Establishment of Imaging Spectroscopy of Nuclear Gamma-Rays based on Geometrical Optics [CL]

http://arxiv.org/abs/1702.01483


Since the discovery of nuclear gamma-rays, its imaging has been limited to pseudo imaging, such as Compton Camera (CC) and coded mask. Pseudo imaging does not keep physical information (intensity, or brightness in Optics) along a ray, and thus is capable of no more than qualitative imaging of bright objects. To attain quantitative imaging, cameras that realize geometrical optics is essential, which would be, for nuclear MeV gammas, only possible via complete reconstruction of the Compton process. Recently we have revealed that “Electron Tracking Compton Camera” (ETCC) provides a well-defined Point Spread Function (PSF). The information of an incoming gamma is kept along a ray with the PSF and that is equivalent to geometrical optics. Here we present an imaging-spectroscopic measurement with the ETCC. Our results highlight the intrinsic difficulty with CCs in performing accurate imaging, and show that the ETCC surmounts this problem. The imaging capability also helps the ETCC suppress the noise level dramatically by ~3 orders of magnitude without a shielding structure. Furthermore, full reconstruction of Compton process with the ETCC provides spectra free of Compton edges. These results mark the first proper imaging of nuclear gammas based on the genuine geometrical optics.

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T. Tanimori, Y. Mizumura, A. Takada, et. al.
Fri, 10 Feb 17
40/46

Comments: 22 pages, 8 figures

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Radiogenic Neutron Yield Calculations for Low-Background Experiments [CL]

http://arxiv.org/abs/1702.02465


Nuclear recoil backgrounds are one of the most dangerous backgrounds for many dark matter experiments. A primary source of nuclear recoils is radiogenic neutrons produced in the detector material itself. These neutrons result from fission and $(\alpha,n)$ reactions originating from uranium and thorium contamination. In this paper, we discuss neutron yields from these sources. We compile a list of $(\alpha,n)$ yields for many materials common in low-background detectors, calculated using NeuCBOT, a new tool introduced in this paper, available at https://github.com/shawest/neucbot. These calculations are compared to computations made using data compilations and SOURCES-4A

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S. Westerdale and P. Meyers
Thu, 9 Feb 17
64/67

Comments: N/A

Concurrent application of ANC and THM to assess the $^{13}{\rm C}(α,n)^{16}{\rm O}$ absolute cross section at astrophysical energies and possible consequences for neutron production in low-mass AGB stars [SSA]

http://arxiv.org/abs/1702.01589


The $^{13}{\rm C}(\alpha,n)^{16}{\rm O}$ reaction is considered to be the main neutron source responsible for the production of heavy nuclides (from ${\rm Sr}$ to ${\rm Bi}$) through slow $n$-capture nucleosynthesis ($s$-process) at low temperatures during the asymptotic giant branch (AGB) phase of low mass stars ($\lesssim 3-4\;{\rm M}_{\odot}$, or LMSs). In recent years, several direct and indirect measurements have been carried out to determine the cross section at the energies of astrophysical interest (around $190\pm40\;{\rm keV}$). However, they yield inconsistent results causing a highly uncertain reaction rate and affecting the neutron release in LMSs. In this work we have combined two indirect approaches, the asymptotic normalization coefficient (or ANC) and the Trojan Horse Method (THM), to unambiguously determine the absolute value of the $^{13}{\rm C}(\alpha,n)^{16}{\rm O}$ astrophysical factor. Therefore, we have determined a very accurate reaction rate to be introduced into astrophysical models of $s$-process nucleosynthesis in LMSs. Calculations using such recommended rate have shown limited variations in the production of those neutron-rich nuclei (with $86\leq A\leq 209$) receiving contribution only by slow neutron captures.

