENELOPE (Salvat, 2015) and PENNUC codes (Garcla-Torano et al., 2017). The combination of both codes allows simulating the decay of a nucleus as a random sequence of transitions. Nuclear data are taken from the DDEP data base from which data sets can be exported in a compatible format.

3.3 ENEA

The detector used to measure the U source was a p-type HPGe. The source was placed at 9.8 cm distance from the detector end-cap. The detection efficiency calibration was done using ENEA point sources. The efficiency transfer from the point sources of standard geometry to the U source has been realised with the GESPECOR 4.1 (Sima et al., 2001) software. The same software has been used to calculate the coincidence summing correction coefficients.

3.4 CMI

Measurements were performed with a HPGe detector. The source-to-detector distance was set at 3 cm. Dead-time and pile-up effect corrections were made electronically and were smaller than 0.03 %. The FEP and total efficiencies of photon detection were calculated by the MCNP (Goorley et al., 2012) simulation code and experimentally validated. The method was validated by comparing computations and experiments. A set of EFS (CMI) standards was

used, including 241Am, 109Cd, 57Co, 177Lu, 203Hg. The peak areas were evaluated using the programme Canberra GENIE 2000, which applies the summation method to determine the area of isolated peaks. Multiplets were fitted with a Gaussian function and a linear background.

4 Results

A y-ray spectrum from a U source obtained at the JRC is shown in Fig. 2. Some of the identified peaks in the graph are emitted in the decay of its daughter nuclide Th. In total 33 y-ray lines were analysed and 53 y-ray emission probabilities with their associated uncertainties have been reported by all the 4 laboratories.

For those y-lines of which results were reported by more than one lab, the mean value was calculated using the power-moderated mean (PMM) method (Pommé and Keightley, 2015). The default setting of the PMM formalism was used, assuming that uncertainties have a tendency of being underestimated, which implies that for N measurement data the power of the weighting factors is reduced to a=2-3/N.

In Table 4 information from each laboratory is given on the coincidence correction factors (CCF) - i.e. presented as CCF minus 1 - and their assigned uncertainties for some of the main y-rays. It is easily discerned that each laboratory followed a different approach to evaluate their uncertainties, which complicated a balanced weighting of the mean values. A more homogeneous treatment of the CCF uncertainties was imposed in the evaluation phase, by initially excluding this uncertainty component for the calculation of the PMM values and afterwards adding a common 1% relative uncertainty on the CCF in quadrature. This compensated for disproportionate weighting and helped avoiding the reduction of this correlated uncertainty component through the calculation of a mean value.

An overview of the PY values is presented in Table 5 and a typical uncertainty budget in Table 6. The uncertainties on the mean PY values in the far right column of Table 5 correspond to the quadratic sum of the PMM uncertainty and the 1% uncertainty on the CCF. No extreme values have been identified by means of a k=2 exclusion criterion in the PMM formalism. In Fig. 3, the PMM values are presented against the experimental ones.

5 Discussion

When comparing the emission probabilities reported in this work with evaluated data from the DDEP (2017), it appears that 25 out of the 33 agree. The statistical significance can be interpreted through the use of the Z-score, shown in Fig. 4, and defined as

pm _ pev

a2 +a2

where PY and a are the y-ray emission probabilities and their associated standard uncertainties, for which the index m refers to values measured in this work and ev to the

DDEP evaluated data. If |Z| < 2, % probability.

then the agrees within the with a 95

pm ° pev

Eight out of the thirty-three P™ values presented in this work do not agree with the DDEP

recommended data. These include Py values which have been measured only once or/and have no assigned uncertainties, such as the ones for the 72.7 keV (Z=9), 356.03 keV (Z=3.6) and 289.56 keV (Z=2.1) y-rays measured by Vano et al. (1975) and the 54.25 keV (Z=2.5) Py by Baranov et al. (1977). The Py for the 275.49 keV y-ray (Z=26 for the 275.35+275.49 keV doublet) are published in three studies without including any uncertainties. The PY for the 31.6 keV (Z=6), and 51.21 keV (Z=2.8), y-rays have been measured 40 years ago.

6 Conclusions

Emission probabilities for 33 y-lines in the decay of U are reported in this paper and uncertainties have been reduced in seven cases. The photon emission probabilities higher than 0.3 % obtained in this work are in agreement with the recommended values from the Decay Data Evaluation Project (DDEP). For example, the measured 57.2 (7) % emission probability for the main y-ray of 185.72 keV agrees well with the currently DDEP recommended value of 57.1 (3) %. Discrepancies were observed for y-ray intensity values measured only once before 1980. New values for these cases are proposed.


