Two-particle correlation functions in projectilelike emission experiments at very forward angles

Projectilelike fragment emission is studied via two-particle correlation functions, constructed from coincidence data detected at very forward angles $\left(0.7°⩽{\theta }_{\mathrm{lab}}⩽7°\right)$ in the $E∕A=44$ and $77\mathrm{MeV}$ ${}^{40}\mathrm{Ar}+{}^{27}\mathrm{Al}$ reactions. The shape of the proton-proton correlation function is unexpected. Possible origins of this effect are discussed and a way to reestablish the expected shape is presented.

Figure 1

From $E∕A=44$ (left) and 77 (right) MeV ${}^{40}\mathrm{Ar}+{}^{27}\mathrm{Al}$ collisions, distributions of protons (upper panels) and $\alpha$-particles (lower panels) laboratory kinetic energy∕nucleon. The particles are detected at $0.7°⩽{\theta }_{\mathrm{lab}}⩽7°$ in coincidence with one forward emitted fragment.

Figure 2

Data from $E∕A=44$ (left) and 77 (right) MeV ${}^{40}\mathrm{Ar}+{}^{27}\mathrm{Al}$ collisions. Upper panels: Proton-proton coincidence yield (black dots) and non-correlated background yield generated with the “event-mixing” technique (open circles). Lower panels: corresponding proton-proton correlation functions.

Figure 3

Same as in Fig. 2, after applying a low energy threshold for protons ($E_p>35\mathrm{MeV}$ at $E∕A=44\mathrm{MeV}$, and $E_p>65\mathrm{MeV}$ at $E∕A=77\mathrm{MeV}$).

Figure 4

From $E∕A=44$ (top) and 77 (bottom) MeV ${}^{40}\mathrm{Ar}+{}^{27}\mathrm{Al}$ collisions, $\alpha \alpha$ correlation function, without and with an energy threshold, as indicated in the panels. The position of the resonances from excited states of ${}^{8,9}\mathrm{Be}$ that decay into the $\alpha \alpha$ channel is indicated by the arrows.