Cross section of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mmultiscripts><mml:mi>He</mml:mi><mml:mprescripts/><mml:none/><mml:mn>3</mml:mn></mml:mmultiscripts><mml:mo>(</mml:mo><mml:mi>α</mml:mi><mml:mo>,</mml:mo><mml:mi>γ</mml:mi><mml:mo>)</mml:mo><mml:mmultiscripts><mml:mi>Be</mml:mi><mml:mprescripts/><mml:none/><mml:mn>7</mml:mn></mml:mmultiscripts></mml:mrow></mml:math> around the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Be</mml:mi><mml:mprescripts/><mml:none/><mml:mn>7</mml:mn></mml:mmultiscripts></mml:math> proton separation threshold

Background: The $^{3}\mathrm{He}(\ensuremath{\alpha},\ensuremath{\gamma})^{7}\mathrm{Be}$ reaction is a widely studied nuclear reaction; however, it is still not understood with the required precision. It has a great importance both in Big Bang nucleosynthesis and in solar hydrogen burning. The low mass number of the reaction partners makes it also suitable for testing microscopic calculations.Purpose: Despite the high number of experimental studies, none of them addresses the $^{3}\mathrm{He}(\ensuremath{\alpha},\ensuremath{\gamma})^{7}\mathrm{Be}$ reaction cross sections above 3.1-MeV center-of-mass energy. Recently, a previously unobserved resonance in the $^{6}\mathrm{Li}(\mathrm{p},\ensuremath{\gamma})^{7}\mathrm{Be}$ reaction suggested a new level in $^{7}\mathrm{Be}$, which would also have an impact on the $^{3}\mathrm{He}(\ensuremath{\alpha},\ensuremath{\gamma})^{7}\mathrm{Be}$ reaction in the energy range above 4.0 MeV. The aim of the present experiment is to measure the $^{3}\mathrm{He}(\ensuremath{\alpha},\ensuremath{\gamma})^{7}\mathrm{Be}$ reaction cross section in the energy range of the proposed level.Method: For this investigation the activation technique was used. A thin window gas-cell target confining $^{3}\mathrm{He}$ gas was irradiated using an $\ensuremath{\alpha}$ beam. The $^{7}\mathrm{Be}$ produced was implanted into the exit foil. The $^{7}\mathrm{Be}$ activity was determined by counting the $\ensuremath{\gamma}$ rays following its decay by a well-shielded high-purity germanium detector.Results: Reaction cross sections have been determined between ${E}_{\text{cm}}=4.0\phantom{\rule{4.pt}{0ex}}\text{and}\phantom{\rule{4.pt}{0ex}}4.4$ MeV with 0.04-MeV steps covering the energy range of the proposed nuclear level. One lower-energy cross-section point was also determined to be able to compare the results with previous studies.Conclusions: A constant cross section of around 10.5 $\ensuremath{\mu}\mathrm{b}$ was observed around the $^{7}\mathrm{Be}$ proton separation energy. An upper limit of 45 neV for the strength of a $^{3}\mathrm{He}(\ensuremath{\alpha},\ensuremath{\gamma})^{7}\mathrm{Be}$ resonance is derived.

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