Physics Procedia 31 ( 2012 ) 99 – 109

1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of Institute for Reference Materials and Measurements.doi: 10.1016/j.phpro.2012.04.014

GAMMA-1 Emission of Prompt Gamma-Rays in Fission and Related Topics

Prompt gamma activation analysis at the Budapest Research Reactor

T. Belgya a* Institute of Isotopes, Hungarian Academy of Sciences, Konkoly T. u. 29-33, H-1121 Budapest, Hungary

Abstract

Prompt gamma activation analysis is a powerful method, but it is not yet well enough known in the nuclear data community. For this reason, we describe the PGAA measurement and data evaluation processes, as well as the energy and spatial distribution measurements on our cold neutron beam at the PGAA-NIPS facilities of the Budapest Research Reactor. We emphasize that the strong spatial inhomogeneity of the neutron beam puts a constraint on the samples to be analysed if it is important to obtain accurate quantitative measurement results. Finally, an overview of our recent experiments is given. © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the organizer Keywords: Prompt Gamma Activation Analysis; Reactor, cold neutron spectrum; gamma spectroscopy; radiative capture cross section.

1. Introduction

Prompt gamma activation analysis (PGAA) is a nuclear analytical method which utilizes the prompt gamma radiation released in the radiative neutron capture reaction to identify and quantify the elemental or isotopic content of samples. Its application was retarded compared to the instrumental neutron activation analysis due to the much more complex gamma ray spectra obtained and the lack of a suitable analytical database [1]. The first complete database was published in 2004 [2].

PGAA is a non-destructive analytical method, which can provide instantaneous, multielemental results even during the acquisition of the gamma ray spectrum. It provides bulk analytical results for the

* Corresponding author. Tel.: +36-1-392-2539; fax: +36-1-392-2533. E-mail address: tamas.belgya@energia.mta.hu

Available online at www.sciencedirect.com

© 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of Institute for Reference Materials and Measurements.

Open access under CC BY-NC-ND license.

Open access under CC BY-NC-ND license.

100 T. Belgya / Physics Procedia 31 ( 2012 ) 99 – 109

irradiated volume due to the large penetrability of neutrons and gamma rays. The method is suitable for analysis of major components of the sample but for a few elements with large cross sections, it is able to determine them at trace levels i.e. below ppm. It is unique in the observation of hydrogen either in bound or unbound states and is very sensitive for boron, a trace element that correlates well with volcanic activities at volcanic arcs near subduction zones. Its non-destructive feature makes it an ideal tool for studying the elemental composition of archaeological samples, which helps archaeologists to find the origin of objects and their trading routes.

The best-suited places to perform PGAA are at research reactors or at intense pulsed spallation sources. Here we will concentrate on the results from PGAA facility built at the Budapest Research Reactor. Since starting its operation in 1996, several major upgrades were accomplished at the Budapest prompt gamma facility in order to improve its performance and productivity [3]. Our intention is to utilize this cold neutron beam facility as fully as possible. Currently we operate two experimental stations utilizing spatially separated upper and lower beams. On the upper beam, the PGAA station is installed close to the end of the supermirror neutron guide and the NIPS station is situated about 1 m downstream on the lower beam. The modular neutron flight tube can host various arbitrary setups beside its regularly used ones [3]. Especially, the NIPS station is intended to host such special equipment as a fission chamber or time and space resolved wire chamber etc. to server users coming to us in Transnational Access programs supported by our EU projects through the Budapest Neutron Centre [4].

The PGAA method has significantly advanced at our laboratory in the past 15 years [2, 5-7] and not only the method, but also the supporting data library was developed [8-10]. Beside the development of the method, we constantly try to expand its applications in various fields such as archaeomerty [11, 12], geology [13-15], in-beam catalysis [16-19], material sciences [20-24] and even in the explanation of megafaunal extinctions and the Younger Dryas cooling ca. 12.9 ky ago [25].

The goal of this article is to introduce the PGAA method to non-specialists and possible industrial partners, and its applications for nuclear data, nuclear safeguards and nuclear energy. We start by describing the neutron beam used to initiate the capture reactions.

