Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 3, pp. 283-286, 2005

Cross Section of Thermal-Neutron Capture Reaction by 99Tc
Kazuyoshi Furutaka,*, a Hideo Harada,a Shoji Nakamura,a Toshio Katoh,a Toshiyuki Fujii,bHajimu Yamana,b and Subramanian RamancaTokai works, Japan Nuclear Cycle Development Institute, 4-33 Muramatsu, Tokai-mura, Naka-gun, Ibaraki
319-1194, Japanb
Research Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka 590-0494, Japanc
Physics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.
Received: July 31, 2005; In Final Form: September 30, 2005
An account is given of a series of study to accurately determine the cross section of the 99Tc(n,γ)100Tc reaction forthermal neutrons. Methods used are an activation analysis in which γ rays in 100Ru are examined, and analyses ofground-state transitions in 100Tc. To reduce uncertainty of the cross section obtained in the activation measurement, an attempt was made to accurately determine emission probabilities of γ rays in 100Ru.

1. Introduction

30*Corresponding author. E-mail: furutaka.kazuyoshi@jaea.go.jp.
FAX: +81-29-282-5458.

JENDL–3.3

24
22

23

JENDL–3.2

INDC(NDS) –440

21(dec ay )ENDF/B–VI(prompt, lower limit)

Mo l n a r e t a l .

18(dec ay )18

Mo l n a r e t a l .

(prompt , lower limit)

19

Present (Furutaka et al .)Present (Furutaka et al .)12

L u ca s e t a l .

Present (Harada et al .)10

11

Ovechkin et al .

Watanabe et al .

89Pattenden et al .

7

Tattersal l et al .

0

Pomerance et al .

6

10

19

20

σ0 / barns

Technetium-99 (half-life T1/2 = 2.1 × 105 years)1 is one of thelong-lived fission products (LLFP) that are produced in commercial nuclear power plants with relatively large yields (cumulative yield for 235 U: 6.11%). 2 Technetium-100, which is
produced in neutron-capture reaction by 99Tc, β-decays to its
stable daughter nuclide 100Ru, with a short half-life of 15.3 s.3
Also, cross section of 99Tc(n,γ) reaction is reported to be relatively large. These facts make the nuclide 99Tc among possible
candidates for transmutation using neutron.
In order to design transmutation systems for 99Tc using neutron-capture reaction and to study their feasibilities, accurate
cross section of the 99Tc(n,γ) reaction is in need (in Reference
4, the cross section is requested to be determined as accurate
as 5% in an energy range between thermal and 20 MeV), andtherefore in recent years many efforts have been put to measure
the cross section experimentally (see Reference 5 and the references therein). Among cross section of the (n,γ) reaction, theone at thermal energy (2200 m/s), σ0, is important, because not
only is it used by itself to calculate quantities such as reaction
rate for thermal neutrons, but also it is often used to processexperimental data obtained in the so-called resolved resonanceregion, namely to normalize, to check consistencies of experimentally obtained resonance parameters and so on.
Several authors have reported the cross section of the 99Tc(n,γ)
reaction for thermal neutrons. Pomerance 6 and Tattersall7
reported the cross sections obtained by using the pile oscillator
method in which a sample is oscillated between a center of areactor and a position outside the reactor and the resulting
modulation of the pile power is observed as a measure of thecross section. Pattenden8 and Watanabe et al.9 measured total
neutron cross sections as functions of neutron energy, anddeduced capture cross sections at thermal energy by subtracting scattering cross sections. Some researchers employed activation methods to deduce the cross section. Ovechkin et al.10
detected γ rays in 100Ru emitted in the capture reaction to deduce
rate of the reaction and the cross section. Lucas et al.11 examined changes in isotopic composition of Ruthenium in the irradiated samples by a mass-spectroscopic method after neutron
irradiations. The leftmost quarter of Figure 1 shows the reported
values of σ0 of the 99 Tc(n,γ) reactions. As can be seen in the

Figure 1. The cross sections of the 99 Tc(n,γ) reaction for thermal
neutrons reported in References 12 and 18 are compared with otherpublished experimental data as well as evaluated ones. The filled circles (●) represent experimental data, whereas the filled diamonds(◆) indicate evaluated ones.

