Invariant-mass spectroscopy in projectile fragmentation reactions

The fragmentation of a projectile into a number of pieces can lead to the creation of many resonances in different nuclei. We discuss application of the invariant-mass method to the products from such reactions to find some of the most exotic resonances located furthest beyond the proton drip line. We show examples from fragmentation of a fast O13 beam including the production of the newly identified N9 resonance. In extracting resonance parameters from invariant-mass spectra, accurate estimates of the background from nonresonant prompt protons are needed. These prompt protons are correlated with the resonances due to long-range Coulomb interactions and will suppress events when resonance decay products and the prompt protons have small invariant masses. The shape of this background is especially important in determining the widths of wide resonances typically found at the edge of the chart of nuclides. An event-mixing recipe is proposed to describe this background, where the mixed events have reduced weighting for the smaller invariant masses to account for the effect of the Coulomb final-state interactions. The weighting is based on the measured correlations of heavier hydrogen isotopes with the resonances or the projectile residues. We also show that the relative magnitude of the background can be reduced in some cases by selecting events where the resonance decay products are accompanied by a deuteron or triton cluster. The deuteron and triton clusters are largely not products of resonant decay and thus events with an accompanying cluster are associated with fewer background protons than events without them. The width of the 5/2− state in C9 with the new background prescription was found to be consistent with the value obtained using proton elastic scattering on B8. Improved values for the widths of C10, N10,11, and O12 resonances were also obtained using the new background prescription.10 MoreReceived 15 March 2023Accepted 27 July 2023DOI:https://doi.org/10.1103/PhysRevC.108.044318©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBreakup reactionsEnergy levelsLifetimes & widthsNuclear fragmentationNuclear reactionsNuclear spin & parityNuclear structure & decaysProton emissionRare & new isotopesProperties6 ≤ A ≤ 19Nuclear Physics