2. The neutron flux distribution at the Budapest PGAA-NIPS facilities

A current view of the PGAA-NIPS facilities is shown in Figure 1. The modular aluminium neutron flight tube lined inside with neutron absorbent shielding can be evacuated. This modular flight tube surrounds the neutron beam and hosts a beam chopper as well as holding the target to be bombarded. The chopper provides a pulsed beam, which was recently used to measure the neutron spectrum recently with a 2D position-sensitive multi-wire, 3He filled neutron detector obtained from János Füzy [26].

A neutron time of flight measurement was made with the chopper in pin-hole geometry with a hole-diameter of about 1 mm situated at the middle of the upper beam and with a flight distance of 190 cm from chopper to detector. The measured, efficiency corrected [26] cold neutron spectrum is shown in Fig. 2.

After the transformation of the efficiency corrected TOF spectrum we obtained the neutron energy spectrum (see Fig. 3). The valleys in the neutron spectrum are due to monochromator crystals placed down stream in the beam that serve other instruments. To obtain correct energy scale the determination of time zero is very important. This was done by inserting polycrystalline graphite and BeO filters in the beam and changing the start time to obtain the literature values of their Bragg edges. The average energy of the neutron spectrum is 0.012 eV, which corresponds to 140 K.

This strong spatial dependence of the neutron beam puts geometrical constraints on the target to be analysed, which was discussed in Ref. [27]. Briefly, it means that unique results can be obtained for elemental mass ratios without any spatial dependence correction if the target is thin and homogeneous. A

T. Belgya / Physics Procedia 31 ( 2012 ) 99 – 109 101

second requirement is that the elemental cross sections must follow the 1/v rule to avoid a necessary correction due to the energy dependence of the beam.

Figure 1: Panel (a) shows the PGAA flight tube and the sample chamber and the gamma-ray spectrometer. Panel (b) shows the whole PGAA-NIPS facility enclosed in a cabin.

120x103

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Cou

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nnel

14121086420Wavelength AA

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6

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Effi correction factor

tproject17 t17_effi_corrected Absorption_in_3cm_3bar_He3

Figure 2: The (green-dotted) line is the measured time-projected neutron distribution, the higher amplitude (red) curve is efficiency corrected, while the (blue-dashed) curve shows the efficiency correction factor as a function of the neutron wavelength in Ǻngstrom.

a b

Chopper

102 T. Belgya / Physics Procedia 31 ( 2012 ) 99 – 109

Figure 3: The neutron spectrum as a function of the energy. The quantity mc2 in the formula is the neutron mass in eV, t is the neutron arrival time, l is its flight distance, and c is the speed of light.

The spatial distribution of the neutron beam, which was measured earlier using a neutron radiograph in our EU FP6 ANCIENT CHARM project. It is shown in Figure 4.

Figure 4: False color spatial distribution of the beam intensity. The red (darker in the middle) area belongs to the high value. Size of 2 cm is shown by a horizontal black line.

0

2000

4000

6000

8000

10000

12000

14000

16000

0 0.02 0.04 0.06 0.08 0.1

E (eV)

dN/d

E

22

23

lmc

ctdtdN

dEdN

dEdt

dtdN

dEdN

−=

=

T. Belgya / Physics Procedia 31 ( 2012 ) 99 – 109 103

3. PGA analysis of samples

The measurement of a sample is a simple process. The sample can be in solid, powder, liquid or gaseous form. The sample is placed – usually with a minor or no sample preparation – into the target chamber where it is irradiated with the neutron beam. The emitted gamma rays are detected with a BGO guarded Compton-suppression HPGe detector. The processed and digitized pulse height signals are recorded in a PC driven multi-channel analyser. A typical spectrum from wood ash obtained from a bio-energy plant is shown in Fig. 5. The spectrum contains about 1200 peaks.

