In situ Measurement Method of Positron Annihilation in Nuclear Astrophysical Experiments

In thermonuclear reactions of nuclear astrophysical interest, some can produce short-lived products that emit positrons. These positrons will annihilate with electrons in the target and then produce a pair of 511 keV γ-rays, which can be used to determine the reaction yield and calculate the cross-section as well as the astrophysical S-factor. Recently, an in situ measurement method for positron annihilation on experimental terminals has been proposed. This method takes advantage of the characteristic opposite direction of the 511 keV γ-ray pairs and uses the opposite units in the detection array for spatial coincidence measurements to suppress background. In this study, we investigated this method using the newly developed large modular BGO detector array LAMBDA-II. The results show that the detection efficiency of LAMBDA-II for in situ β⁺ decay of reaction products is (7.6±0.2)%, which is in good agreement with the value given by Monte Carlo simulations. The yield of the ¹⁴N(p, γ)¹⁵O 259 keV resonance determined by in situ measurement agrees well with that derived from prompt γ-ray measurement, verifying the reliability of this method and providing a solid foundation for its further application in nuclear astrophysics research.