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O. Trippella and M. Cognata
Tue, 7 Feb 17
37/64

Comments: N/A

Antonella: A nuclear-recoil ionization-efficiency measurement in silicon at low energies [CL]

http://arxiv.org/abs/1702.00873


We have measured the ionization efficiency of silicon nuclear recoils with kinetic energy between 1.8 and 20 keV. We bombarded a silicon-drift diode with a neutron beam to perform an elastic-scattering experiment. A broad-energy neutron spectrum was used and the nuclear recoil energy was reconstructed with the time-of-flight technique. The overall trend of the results of this work are well described by the theory of Lindhard et al. above 4 keV of recoil energy. Below this energy, the presented data shows a deviation from the model. The data indicates a faster drop than the extrapolation of the Lindhard theory to low energies.

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F. Izraelevitch, D. Amidei, A. Aprahamian, et. al.
Mon, 6 Feb 17
12/43

Comments: N/A

Impacts of nuclear-physics uncertainty in stellar temperatures on the s-process nucleosynthesis [SSA]

http://arxiv.org/abs/1701.06978


We evaluated the uncertainty relevant to s-process nucleosynthesis using a Monte-Carlo centred approach. We are based on a realistic and general prescription of temperature dependent uncertainty for the reactions. We considered massive stars for the weak s-process and AGB stars for the main s-process. We found that the adopted uncertainty for (n,$\gamma$) rates, tens of per cent on average, affect the production of s-process nuclei along the $\beta$-stability line, while for $\beta$-decay, for which contributions from excited states enhances the uncertainty, has the strongest impact on branching points.

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N. Nishimura, G. Cescutti, R. Hirschi, et. al.
Wed, 25 Jan 17
58/74

Comments: 3 pages, 3 figures, to be published in the Proceedings of “the 14th International Symposium on Nuclei in the Cosmos (NIC-XIV)”; see arXiv:1701.00489, for the completed results

The Sun as a probe of Fundamental Physics and Cosmology [CEA]

http://arxiv.org/abs/1701.03926


The high quality data provided by helioseismology, solar neutrino flux measurements, spectral determination of solar abundances, nuclear reactions rates coefficients among other experimental data, leads to the highly accurate prediction of the internal structure of the present Sun – the standard solar model. In this talk, I have discussed how the standard solar model, the best representation of the real Sun, can be used to study the properties of dark matter, for which two complementary approaches have been developed: – to limit the number of theoretical candidates proposed as the dark matter particles, this analysis complements the experimental search of dark matter, and – as a template for the study of the impact of dark matter in the evolution of stars, which possibly occurs for stellar populations formed in regions of high density of dark matter, such as stars formed in the centre of galaxies and the first generations of stars.

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I. Lopes
Tue, 17 Jan 17
77/81

Comments: 11 pages; 6 figures; Nuclear Physics in Astrophysics VI (NPA6)

Reduced $E$1 $S$-factor of $^{12}$C($α$,$γ_0$)$^{16}$O [CL]

http://arxiv.org/abs/1701.02848


The astrophysical $S$-factor of $E$1 transition for $^{12}$C($\alpha$,$\gamma_0$)$^{16}$O is discussed in the $R$-matrix theory. The reduced $\alpha$-particle widths of the 1$^-_1$ ($E_x= 7.12$ MeV) and 1$^-_2$ ($E_x= 9.59$ MeV) states are extracted from the result of the potential model. The formal parameters are obtained without the linear approximation to the shift function. The resultant $E$1 $S$-factor is not strongly enhanced by the subthreshold 1$^-_1$ state if the channel radius is 4.75 fm. The calculated $\beta$-delayed $\alpha$-particle spectrum of $^{16}$N and the p-wave phase shift of $\alpha$+$^{12}$C elastic scattering are also found to be consistent with the previous studies. The small channel radius leads to the low penetrability to the Coulomb barrier, and it makes the reduced $E$1 $S$-factor below the barrier. Owing to the large reduced width from the molecular structure, the $R$-matrix pole of the 1$^-_2$ state is shifted in the vicinity of 1$^-_1$. The proximity of the two poles suppresses the interference between the states. The weak coupling feature of the $\alpha$+$^{12}$C system appears to be expressed as the shrinking strong interaction region.

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M. Katsuma
Thu, 12 Jan 17
12/60

Comments: 17 pages, 8 figures, 2 tables. Submitted to PTEP