figure, the reported values are not accurate enough (the uncertainty of the most precise one being as large as 8%) 8 and at thesame time scattering around about 20 barns.
In order to accurately determine the cross section of thermalneutron capture reaction by 99Tc experimentally, a series of study
has been performed:• Measurement of σ0 using an activation method in which γ rays
in 100Ru were observed,• Measurement of emission probabilities of γ rays in 100Ru, and• Determination of σ0 from analyses of the prompt γ rays emitted in the capture reaction.
In the following, the research is described, and the obtained
results are compared to others. Possible improvements of theexperimental methods are also presented.
2. Measurement of the σ0 Using an Activation Method
The product of the 99Tc(n,γ) reaction, 100Tc, β-decays to 100Ru© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2005

284 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005
with T1/2 = 15.3 s; most of the decays (93%) directly go to theground state of 100Ru, but a part of the intensity feeds its excitedstates. The excited states eventually deexcite by emitting some
γ rays, the most intense ones being 540-keV and 591-keV ones,
and produce stable 100Ru nuclei in its ground state. Therefore,by counting these γ rays in 100Ru, one can deduce the numberof 100Ru nuclei produced, and consequently reaction cross section, if the numbers of target nuclei and irradiated neutrons aswell as the emission probabilities of the γ rays are known.
The details of the experiment have been presented elsewhere,12 and so here it’s described only briefly. Four irradiation
samples of ammonium pertechnetate containing ~370 kBq of99
Tc were used in the measurement; the number of 99Tc nuclei
in each sample was determined through β-ray counting using aliquid scintillation detector and weighing. Each sample wasirradiated by neutrons for 2 minutes using a rotary specimen
rack of the TRIGA MARK II reactor of Rikkyo University. At
the irradiation position, the thermal neutron flux was 5 × 1011
n/cm2 /s and the epithermal index in the Westcott’s convention
was 0.033. Each sample was irradiated four times, two with asurrounding Cd capsule and two without, with 40 minutes ofcooling in between. The neutron flux at the irradiation position was measured separately between the Tc irradiations byusing Co/Al and Au/Al alloys as activation detectors. Aftereach irradiation, γ rays emitted from the sample were measured
using a high purity Ge detector with 90% relative efficiency
and 2.1 keV FWHM at 1.33 MeV. The distance between thesample and the front surface of the detector endcap was 100 mm.
In the observed spectra, peaks due to the 540- and 591-keV γ
rays were clearly identified. From the peak counts of these γrays, reaction rates of the capture reaction were deduced andanalyzed based on a modified version of Westcott’s convention.
With the emission probabilities for the two γ rays listed in
Reference 13, after a correction of summing-coincidence effect,
the cross section was deduced. We have adopted the cross section obtained from the analysis of the 540-keV γ ray. The value
obtained is 22.9 ± 1.3 b.12 The result agrees well with the values
reported in References 8 and 10, but about 3 barns higher thanthat listed in References 6, 7, 9, and 11. Note that the uncertainty of the emission probability is not included in the error.
By including the uncertainty, the result would be 22.9 ± 2.6 b.12
As is easily seen, the overall accuracy can be largely improvedby reducing the uncertainty of the emission probability.
3. Measurement of Emission Probabilities of γ rays in 100Ru
The emission probabilities listed in Reference 13 are basedon the study done by Berzins et al.14 In the study, the number of(β, 591-keV γ) coincidences per emitted γ ray were measured byusing a NaI(Tl) and a plastic scintillator as a γ- and β-ray detector, respectively. Neglecting indirect β feeding to the 1131-keV
state, the authors have deduced the β feeding to the 1131-keVlevel as 5.7%. The authors did not quote the uncertainty of thevalue; in Reference 15 the uncertainty was estimated to beabout 16%. The uncertainties of the absolute intensity of the
540-keV γ ray quoted in References 1 and 13 are 17.14% and10%, respectively, which are rather large. In order to determine
emission probabilities of the γ rays in 100Ru and to estimate theiruncertainties accurately, a β−γ coincidence system has beendeveloped which consist of a plastic scintillation detector as a β
detector and a large-volume high-purity germanium detector as
a γ detector,16 and applied to an experiment.3
The experiment has been performed at the Kyoto University
Research Reactor Institute. Samples of technetium containing
5 kBq of 99Tc were irradiated for 15 s using one of the pneumatic irradiation tubes of the research reactor in the institute.
The neutron flux at the irradiation position is 2.3 × 1013 n/cm2 /s
for thermal and 8.4 × 1011 n/cm 2 /s for epithermal neutrons.
After each irradiation, the sample was placed between the

