Speaker
Description
When a material or sample becomes activated $\beta$-emitting isotopes will often provide a significant contribution to nuclear responses. As such, accurate values for $\beta$-decay observables (decay energies, heating contributions) form an import part of decay data library development. Unfortunately, much of current evaluated $\beta$-decay data is incomplete.
During $\beta$-decay the transmuted nucleon will emit a $\beta$ particle (i.e. $e^-$ or $e^+$) and an associated neutrino, $\bar{\nu}$ or $\nu$ accordingly. These two emissions will share the energy released during the decay (minus the daughter recoil). As a result neither the $\beta$ or $\nu$ are released with a discreet defined energy. The 3-body $\beta$-decay system gives rise to a continuum energy spectrum for released $\beta$'s, the inverse of which is the equivalent spectrum for $\nu$'s. The continuum $\beta$ spectra directly provide knowledge of the range of expected $\beta$-$\nu$ energies and intensities.
$\beta$ continuum spectra are inconsistently included in decay data libraries, currently there are 33 nuclides with continuum spectra in JEFF3.3 [1], 276 in ENDF/BVIII [2] and 160 nuclides in Decay2020. These are not sufficient numbers to allow for detailed inventory analysis and often confined to $\beta$-delayed neutron decay transitions of interest to fission. Thus, despite spectra being available, complete spectra are often missing. $\nu$ spectra are not included in modern decay data and will require equivalent treatment to $\beta$ spectra if a nuclide can undergo multiple $\beta$-decay transitions. The consequence of this low coverage is that detailed studies using continuum spectra have not been previously viable.
To allow for detailed $\beta$ spectroscopy studies, including those of complex nuclear inventories such as those arising from activated fusion materials, knowledge of a greater number of $\beta$ continuum spectra is required. To enable this, and to complement existing decay data, a library of continuum $\beta$-$\nu$ spectra has been developed. This library currently contains entries for almost 1600 $\beta$-emitting nuclides, each entry contains $\beta$ and $\nu$ spectra for each decay transition a given nuclide is believed to undergo and a summed total spectra. Different available codes were investigated but ultimately the library was developed using BetaShape [3,4,5] and inputs taken form ENSDF data of the National Nuclear Data Center (NNDC) [6].
Validation efforts will be shown that demonstrate that the spectra within the library compare well with experimental results, accepted spectra and spectra mean energies from standard decay data, but their are cases where the agreement is poor. These suggest that there can be improvements in either ENSDF data entries and/or $\beta$-decay theory, as well as additional direct experimental measurements being desired.
Methodologies have been developed to allow the calculation of complete $\beta$ and $\nu$ spectra for complex nuclear inventories. These are being included in the transmutation-activation code FISPACT-II [7] to allow inventory $\beta$-$\nu$ spectra to be output for high fidelity irradiation studies. An example inventory $\beta^-$ spectrum, from SS316L(n)-IG steel after a simulated 10 year irradiation in a fusion reactor vacuum vessel, is shown in figure 1 (see attached pdf). Results such as this will be presented as discussed. From such spectra more complete information on $\beta$ emissions can be determined for a given inventory. For example, accurate apportionment of $\beta^-$/ $\beta^+$ decay heating or $\nu$ source terms could be derived.
The contributions to $\gamma$-spectra from internal $\beta$-decay bremsstrahlung have been found using the available spectra and will be shown. These allow not only for more accurate $\gamma$-spectra, but the $\gamma$ dose rate contribution from $\beta$-decay bremsstrahlung to be quantified. The developed $\beta$-$\nu$ continuum spectra library is believed to be able to enhance modern decay data and activation calculations. It is hoped that the increased availability of continuum spectra, from nuclides and complex inventories, will prove to be beneficial for $\beta$-spectroscopy and material activation studies.
(see attached pdf for references)