The acquired spectra are analysed with the Hypermet PC program [28-30] that provides peak energies and areas. The evaluation continues with homemade EXCEL software, which calculates elemental masses from the irradiated part of the sample [5, 6]. The analysis assumes the validity of the above constraints and uses the following simple form

)()(; 0 γγ

σ

γγ ⋅ε⋅φ⋅⋅σ⋅θ⋅=⋅⋅=

γ

EfEPMNStSmA A , (1)

where Aγ is the measured gamma-peak area at energy Eγ, m is the mass of the element that emitted the

gamma-ray, S is the sensitivity and t is the irradiation and acquisition time. The sensitivity depends on the Avogadro number NA, the molar mass M, the elemental partial gamma-ray production cross section σγ, the neutron flux φ, the detector efficiency ε(Eγ) and neutron and gamma absorption correction factor f(Eγ).

Figure 5: PGAA spectrum of ~1 g of wood ash.

The simplified flow diagram of the EXCEL software is shown In Fig. 6. The partial gamma-ray production cross sections σγ, directly measured in our laboratory and published

in [7, 31], can be used for PGAA analysis.

1E+0

1E+1

1E+2

1E+3

1E+4

1E+5

1E+6

0 2000 4000 6000 8000 10000 12000

E (keV)

Cou

nts/

0.7

keV

104 T. Belgya / Physics Procedia 31 ( 2012 ) 99 – 109

Figure 6: Simplified flow diagram of the PGAA analysis software.

The output for the wood ash is shown in Fig. 7. It contains the most important input parameters of the analysis and the results. The c% atom means atomic percent while the other three columns with c% are weight percent of element per all elements, element per all oxide and oxides per all oxides. Oxygen has a very low neutron capture cross section and thus it is difficult to measure. Instead of the measured value, the value is calculated for the maximum oxidation number.

Figure 7: Output of the PGAA analysis software. Masses are in gram unit.

Spectrum: N22HAM1.MCA Peak list: N22HAM1.pkl Uncertainty calculation: statisticalLive time: 39126.63 s Neutron Flux: 1.50E+8 ±2 %, temp 35 K, BKG: 13 vac08jan Conc. format: ppm / %

Z El M mmeas

unc%

mBkg

unc%

mnet

ox.st.

mox

unc%

c%atom

unc%

c%el/el

unc%

c%el/ox

unc%

c%ox/ox

unc%

1 H 1.008 6.82E-4 0.6 1.15E-5 2.0 6.70E-4 1 5.99E-3 0.6 4.02 1.9 0.187 1.5 0.093 0.9 0.83 1.86 C 12.01 0.080 3.8 0.0 0.08 4 0.29 3.8 40 2.5 22 3.2 11.1 3.4 41 2.49 F 19 4.89E-2 5. 2.06E-4 20. 4.87E-2 -1 4.87E-2 5. 16 4. 14 4. 6.8 5. 6.8 5.