Furutakadetectors and coincidence measurement was done for at least
120 s. The distances between the sample and the front surfacesof the detectors were 30 mm and 100 mm for γ- and β-ray detectors, respectively. In some of the measurements an acrylic plate(16 mm in thickness) was inserted between the sample and theβ-ray detector to prevent the β-ray from entering the detector
and to estimate the sensitivity of the detector to γ rays. A total of
99 measurements were done.
From the observed β and γ spectra, ratios of the number of
β−γ coincidences to the number of β rays are deduced for 540and 591-keV γ rays as functions of applied β-ray detection
thresholds. By extrapolating the functions to vanishing threshold value, the effect of the β detection threshold to the ratio
was eliminated. After making corrections of summing-coincidence effect and γ-ray sensitivity in the β detector, the emission
probabilities of the 540- and 591-keV γ rays were obtained as
6.6 ± 0.5% and 5.5 ± 0.3%, respectively. The obtained results
agree with that reported in Reference 1 within limits of errors,
but are a little bit smaller. The estimated errors are muchsmaller than those quoted in Reference 1.
4. Measurement of the σ0 from Yields of the Prompt γ rays
In a separate series of study, one of the present authors has
demonstrated the great capabilities of determining the crosssections of the neutron capture reactions, by detecting γ rays insingles measurements with a sensitive detection system andconstructing level schemes.17 In the study, prompt γ rays emitted in neutron capture reactions by nuclides with masses up to
60 were measured, and almost complete decay schemes wereconstructed by analyzing the obtained data based on the
Rydberg-Ritz combination principle. By summing up intensities of primary transitions from a capturing state or of secondary ones to the ground state of a nuclide, one can deduce crosssection of the reaction. The method was applied to a measurement of σ0 of the 99Tc(n,γ) reaction. Results of a study along
the same line have been published recently.18
The details of the study have already presented in Reference
19 and here only brief description is given. The measurementof γ rays was done at the Los Alamos National Laboratory
Omega West Reactor. A metallic sample of 99Tc (84.6 mg) was
placed in the thermal column of the internal target facility at
the reactor. The target position was 1.5 m from the edge of thereactor core and the thermal neutron flux was ~6 × 1011 n/cm2 /sand the Cd(In) ratio was ≈2000. γ rays were measured with aspectrometer consisting of 30-cm3 coaxial intrinsic Ge detector
positioned inside a 20-cm-diameter by 30-cm-long NaI(Tl)annulus; The annulus is optically divided into two halves. The
spectrometer was located 6.3 m from the target position andwas operated either in the Compton-suppressed(CS) mode orin the pair-spectrometer(PS) mode. γ rays with energies of up
to ~3 MeV (~11 MeV) were measured in the CS (PS) mode.
Energies of the γ rays were calibrated with the prompt γ rayfrom the 1H(n,γ) reaction plus the annihilation radiation in the
CS mode; in PS mode, the calibration was done with the prompt
γ rays emitted in the 14N(n,γ) reaction using a melamine (C3H3N6)
sample. Calibration of detection efficiencies were done with aset of standard sources (CS) and relative intensities of the γrays in 14N(n,γ) reaction (PS). Absolute intensities were determined through measurements in which the target was irradiated together with a 100.0-mg CH2 standard; the recommended
value of σ0 = 332.6 ± 0.7 mb was used for the 1H(n,γ) reaction.
Figure 2 shows a selected portion of the observed spectrum
in the CS mode. Many γ rays were observed in both modes,
including the 540-and 591-keV γ rays in 100 Ru. Gamma-ray
peaks observed in both modes are analyzed and the energies aswell as the intensities were determined. The obtained data inboth modes are combined, and after elimination/correction of
background, a total of 1086 γ rays were assigned to be emitted