This work has been supported by the European Metrology Research Programme (EMRP),

JRP-Contract IND57 MetroNORM ( The EMRP is jointly funded by

the EMRP participating countries within EURAMET and the European Union.

The authors would like to thank Gerd Marissens, Jan Paepen and Heiko Stroh from JRC-Geel

for their technical support.


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Figure Captions

Fig. 1: Simplified decay scheme of 235U into 231Th, using data from (DDEP, 2017).

Fig. 2: Gamma-ray spectrum from a U source taken by the JRC, using a BEGe detector in the underground facility HADES.

Fig. 3: The PMM values (solid lines) of the measured emission probabilities for y-rays with intensities higher than 0.1 % and energies above 100 keV, measured by more than one laboratory. The standard uncertainties of the mean values (dotted lines) include the 1% uncertainty on the CCFs. The PY from the DDEP data base (open circles) are included in the graphs for easy comparison.

Fig. 4: Comparison of y-ray emission probabilities values from this work with recommended data from the DDEP data base using the Z-score. The green triangle data points represent emission probabilities from this work whose uncertainties have been reduced compared to the DDEP recommended values. The blue star data points represent emission probabilities for which uncertainties are assigned for the first time. The Z score for the doublet (275.35+275.49 keV) is not included in the graph (Z = -25.5).

Table 1: Isotope amount ratios in the enriched U material used for measurement (Ref. date: 9th September 2006).

Isotope amount ratios

n(233U)/n(235U) n(234U)/n(235U) n(236U)/n(235U) n(238U)/n(235U)

<0.0000000005 0.000019552(31) 0.000038657(30) 0.000003367(75)

Table 2: Activity of 235U in the sources standardised by Defined Solid Alpha counting (k=1) at JRC and CIEMAT (ref. date: 24/07/2009 09:30:00 GMT) which were subsequently used in the indicated laboratories for the PY measurements.

Activity of 235U(Bq)

Laboratory Source JRC CIEMAT

JRC U235G0904 100.75(18) -

CMI U235G0916 63.49(15) 63.44(20)

ENEA U235G0919 38.46(9) 38.23(10)

CIEMAT U235S0924 45.50(10) 45.44(23)

Table 3: Overview of experimental setups and analysis methods.



Measurement time (days)

Low background HPGe, Planar BEGe, Thin dead layer,

50% relative efficiency

Extended range HPGE


Thin dead layer, 40 % relative efficiency

HPGe (p-type), FWHM 0.9 keV @ 122 keV and 1.9 keV @ 1332.5 keV, 40% relative efficiency

HPGe 4018


FWHM 1.8 keV @

1332 keV, Pb


40% relative


Source- detector distance (cm)

FEP Efficiency

PTB point sources + EGSnrc

Point and volume sources+MC

Point sources +GESPECOR 4.2

Point sources +MCNP

Coincidence corrections



Table 4: Summing coincidence correction factors (presented as CCF minus 1) and their relative uncertainties (urel) calculated for the determination of the U absolute y-ray emission probabilities reported by the laboratories.


Energy (keV) CCF-1 (%) urel(%) CCF-1 (%) urel(%) CCF-1 (%) urel(%) CCF-1 (%) urel(%)

109.19 0 1 0.4 0.4 0.07 1 0.56 0.1

143.77 0 1 0.7 0.05 0.097 1

163.36 0 1 0.2 1.2 0.035 1 0.23 0.1

182.62 0 1 1.5 0.6

185.72 0 1 0.4 0.3 0.002 1 0.56 0.1

194.94 0 1 0.5 0.5

202.12 0 1 1.2 0.2 1.557 1 8.95 0.3

205.316 0 1 0.1 0.7 0.023 1 0.22 0.1

221.39 0 1

Table 5: Absolute y-ray emission probabilities (%) measured in the 235U decay. A common relative uncertainty of 1% for the CCF has been added to the final result in the last column.