12 Mg 24.31 1.08E-2 5. 0.0 1.08E-2 2 1.79E-2 5. 2.7 5. 3.0 5. 1.5 5. 2.5 5.13 Al 26.98 1.06E-2 1.6 1.73E-3 3.0 8.84E-3 3 1.67E-2 2.0 1.98 2.7 2.47 2.4 1.23 2.1 2.32 2.614 Si 28.09 0.064 2.1 0.0 0.06 4 0.14 2.1 13.7 2.5 17.8 2.2 8.8 2.1 18.9 2.415 p 30.97 7.23E-3 5. 0.0 7.23E-3 5 1.66E-2 5. 1.4 6. 2.0 6. 1.0 5. 2.3 6.17 Cl 35.45 7.99E-4 39. 0.0 7.99E-4 -1 7.99E-4 39. 0.1 39. 0.2 39. 0.11 39. 0.11 39.19 K 39.1 3.78E-2 2.4 0.0 3.78E-2 1 4.56E-2 2.4 5.9 2.9 10.6 2.5 5.3 2.4 6.3 2.820 Ca 40.08 0.092 2.4 0.0 0.09 2 0.13 2.4 13.9 2.7 26 2.1 12.8 2.2 18.0 2.622 Ti 47.87 8.30E-4 2.4 0.0 8.30E-4 4 1.39E-3 2.4 0.105 3.0 0.23 2.7 0.115 2.5 0.193 2.924 Cr 52 1.21E-5 49. 0.0 1.21E-5 3 1.77E-5 49. 10 ppm 49. 30 ppm 49. 20 ppm 49. 20 ppm 49.25 Mn 54.94 2.35E-3 3.8 3.18E-6 10. 2.35E-3 3 3.37E-3 3.8 0.26 4. 0.65 4.0 0.33 3.8 0.47 4.27 Co 58.93 1.08E-5 15. 0.0 1.08E-5 2 1.38E-5 15. 11 ppm 15. 30 ppm 15. 15 ppm 15. 19 ppm 15.28 Ni 58.69 3.35E-5 11. 0.0 3.35E-5 2 4.27E-5 11. 35 ppm 11. 90 ppm 11. 50 ppm 11. 60 ppm 11.34 Se 78.96 2.59E-4 8. 0.0 2.59E-4 4 3.64E-4 8. 200 ppm 9. 0.07 8. 360 ppm 8. 0.051 8.38 Sr 87.62 9.19E-4 5. 0.0 9.19E-4 2 1.09E-3 5. 0.063 5. 0.26 5. 0.13 5. 0.15 5.48 Cd 112.4 1.07E-6 1.9 0.0 1.07E-6 2 1.22E-6 1.9 0.58 ppm 2.6 3.0 ppm 2.3 1.49 ppm 2.0 1.70 ppm 2.649 In 114.8 3.63E-6 8. 0.0 3.63E-6 3 4.39E-6 8. 1.9 ppm 8. 10 ppm 8. 5.0 ppm 8. 6.1 ppm 8.50 Sn 118.7 3.13E-3 6. 0.0 3.13E-3 2 3.55E-3 6. 0.16 6. 0.87 6. 0.43 6. 0.49 6.60 Nd 144.2 1.13E-5 8. 0.0 1.13E-5 3 1.32E-5 8. 4.7 ppm 9. 31 ppm 8. 16 ppm 8. 18 ppm 9.62 Sm 150.4 6.74E-7 1.6 0.0 6.74E-7 3 7.82E-7 1.6 0.271ppm 2.4 1.88 ppm 2.1 0.94 ppm 1.8 1.09 ppm 2.464 Gd 157.3 9.60E-7 19. 0.0 9.60E-7 3 1.11E-6 19. 0.4 ppm 19. 3 ppm 19. 1.3 ppm 19. 1.5 ppm 19.

000

0.35818 2.6 0.71964 1.3 100.31 100.36 49.78 100.43Quantification limit for 50 % - O calculated 0.36146 50 % O/ total

mass without O 0.35818self-abs.: no (recalc.: Ctrl+Shift+S) thickness (mm) : 1 density: 2.7 oxide: yes

version: 3.2.2 (2008.04.21)

INPUT MODULE Peak list from Hypermet PC (up to 1200 lines)

PGA library-25 (~1800 lines)

PGA library-1% (~6200 lines)

Element data (Molar mass, oxidation number, γ and n self-absorption)

Background in equivalent mass vs. range of dates.

Efficiency vs. range of dates

Auxiliary data (Beam temperature and flux, sample density and thickness)

DATA EVAULATION MODULE

OUTPUT MODULE

Iterative edit of the data by the analyst

T. Belgya / Physics Procedia 31 ( 2012 ) 99 – 109 105

To check the goodness of our partial gamma-ray production cross section library values we made measurements on seven certified geological standards. The results are shown in Fig. 8 for the major components as oxides.

0

10

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30

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0 10 20 30 40 50 60 70 80

PGAA w%

Cer

tifie

d w

%

Al2O3CaOFe2O3K2OMgONa2OSiO2TiO2Line

02468

101214161820

0 10 20

PGAA w%

Cer

tifie

d w

%

Al2O3CaOFe2O3K2OMgONa2OSiO2TiO2Line

Figure 8: Left diagram has all of the 52 data, while the right diagram shows an enlarged portion of the left diagram.

The agreement is good, which means that the PGAA database gives reliable results from the analysis.

