Comparison of midvelocity fragment formation with projectilelike decay

The characteristics of intermediate mass fragments (IMFs: $3\le Z\le 20)$ produced in midperipheral and central collisions are compared. We compare IMFs detected at midvelocity with those evaporated from the excited projectilelike fragment (PLF${}^{*}$). On average, the IMFs produced at midvelocity are larger in atomic number, exhibit broader transverse velocity distributions, and are more neutron rich as compared to IMFs evaporated from the PLF${}^{*}$. These characteristics of midvelocity fragments are consistent with the low-density formation of the fragments. We present in the different kinematical regions studied, the $\langle E_{\perp }\rangle$ for isotopically identified IMFs. For a given $Z,\langle E_{\perp }\rangle$ is either constant or decreases slightly with increasing A, in contradiction with a mass-dependent collective expansion in which all IMFs are emitted on average at the same time. Neutron-deficient isotopes of even Z elements manifest higher kinetic energies than heavier isotopes of the same element for both PLF${}^{*}$ and midvelocity emission. This result may be because of the charged-particle decay of long-lived excited states.

Figure 1

Element distribution measured by the ring detector for the angular range 2.1${}^{°}\le {\theta }^{\mathrm{lab}}\le 4.2^{°}$. The arrow indicates the atomic number of the beam.

Figure 2

Isotopic resolution achieved in LASSA for isotopes of Li and Be. The spectra have been summed over all nine LASSA telescopes.

Figure 3

(a) Charged-particle multiplicity distribution associated with the detection of a PLF with $30\le Z\le 46$ in the RD; (b) velocity distribution for $30\le Z\le 46$ detected in RD; (c) atomic number distribution of fragments detected in the RD.

Figure 4

(Color online) Invariant cross-section plots of the emission of ${}^6\mathrm{Li}$ fragments in the center-of-mass frame. (a) Associated with $30\le Z_{\mathrm{PLF}}\le 46$. (b) Associated with $N_C⩾20$. The dashed arrow indicates $\langle V_{\parallel }^{\mathrm{PLF}}\rangle$ while the solid arrow indicates $V_{\mathrm{BEAM}}$. The differential yield is presented on a logarithmic scale.

Figure 5

(a) Z distribution of the largest fragment detected in the RD associated with events for which $N_c⩾20$. (b) Cumulative yield distribution of the largest fragment in the RD associated with events for which $N_c⩾20$.

Figure 6

(Color online) Correlation between atomic number and velocity (in the COM) of the largest fragment detected in the RD. The most probable velocity for each Z is indicated by a solid circle while the beam velocity is indicated by the arrow.

Figure 7

Z distribution of fragments associated with midperipheral collisions emitted in the angular range 85${}^{°}\le {\theta }^{\mathrm{PLF}}\le {95}^{°}$ in the PLF frame (solid circles); midperipheral collisions and at midvelocity (open triangles); and fragments associated with central collisions at midvelocity (open squares). The lines depict the Z distribution predicted by the statistical model GEMINI (see text for details). The yield for each case has been normalized to unity for the interval shown.

Figure 8

Distributions of $V_{\perp }$, for ${}^7\mathrm{Be}$ and ${}^{10}\mathrm{Be}$ fragments in the center-of-mass frame. (a) Midperipheral collisions and 80${}^{°}\le {\theta }^{\mathrm{PLF}}\le {100}^{°}$; (b) midperipheral collisions and 0$\le V_{\parallel }\hspace{1em}⩽1$ cm/ns; (c) central collisions and $0\le V_{\parallel }\le 1$ cm/ns. All distributions have been normalized to unity.

Figure 9

Distributions of $V_{\perp }$, for ${}^6\mathrm{Li}$ and ${}^9\mathrm{Li}$ fragments in the center-of-mass frame. (a) Midperipheral collisions and 80${}^{°}\le {\theta }^{\mathrm{PLF}}\le {100}^{°}$; (b) midperipheral collisions and $0\le V_{\parallel }\hspace{1em}⩽1$ cm/ns; (c) central collisions and $0\le V_{\parallel }\le 1$ cm/ns. All distributions have been normalized to unity.

Figure 10

Average transverse energies for $2\le Z\le 6$ selected under different criteria. For the PLF${}^{*}$, fragments were selected in the angular range ${85}^{°}\le {\theta }^{\mathrm{PLF}}\le {95}^{°}$, whereas midvelocity fragments have $0\le V_{\parallel }\le 1$ cm/ns in the center-of-mass frame.

Figure 11

Average transverse energies as a function of mass number for $2\le Z\le 6$ for midperipheral and central collisions with different selection criteria. For the PLF${}^{*}$, fragments were selected in the angular range ${85}^{°}\le {\theta }^{\mathrm{PLF}}\le {95}^{°}$, whereas midvelocity fragments have $0\le V_{\parallel }\le 1$ cm/ns in the center-of-mass frame.

Figure 12

Average center-of-mass energies for $2\le Z\le 4$ as a function of mass selected by element following incomplete fusion in the reaction ${}^{64}\mathrm{Ni}$+${}^{100}\mathrm{Mo}$ at $E/A=11\text{ MeV}$ [39].

Figure 13

(a) $\langle E_{\perp }\rangle$ for helium isotopes as a function of $\langle V_{\mathrm{PLF}}\rangle$ and deduced excitation energy (upper axis). ${}^3\mathrm{He}$ (solid circles), ${}^4\mathrm{He}$ (open circles), and ${}^6\mathrm{He}$ (solid squares). (b) Average transverse energy of ${}^3\mathrm{He}$ and ${}^6\mathrm{He}$ with reference to ${}^4\mathrm{He}$.