Energy (keV) DDEP JRC CIEMAT ENEA CMI This work

31.6 0.017(6) 0.079(9) 0.079(9)

42.01 0.056(9) 0.070(8) 0.070(8)

51.21 0.034(7) 0.010(4) 0.010(5)

54.25 0.0285 0.016(5) 0.016(5)

72.7 0.116 0.275(20)a 0.275(18)

74.94 0.051(6) 0.073(13)b 0.073(13)

109.19 1.66(13) 1.59(7) 1.84(30) 2.09(16) 1.82(17)

120.35 0.026 0.022(9) 0.022(10)

140.76 0.20(1) 0.224(15) 0.224(14)

143.767 10.94(6) 11.0(4) 10.64(26) 10.9(4) 10.92(13) 10.87(16)

150.936 0.09(3) 0.076(9) 0.076(9)

163.356 5.08(3) 5.20(19) 5.15(13) 5.13(25) 5.11(7) 5.14(8)

182.62 0.39(5) 0.367(15) 0.47(13) 0.39(4)

185.72 57.1(3) 58.3(21) 57.32(29) 57.6(21) 56.6(7) 57.2(7)

194.94 0.63(1) 0.635(24) 0.71(10) 0.66(4)

198.894 0.036(2) 0.044(3) 0.044 (3)

202.12 1.08(2) 1.07(4) 1.06(12) 1.08(11) 1.051(24) 1.06(3)

205.316 5.02(3) 5.08(19) 5.06(12) 5.08(23) 5.00(7) 5.04(8)

215.28 0.029(3) 0.030(2) 0.030(2)

221.386 0.118(5) 0.117(5) 0.101(10) 0.110(8)

228.76 0.0074 0.0067(10) 0.0067(10)

233.5 0.038(4) 0.032(2) 0.032(2)

240.88 0.074(4) 0.064(3) 0.064(4)

246.83 0.055(3) 0.048(2) 0.048(3)

266.47 0.0078(6) 0.0067(10) 0.0067(10)

275.35 275.49 0.051(6) 0.032 J 0.032(3)c 0.032(2)

289.56 0.0074 0.0053(10) 0.0053(10)

291.65 0.040(6) 0.0273(18)d 0.0273(19)

345.92 0.040(6) 0.038(3)e 0.038(3)

356.03 0.0053 0.0024(7) 0.0024(8)

387.84 0.040(6) 0.026(2) 0.026(2)

410.29 0.0032 0.0023(6) 0.0023(6)

a Possible interference with the 73.72 keV and 74.94 keV y-rays. b Possible interference with the 73.72 keV and 72.7 keV y-rays. c PT for the 275.35 keV and 275.49 keV y-ray doublet.

d Possible interference with the 291.2 keV y-ray discussed in the DDEP database. e Possible interference with the 345.4 keV y-ray discussed in the DDEP database.

Table 6: Typical relative uncertainties for the emission probability of the 185.72 keV and 202.12 keV y-rays (k=1).

185.72 keV - 57.1% 202.12 keV - 1.08%


Counting statistics 0.08 0.5 0.55 0.12 0.65 10.9 9.6 2

Efficiency 3.6 0.41 3 1.1 3.6 0.2 3 1.1

Coincidence 1 0.3 1 0.1 1 0.2 1 0.3


Source activity 0.3 0.5 0.5 0.3 0.3 0.5 0.5 0.3

Dead Time/pile-up 0.04 0.52 0.01 0.04 0.54 0.01

Total 3.7 0.9 4.3 1.2 3.8 10.9 10.1 2.3

Fig. 1

235U (704 (1) 106 a)

387.841 keV

cr, to

Ci 1-H

j/ 5.95 (1) %

3 <N 00 Q

277.56 keV

236.902 keV

r\| ta rvj

221.3S6 JteV

205.313 keV

185.721 keV

96.17 keV

¡N ^ ^ ■M ftj JE

41.954 keV

V3.33 (6) %

18.8 (13)%

_iT 0.106 (16) %

■g i/57.19 (20) %

" jT 3.01 (16) %

28 (5) %

3.79 (6) % j/4.74 (6)%

Th (25.522 h)

Fig. 2




Energy (keV)

+CT 10£ ).19 keV

•CT1 1 1

Q- O I- < ^

ljj a: < ljj ^

Q ^ ^ Z O


182.62 keV

202.1 keV






143.7 keV

Q- O I- < 5

ljj a: < ljj ^

Q -5 ^ z O


+CT ' 181 .72 keV

205.3 keV

< ^ £ 8

163.36 keV

I ____i.........

0- O I- < 5

ljj a: < ljj ^

q -5 ^ z o


+CT 1 194.94 keV

Ca i mPTM--

221.39 keV

Fig. 4

O O (D

I 0 1 -J

1E-3 0.01

0.1 1 P(%)

10 100


y C=-2

100 200 300 400

Energy (keV)


• Measured gamma-ray emission probabilities per decay of 235U.

• Mean values taken from 4 laboratories.

• Source activity determined by primary standardisation and mass spectrometry.

• Results reported for 33 y-ray lines.

• Generally in good agreement with literature values.