4. Measurement of other quantities

Equation (1) can be used to determine any single quantity which it depends on. Table 1 lists these quantities we can measure using Eq. (1). Any of the listed measurement types can be interesting for the nuclear data or nuclear structure experimentalists. In the rest of the article we briefly summarise the measurements that have already been performed at our PGAA-NIPS facilities.

Table 1. List of the type of measurement based on Eq. (1).

Unknown Type of measurement

m PGAA

m(r) PGAI

θ Isotopic composition

σ0 Nuclear data, nuclear (astro) physics

Pγ Nuclear structure, nuclear data

φ Average flux

φ(r) Flux spatial distribution

ε(Eγ) Efficiency, intensity calibration

f(Eγ) Gamma and/or neutron absorption in the sample

γ ≡ particle Particle production (capture or fission)

We have already given details of the mass measurement method. Spatial distribution of element mass,

(m(r)), has already been measured in our recent EU FP6 ANCIENT CHARM project. The first quasi-3D element mass distribution measurement based on PGAA was performed in our laboratory [32, 33]. In the

106 T. Belgya / Physics Procedia 31 ( 2012 ) 99 – 109

EU FP6 EFNUDAT project, we have performed several experiments on enriched target compositions (θ )on Fe, Zr, and Hf samples [34]. The determination of the total radiative neutron capture-cross sections (σ0) of isotopes is the most demanded quantity by the nuclear data and astrophysics community. The ways of evaluation methods using (n,γ) reactions were discussed in [35-37] and many values have already been determined for long-lived fission fragments [38, 39], for U [40, 41], for structural materials [42-44] and for many isotopes in the Evaluated Gamma-ray Activation File (EGAF) [37]. The absolute decay probabilities (Pγ) are frequently used in low energy nuclear physics to check the predictive power of nuclear model calculations. These values are also used in efficiency calibrations especially for high gamma ray energies for which no radioactive sources can be found [45-48].

At high level-energies, the gamma-ray decay probabilities behave statistically; their values fluctuate widely and follow the Porter-Thomas distribution. We have started to study these nuclear properties as well [36, 49-51] and recently our study became more focused in the EU FP6 EFNUDAT and EU FP7 ERINDA projects [52]. Average flux determination (φ) is a routine job at neutron sources; however, the high resolution spatial and energy resolution flux measurements developed a lot in the past few years due to the high-resolution radiography systems. This topic was covered in detail in the previous part of this paper. The measurement of the efficiency (ε(Eγ)) at high energy of detectors require the knowledge of absolute decay probabilities (Pγ) and vice-versa. This problem has been addressed in Ref. [42]. In strong gamma and/or neutron absorbers, it is important to obtain an absorption correction that can be measured [53] or calculated [2].

Finally, if instead of the gamma a particle is emitted; many other kinds of measurements can be thought of. There are some works in which we have already studied particle emissions. One of the most important particle-emitting reactions is fission, which is very important for the nuclear data and structure communities. Recently in the EFNUDAT project, we have re-measured the neutron emission energy and angular distribution in the 235U(n,f) reaction [54]. In an other experiment, also in the EFNUDAT project, we performed a test experiment using the VERDI 2E-2v fission fragment spectrometer on the 235U(n,fγ) reaction [55, 56]. At the same time we also measured the prompt fission gamma-ray distribution using fast La-containing detectors [57].

5. Summary

In this article, we have described PGAA measurement and data evaluation methods, and the energy and spatial distribution of our cold neutron beam. We emphasize that the strong spatial distribution of the neutron beam puts constraint on the sample if it is important to obtain accurate quantitative measurement results. Finally, an overview of our experimental capabilities was given.

Acknowledgements

Financial support of the EU FP6 ANCIENT CHARM (No. 15311), the EU FP6 EFNUDAT (No. 036434), the EU FP6 ERINDA (No. 269499) and the NAP VENEUS05 (OMFB-00184/2006) projects are greatly acknowledged. Contribution of L. Szentmiklósi and Z. Kis in the neutron spectrum experiments is acknowledged.

