**65-Tb-159 Review**

By V.G. Pronyaev, NDS, IAEA

28 January 2003

**General Comment:**

Taking into account that the accuracy of nuclear data for fission
products is most important in neutron transport and nuclear
transmutation calculations, the following priority to the quality of the
data can be given when we compare different evaluations:

1.  Most important energy region is a region of spectrum with maximum
    neutron fluxes in thermal and fast fission reactors, namely around
    0.0253 eV and below 5 MeV.

2.  Most important cross sections in order of reducing of their
    importance are integral cross sections: capture, total, elastic,
    inelastic with excitation of levels and other threshold cross
    sections; differential cross sections: secondary neutron spectrum
    from inelastic scattering and elastic scattering differential cross
    sections, gamma production cross section and spectra.

##### **Files**

ENDF/B-VI -- 1980 evaluation, based on Mughabghab81 23 SLBW resonances
below 99.634 eV and theoretical model calculations in the energy region
above.

JEF-2 -- 1983 evaluation, with wit 30 resolved resonances up to 170 eV.
Total spin to resonances (when not known) was assigned by random number
method. Results of optical and statistical model calculations are for
given energy above 170 eV.

JENDL-3.2 (and JENDL-3.3) -- 1990 evaluation, based on 228 s-wave MLBW
resonances (same as in JENDL-2) in the region up to 1188 eV. Random
number method was used for assigning the spin to resonances when it was
unknown. Unresolved resonance region with average resonance parameters
for s-, p- and d-waves covers the energy between 1.188 and 100 keV.
Results of optical, statistical compound and pre-equilibrium model
calculations are givenfor energy above 100 keV.

###### Thermal and resonance region

Table below shows the thermal cross section values (barns at 0.0253 eV)
and RI (barns) for all libraries:

  ------------- ------------- ------------- -------------- ----------------
                ENDF/B-VI     JEF-2         JENDL-3.2 (and Mughabghab81
                                            3.3)           

  Total         29.72         29.86         47.39          

  Elastic       4.21          6.57          20.87          6.92±0.06

  Capture       25.51         23.21         26.52          23.4±0.4

  RI                          406\.         471\.          418±20
  ------------- ------------- ------------- -------------- ----------------

Total cross section given in JENDL-3.2 below 0.1 eV and calculated from
resonance parameters is in full agreement with all experimental data.
New experimental value 258±3 b at 0.568 meV by KNOPF(1997), X4=22450, is
in excellent agreement with JENDL-3.2 value 258.7 b at 0.552 meV.
ENDF/B-VI and JEF-2 give about 40% too low cross sections. Above 0.1 eV
up to first resonance in eV region JENDL-3.2 values are slightly below
experimental data. This is clear because position and width of negative
and first resonance are not fitted well to describe the cross section
behavior in this region. This drawback of JENDL-3.2 lead also to such
behavior of capture cross section, which overestimates the cross section
below 0.05 eV, underestimates above and gives energy dependence
different from *1/v* clearly seen from the experimental data. JEF-2,
based on Mughabghab81 sigma capture and RI values, gives better
description of capture cross section in the energy region below 170 eV.
JENDL-3.2 values of capture cross section give ratio of RI to capture
thermal value (17.76) very close to Mughabghab91 value 17.86, but both
cross sections are overestimated at 13%. As seems, many missed s-, p-
and d- resonances or their contributions should be added to the
high-energy part of the resolved resonance region (RRR) in JENDL-3.2. At
present, first 100 eV in RRR include 31 resonances and last 188 eV --
only 21. The missing of resonances is evident. These missing resonances
are not compensated through the assigning of larger effective widths to
the resonances in the high-energy part of RRR. Parameters of the average
resonances given in the unresolved resonance region (URR) below 100 keV
in JENDL-3.2 provide good fit of total cross section, but neutron
capture is 30% above the latest BOKHOVKO96 data for 10 to 400 keV energy
range and 10% above GIBBONS61 data for 80 -- 200 keV. At the same time,
one-group fast reactor capture cross section is below ENDF/B-VI value at
12%, because JENDL-3.2 cross section is up to 30% low in the keV region
(see also the problem of missed resonances in the RRR). To resolve the
discrepancy HARKER72, X4=10218, measurements of CFRMF spectrum averaged
ratio ^159^Tm(n,)/^197^Au(n,) (2.22) can be used.

###### Fast region

Concerning the energy region with En \> 100 keV, the total, elastic,
inelastic and (n,2n) cross sections are the best in the JENDL-3.2
library. The results obtained in calculations with optical, statistical
and pre-equilibrium models are supported by all experimental data. The
presentation of data (on number of reactions included and secondary
angular and energy distributions given) is more detailed in the
JENDL-3.2 evaluation, where MF=4 and MF=5 files are given for all
neutron-production channels. The fast neutron capture cross section goes
close to the existing experimental data for exclusion of 14 MeV point,
where contribution of direct capture is underestimated.

###### Recommendation and conclusion

The JENDL-3.2 can be recommended as the best in the whole energy region
10-5 eV -- 20 MeV.

###### Further possible improvements

The parameters of the first few resolved resonances (including the
negative-energy resonance(s)) should be revised in JENDL-3.2 to fit
existing experimental data. This will lead to the revision of the
thermal capture cross section, RI and total and elastic scattering cross
sections in the vicinity of the first resonance. The background
contribution (or simulated pseudo-levels) for missed s-resonances as
well as for absent p- and d-wave resonances should be evaluated and
added for RRR in JENDL-3.2. The discrepancy between experimental and
evaluated data in the capture cross section for keV region should be
resolved with re-analysis of microscopic experimental data and testing
in the integral experiment (one-group CFRMF spectrum averaged data).

The contribution of direct processes should be added to the inelastic
scattering cross sections with excitation of levels of the collective
nature. Gamma-production cross sections and spectra should be evaluated.