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We give the LUNA results on the s͑E͒ cross section measurement of a key reaction of the protonproton chain strongly affecting the calculated neutrino luminosity from the Sun: 3 He͑ 3 He, 2p͒ 4 He. Because of the cosmic ray suppression provided by the Gran Sasso underground laboratory, it has been possible to measure s͑E͒ throughout the energy range in which this reaction occurs in the Sun, i.e., down to 16.5 keV center of mass energy. The data clearly show the cross section increase due to the electron screening effect but they do not exhibit any evidence for a narrow resonance suggested to explain the 8 B and 7 Be solar neutrino flux reduction. [S0031-9007(99)09440-5] PACS numbers: 26.65. + t, 25.55.HpThe nuclear reactions which generate the energy of stars and, in doing so, synthesize elements occur inside stars at energies within the Gamow peak: E 0 6 dE 0 . In this region, which is far below the Coulomb energy E c (approximately E 0 ͞E c 0.01), the reaction cross section s͑E͒ drops nearly exponentially with decreasing energy E [1]:where S͑E͒ is the astrophysical factor and h is the Sommerfeld parameter, given by 2ph 31.29Z 1 Z 2 ͑m͞E͒ 1͞2 . Z 1 and Z 2 are the nuclear charges of the interacting particles in the entrance channel, m is the reduced mass (in units of amu), and E is the center of mass energy (in units of keV). The extremely low value of s͑E͒ within the Gamow peak has always prevented its measurement in a laboratory at the Earth surface. The signal to background ratio would be too small, even with the highest current beam, because of the cosmic ray interactions. Instead, the observed energy dependence of s͑E͒ at high energies is extrapolated to the low energy region, leading to substantial uncertainties. In particular, a possible resonance in the unmeasured region is not accounted for by the extrapolation, but it could completely dominate the reaction rate at the Gamow peak.In addition, another effect can be studied at low energies: the electron screening. The beam and target used in an experiment are usually ions and neutral atoms, respec-tively. The electron clouds surrounding the interacting nuclei act as a screening potential, thus reducing the height of the Coulomb barrier and leading to a higher cross section, s s ͑E͒, than would be the case for bare nuclei, s b ͑E͒, with an exponential enhancement factor [1],where U e is the electron-screening potential energy. It should be pointed out that the screening effect has to be measured and taken into account to derive the bare nuclei cross section, which is the input data to the models of stellar nucleosynthesis. Therefore both the search for narrow resonances and the study of electron screening demand the direct measurement of the nucleosynthesis cross sections in the low energy region (few tens of keV). In order to start exploring this new and fascinating domain of nuclear astrophysics we installed an accelerator facility deeply underground where the cosmic rays, which are the limiting background in all of the existing experiments, are strongly ...

The investigation of the 19 F(p, α 0 ) reaction at low bombarding energies allows the study of the spectroscopy of the 20 Ne compound nucleus in an energy region where the existence of quartet excitations has been suggested in the literature. Moreover, this reaction plays a major role in the fourth branch of the CNO cycle since it is relevant for the correct description of the hydrogen burning of fluorine in stars. For these reasons, we decided to investigate the 19 F(p, α 0 ) reaction in the E p 0.6-1 MeV energy range. The analysis of angular distributions and excitation functions allows one to improve the 20 Ne spectroscopy in an excitation energy region where some ambiguities concerning J π assignments exist in the literature. In particular, the present data suggest a J π = 0 + assignment to the E x = 13.642 MeV resonance. For this state, both partial and reduced widths for the α 0 channel have been deduced. The trend of the astrophysical factor has been obtained from the integrated cross section. A comparison of the present results with data reported in the literature is also discussed.

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Radiochromic film dosimetry has been widely employed in most of the applications of radiation physics for over twenty years. This is due to a number of appealing features of radiochromic films, such as reliability, accuracy, ease of use and cost. However, current radiochromic film reading techniques, based on the use of commercial densitometers and scanners, provide values of dose only after the exposure of the films to radiation. In this work, an innovative methodology for the real-time reading of radiochromic films is proposed for some specific applications. The new methodology is based on opto-electronic instrumentation that makes use of an optical fiber probe for the determination of optical changes of the films induced by radiation and allows measurements of dose with high degree of precision and accuracy. Furthermore, it has been demonstrated that the dynamic range of some kinds of films, such as the EBT3 Gafchromic films (intensively used in medical physics), can be extended by more than one order of magnitude. Owing to the numerous advantages with respect to the commonly used reading techniques, a National Patent was filed in January 2018.

An excitation function of the ground-state gamma (0)-ray capture transition in C-12(alpha, gamma)O-16 at theta (gamma) = 90 degrees was obtained in far geometry using six Ge detectors, where the study of the reaction was initiated in inverse kinematics involving a windowless gas target. The detectors observed predominantly the El capture amplitude. The data at E = 1.32 to 2.99 MeV lead to an extrapolated astrophysical S factor S-E1(E-0) = 90 +/- 15 keV b at E-0 = 0.3 MeV (for the case of constructive interference between the two lowest E1 sources), in good agreement with previous works. However, a novel Monte Carlo approach in the data extrapolation reveals systematic differences between the various data sets such that a combined analysis of all available data sets could produce a biased estimate of the S-E1(E-0) value. As a consequence, the case of destructive interference between the two lowest E1 sources with S-E1(E-0) = 8 +/- 3 keV b cannot be ruled out rigorously