J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005 285

Ru)
5 9 1 ke V (in the capture reaction. Starting from a skeleton level scheme
built from the available data, a decay scheme was constructedby analyzing the observed γ rays using the Rydberg-Ritz combination principle. A total of 88 γ rays have been incorporated
into the level scheme consisting of 36 bound levels including
eight new ones. Among these, six were assigned as transitions
to the ground state, namely 172.21-, 223.48-, 263.58-, 340.99-,
355.67- and 458.7-keV ones. From the intensities of the groundstate transitions and their internal conversion coefficients calculated using the code HSICC,20 σ0 was obtained as 21.37 ±0.68 b; the value should be regarded as a lower limit, because
the constructed decay scheme is far from complete and there may
be more ground-state transitions which were not incorporated
into the scheme. From the yields of the γ rays in 100Ru observed
simultaneously, the cross section was also deduced; the obtained
value is 22.8 ± 1.8 b. In Figure 1 the obtained results are compared with other experimental/evaluated data.
5. Discussion
Figure 1 compares the values of cross section of the 99Tc(n,γ)
reaction for thermal neutrons reported in References 12 and 19
with other published experimental data as well as evaluatedones. At first glance, the results obtained from the yields ofprompt γ rays in References 18 and 19 look very accurate; thesevalues should be regarded as lower limits of the cross section,
because there may be more ground-state transitions left unobserved/unassigned. Some of the evaluated values are smaller
than these lower limits, reflecting the older values around 20 b;
the present authors recommend to revise the values to larger
ones. Although the results obtained from the prompt γ-ray measurements demonstrate a possibility to accurately determine
cross sections from prompt γ-ray measurements, it seems very
difficult and tedious to construct complete level schemes by
incorporating thousands of observed γ rays. Therefore, subsidiary methods are highly desirable. A possible method would be
the one that enables one to estimate unobserved portion of theintensities of the prompt γ rays from those of the observed ones.
The uncertainties of the cross section reported in References
18 and 19 obtained in the activation measurements are stilllarger than the requested one in Reference 4, because theuncertainties of the emission probabilities of the γ rays in 100Ru
are not yet small enough. Figure 3 shows a comparison of theratios of the intensity of the 591-keV γ ray to that of the 540keV one calculated from the data in References 3, 14, 18, and
19. The ratios obtained from the newer and improved data in
References 18 and 19 published after Reference 3, clearly indicate that the ratio that was obtained from the γ-ray emission
probabilities in Reference 3, is a little bit too large. Thus, theaccuracy of the emission probabilities should be improved further. To determine γ-ray emission probabilities accurately, 2 or
4π β−γ coincidence measurements are usually done; in themethod, it is not difficult to determine emission probabilities

0.6

Present
19(Furutaka et al.)Figure 2. A selected portion of the observed spectrum in the prompt
γ-ray measurement emitted in 99 Tc(n,γ) reaction, in the CS mode.
The assigned ground-state transitions are indicated. The two intense
γ rays in 100Ru, namely 540- and 591-keV γ rays, are also clearly seen.

18

2000

M o ln a r e t a l .

1600

Present
3(Furutaka et al.)

Ru)

1 00

1 00

5 4 0 ke V (511keV (annihilation)

1200

14

800

CHANNEL NUMBER

0.8

Berzins et al.

400(591–keV)/(540–keV)0Intensity ratios

104458.7keV (to g.s.)

1061
340.99keV (to g.s.)355.67keV (to g.s.)172.21keV(to g.s.)

223.48keV (to g.s.)

108263.58keV (to g.s.)

COUNTS PER CHANNEL

Cross Section of Thermal-Neutron Capture Reaction by 99Tc

0.4

Figure 3. Ratios of the intensity of 591-keV γ ray in 100Ru to that of
540-keV one.

with uncertainties of less than a percent. However, the shorthalf-life of 100Tc makes it difficult to apply the technique to thecross section measurements. One possible way would be to do
the measurement with a neutron beam on-line.
6. Conclusion
Results are presented of a series of study to determine crosssections of 99Tc(n,γ) for thermal neutrons. In spite of the efforts,
the attained accuracy doesn’t meet the requests, and further
improved measurements are in need.
Acknowledgements. The present work has been carried out
in part under the Visiting Researcher’s Program of the Research
Reactor Institute, Kyoto University. A part of the present work
has been done as the collaboration between Japan Nuclear
Cycle development institute (JNC) and Oak Ridge National
Laboratory (ORNL) operated by UT-Battelle, LLC, under thecontract No. FERD-01-2094. A part of the experimental workin the present study was sponsored by the U.S. Department ofEnergy, under Contract No. DE-AC05-00OR22725 with UTBattelle, LLC.
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