Direct evaluation of high neutron density environment using $\left(n,2n\right)$ reaction induced by laser-driven neutron source

We demonstrate that $\left(n,2n\right)$ reactions are induced by a high-flux pulse of fast neutrons ($\sim {10}^{10}$ neutrons in $\sim 1\mathrm{ns}$) provided from a laser-driven neutron source (LDNS). The maximum energy of the broadband neutrons reaches a few tens MeV. Several kinds of metal targets are exposed to the fast neutrons. As a result, unstable isotopes, such as ${}^{54}\mathrm{Mn}, {}^{58}\mathrm{Co}, {}^{175}\mathrm{Hf}$, and ${}^{196}\mathrm{Au}$ are produced by $\left(n,2n\right)$ reactions and ${}^{180}\mathrm{Hf}^m, {}^{181}\mathrm{Hf}, {}^{56}\mathrm{Mn}, {}^{198}\mathrm{Au}$, and ${}^{60}\mathrm{Co}$ are produced by $(n,\gamma )$ reactions. We evaluate the neutron fluence and energy spectrum using the activation method in conjunction with a time-of-flight measurement. The neutron fluence is determined to be $\left(4.3\pm 0.5\right)\times {10}^8\mathrm{neutrons}/\mathrm{c}{\mathrm{m}}^2$ in the energy range from approximately 8 to 20 MeV at 8-mm downstream of the neutron source. The present scheme provides a method to evaluate high-density neutrons seen in stellar environments, which are expected to be generated from future LDNSs.

Figure 1

Experimental layout of the fast neutron generation for short L3658. In this experiment, the ${}^9\mathrm{Be}$ target was used as a neutron converter. The neutrons emit almost isotopically. The neutrons were exposed on the activation targets on the laser axis and were measured with a TOF detector located outside of the chamber.

Figure 2

Signals of the laser-accelerated protons and deuterons. (a) Image of the IP located behind the Thomson Parabola Ion Spectrometer, which was recorded on shot L3651. The tracks of the protons and the deuterons clearly appeared. (b) Ion energy spectra of shot L3651 evaluated from the tracks on the IP. The red solid and blue dashed lines present the energy spectra of the protons and the deuterons, respectively.

Figure 3

Results of the neutron TOF measurements. (a) The output from the scintillation detector located at 15 ° away from the laser axis and a distance of 8.3 m on shot L3658. The output was recorded with an oscilloscope shot by shot. Neutron and photon signals were clearly separated. (b) Fast neutron spectrum evaluated from the output of the scintillation detector. The neutrons with energies of up to 26 MeV were measured.

Figure 4

γ-ray spectra for (a) ${}^{180}\mathrm{Hf}^m$, (b) ${}^{196,198}\mathrm{Au}$, (c) ${}^{56}\mathrm{Mn}$, (d) ${}^{175,181}\mathrm{Hf}$, (e) ${}^{54}\mathrm{Mn}$, and (f) ${}^{58,60}\mathrm{Co}$. These spectra were measured using the three HPGe detectors with the shields after laser shots.

Figure 5

Results of half-life measurements of ${}^{196}\mathrm{Au}$ and ${}^{198}\mathrm{Au}$. (a) Measured decay curves of 333-keV and 355-keV γ rays radiated from ${}^{196}\mathrm{Au}$ that is generated by the ${}^{197}\mathrm{Au}(n,2\gamma ){}^{196}\mathrm{Au}$ reaction. (b) Measured decay curve of the 412-keV γ-ray from ${}^{198}\mathrm{Au}$ produced by ${}^{197}\mathrm{Au}(n,\gamma ){}^{198}\mathrm{Au}$ reaction.

Figure 6

Efficiency curves of the Ge detectors used to evaluate the nuclear reaction rate per nucleus. (a)–(c) shows efficiency curve of Ge1–Ge3, respectively.

Figure 7

The $\left(n,2n\right)$ reaction cross sections on ${}^{176}\mathrm{Hf}, {}^{197}\mathrm{Au}, {}^{55}\mathrm{Mn}$, and ${}^{59}\mathrm{Co}$, the measured neutron energy spectra provided from the LDNS and their product. The blue dotted lines show the normalized neutron energy spectrum. The red dashed lines show $\left(n,2n\right)$ cross sections of each nuclide. The black solid lines show the product of the $\left(n,2n\right)$ cross section multiplied by the normalized neutron spectrum for each nuclide.

Figure 8

Evaluated neutron fluxes from the activation method using $\left(n,2n\right)$ reactions. (a) The 8–20 MeV neutron fluence measured by the $\left(n,2n\right)$ activation method, and solid angles of the targets from the neutron generation point in the Be converter and (b) the neutron yield for each target. The dashed line shows that the average yield.