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[40] Trkov A., Molnár G. L., Révay Z., Mughabghab S. F., Firestone R. B., Pronyaev V. G., et al. Revisiting the U-238 thermal capture cross section and gamma-ray emission probabilities from Np-239 decay. Nuclear Science and Engineering 2005;150:336-348.

[41] Wallner A., Buczak K., Quinto F., Steier P., Belgya T., Szentmiklosi L., et al. Neutron-capture Studies on (235)U and (238)U via AMS. J. Korean Phys. Soc. 2011;59:1410-1413.

[42] Belgya T. New gamma-ray intensities for the N-14(n,gamma)N-15 high energy standard and its influence on PGAA and on nuclear quantities. J. Radioanal. Nucl. Chem. 2008;276:609-614.

[43] Krticka M., Firestone R. B., McNabb D. P., Sleaford B., Agvaanluvsan U., Belgya T., et al. Thermal neutron capture cross sections of the palladium isotopes. Physical Review C 2008;77:054615.

[44] Tomandl I., Honzatko J., von Egidy T., Wirth H. F., Belgya T., Lakatos M., et al. Thermal neutron capture cross sections of tellurium isotopes. Physical Review C 2003;68:067602.

[45] Belgya T. Improved accuracy of gamma-ray intensities from basic principles for the calibration reaction 14N(n,g)15N. Physical Review C 2006;74:024603-1-8.

[46] Belgya T. New gamma-ray intensities for the N-14(n,gamma)N-15 high energy standard and its influence on PGAA and on nuclear quantities. Journal of Radioanalytical and Nuclear Chemistry 2008;276:609-614.

[47] Belgya T. and Molnár G. L. Accurate relative gamma-ray intensities from neutron capture on natural chromium. Nucl. Instr. and Methods B 2004;213:29-31.

[48] Molnár G. L., Révay Z. and Belgya T. Accurate absolute intensities for the Cl-35(n,gamma) reaction gamma-ray standard. Nucl. Instr. and Methods A 2004;213:32-35.

[49] Borella A., Belgya T., Kopecky S., Gunsing F., Moxon M. C., Rejmund M., et al. Determination of the 209Bi(n,γ )210Bi and 209Bi(n,γ )210m,gBi reaction cross sections in a cold neutron beam. Nuclear Physics A 2011;850:1-21.

[50] Voinov A., Grimes S. M., Agvaanluvsan U., Algin E., Belgya T., Brune C. R., et al. Level densities of iron isotopes and lower-energy enhancement of gamma-strength function. In: 12nd international Conference on Capture Gamma-Ray Spectroscopy and Related Topics, September 4-9, 2005 University of Notre Dame, Indiana, USA, AIP Melville, New York; 2005. p. 545-547

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[52] Massarczyk R., Birgersson E., Schramm G., Schwengner R., Belgya T., Beyer R., et al. Photon strength function deduced from photon scattering and neutron capture. In: EFNUDAT User and Collaboration workshop: Measurements and Models of Nuclear Reactions, 25-27 May 2010, Paris, France, 2010. p. EPJ Web of Conferences 8, 07008

[53] Borella A., Schillebeeckx P., Molnár G. L., Belgya T., Révay Z., Szentmiklosi L., et al. The 209Bi(nth, g)210Bi and 209Bi(nth, g)210m,gBi Cross Sections Determined At The Budapest Neutron Centre. In: 10th International Conference on Nuclear Data for Science and Technology, September 26 - October 1, 2004, Santa Fe, New Mexico, AIP; 2005. p. 648-651

[54] Kornilov N., Hambsch F. J., Fabry I., Oberstedt S., Belgya T., Kis Z., et al. The U-235(n, f) Prompt Fission Neutron Spectrum at 100 K Input Neutron Energy. Nuclear Science and Engineering 2010;165:117-127.

[55] Oberstedt S., Belgya T., Billnert R., Borcea R., Cano-Ott D., Göök A., et al. Correlation measurements of fission-fragment properties. In: EFNUDAT User and Collaboration workshop: Measurements and Models of Nuclear Reactions, 25-27 May 2010, EPJ Web of Conferences 8, Paris, France, 2010. p. 03005

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