Application of Low Background and High Resolution Measurement Method in Measurement of β Delayed α Decay of ¹⁶N

The ¹²C(α,γ)¹⁶O reaction is the key nuclear reaction in the helium combustion phase of stars. The reaction rate not only directly determines the abundance ratio of carbon and oxygen, but also has an important influence on the subsequent stellar evolution process. However, its cross section at energy relevant for astrophysical applications is only poorly constrained by laboratory data. The cross sections at stellar helium-burning energy (Ec.m.≈0.3 MeV) are dominated by the tails of the Jπ=1- and Jπ=2⁺ subthreshold resonances corresponding to the bound states at 7.12 MeV and 6.92 MeV. The reaction cross section of ¹²C(α,γ)¹⁶O in this energy region is extremely small, and it is difficult to measure directly due to the influence of cosmic ray background. Therefore, it is necessary to extrapolate the contribution of these subthreshold states by indirect measurement. The α particle spectrum following β decay of ¹⁶N can improve the reliability of the extrapolation. β delayed α energy spectrum of ¹⁶N has a low energy peak at Ec.m.≈1.2 MeV. The shape and height of the peak can be used to constrain the extrapolation of the E1 component of ¹²C(α,γ)¹⁶O reaction, which is considered to be of great importance to measure. This work tried to measure α spectrum of ¹⁶N heavy ion injection. ¹⁶N beams from RIBLL1 at Lanzhou National Laboratory of Heavy Ion Accelerator were implanted in double-sided silicon micro strip detector (DSSD). In the experiment of measuring β delayed α decay by the injection method, in addition to the α and recoil nuclei, the β ray generated by the decay will also deposit energy in the detector and lead to an exponentially tailed background, which can distort the energy spectrum and cause great interference to the low-energy alpha peak. Taking advantage of a series of advanced methods such as hit pattern constraint, obviously minimize the distortion of the α spectrum due to β summing which are produced by β decay of 16N, and extend the threshold of the α spectrum down to 800 keV so that the low energy peak at Ec.m.≈1.2 MeV was measured successfully. The ¹⁶N β delay α energy spectrum measured in this work can see a 1.2 MeV α peak, and its shape and relative height are basically consistent with the existing work. This shows that it is feasible to measure the ¹⁶N β delay α energy spectrum with the injection method. This new measurement opens up a new way to study the cross section of ¹²C(α,γ)¹⁶O.