High energy-resolution measurement of the ${}^{82}\mathrm{Se}({}^3\mathrm{He},t){}^{82}\mathrm{Br}$ reaction for double-$\beta$ decay and for solar neutrinos

A high-resolution (${}^3\mathrm{He},t$) charge-exchange experiment at an incident energy of 420 MeV has been performed on the double beta ($\beta \beta$) decay nucleus ${}^{82}\mathrm{Se}$. A detailed Gamow-Teller (${\mathrm{GT}}^{-}$) strength distribution in ${}^{82}\mathrm{Br}$ has been extracted, which provides information to the $\beta \beta$-decay nuclear matrix elements. Three strong and isolated transitions, which are to the 75, 1484 and the 2087 keV states in ${}^{82}\mathrm{Br}$, are found to dominate the low-excitation region below $\approx 2.1$ MeV. Above 2.1 MeV a sudden onset of a strong GT fragmentation is observed. The degree of fragmentation resembles a situation found in the neighboring $A=76$ system ${}^{76}\mathrm{Ge}$, whereas the observed concentration of strength in the three low-lying states is reminiscent of the heavier neighbors ${}^{96}\mathrm{Zr}$ and ${}^{100}\mathrm{Mo}$. The strong GT transition to the 75 keV $(1^{+})$ state makes ${}^{82}\mathrm{Se}$ interesting for solar neutrino detection. The ${}^{82}\mathrm{Se}({\nu }_e,e^{-}){}^{82}\mathrm{Br}$ solar neutrino capture rate in a nonoscillation scenario is therefore evaluated to $668\pm 12\text{(stat)}\pm 60\text{(sys)}$ SNU, and some of the advantages of using selenium for solar neutrino studies are discussed.

Figure 1

Sketch of the $2\nu \beta \beta$ decay process in ${}^{82}\mathrm{Se}$. The transition paths through the intermediate $1^{+}$ states in ${}^{82}\mathrm{Br}$ are indicated. The various $Q$ values and excitation energies are taken from Refs. [1, 30, 31]. All energies are given in keV units.

Figure 2

Excitation-energy spectra of the ${}^{82}\mathrm{Se}$(${}^3\mathrm{He},t){}^{82}\mathrm{Br}$ reaction. The spectra were generated from three different angle cuts (indicated by colors) and overlaid to show the effect of the angular dependence. $\mathrm{\Delta }L=0$ transitions (GT and IAS) are forward-peaked and prominently appear in the spectrum at $0.0^{\circ }\le {\theta }_{\text{lab}}\le 0.5^{\circ }$. Note, the energy scale is modified above 6 MeV. The inset magnifies the IAS region.

Figure 3

Selected angular distributions for the ${}^{82}\mathrm{Se}$(${}^3\mathrm{He},t){}^{82}\mathrm{Br}$ reaction (sorted by $J^{\pi }=1^{+},0^{+},2^{-},3^{+}$ final state excitations).

Figure 4

Angular distributions of the ${}^{82}\mathrm{Se}$(${}^3\mathrm{He},t){}^{82}\mathrm{Br}$ reaction for the energy bins of $\mathrm{\Delta }E=500$ keV compared with DW model calculations. The various angular-momentum-transfer contributions are indicated. The $\theta =0^{\circ }$ cross sections and the extracted GT strengths are given in the Tables 2 and 3.

Figure 5

$B\left(\text{GT}\right)$ strength distribution for the ${}^{82}\mathrm{Se}$(${}^3\mathrm{He},t){}^{82}\mathrm{Br}$ reaction. The inset shows the average $B\left(\text{GT}\right)$ strength per $\mathrm{\Delta }E=100$ keV for an energy bin of 500 keV. The dotted line shows the integration when taking the full spectrum, and the histograms show the sum over the individual states (cf. Table 3). The figure indicates that above $\approx 2.5$ MeV the contribution from the rather structureless GTR tail gets to be appreciable.

Figure 6

Running sum of the $B\left(\text{GT}\right)$ strength by integrating over individual states (full line) and over 500 keV energy bins (broken line).

Figure 7

Cumulative SNU values as a function of excitation energy in ${}^{82}\mathrm{Br}$.