Projectile fragmentation reactions and production of nuclei near the neutron drip line

The reaction mechanism of projectile fragmentation at intermediate energies has been investigated by observing the target dependence of the production cross sections of very neutron rich nuclei. Measurement of longitudinal momentum distributions of projectile-like fragments in a broad range of fragment mass and charge was performed using a 100 MeV/nucleon ${}^{40}\mathrm{Ar}$ beam incident on Be and Ta targets. By measurement of the fragment momentum distribution, a parabolic mass dependence of the momentum peak shift was observed in the results of both targets, and light-fragment acceleration was found only in the Be-target data. The analysis of production cross sections revealed an obvious enhancement of the target dependence except for a target size effect when the neutron excess is increased. This result implies the breakdown of factorization of production cross sections for very neutron rich nuclei near the drip line.

Figure 1

Particle identification in the $\Delta E$–TOF plane at $B\rho =3.708$ Tm with the Be target.

Figure 2

(a) $Z$-projection spectrum for $A/Z=2\pm 0.3$ and (b) $A/Z$-projection spectrum for $Z=13.0\pm 0.5$ at $B\rho =2.523$ Tm (Be target). The arrow in (a) indicates the lack of ${}^8\mathrm{Be}$, which is known to be unbound.

Figure 3

Typical fragment momentum distribution and fitting results for the momentum distribution of ${}^{30}\mathrm{Mg}$ data. The fitting result with a Gaussian function (dotted curve) shows the clearly asymmetric feature of the experimental data. The solid curve indicates a fitting result with the asymmetric Gaussian function.

Figure 4

Typical fragment momentum distributions for (a) ${}^{10}\mathrm{Be}$ and (b) ${}^{30}\mathrm{Mg}$ produced in the $\mathrm{Ar}+\mathrm{Be}$ reaction. The fitting results are also shown with solid curves. The light fragment of ${}^{10}\mathrm{Be}$ has two components (dashes curves), HE and LE; the heavy fragment of ${}^{30}\mathrm{Mg}$ has one component, corresponding to the HE component. The HE component for each isotope has a low-momentum tail and the symmetric parts are shown with dotted curves.

Figure 5

Overall systematic error for the production cross sections (solid line). The dashed line shows error originating from the ambiguity of ${\sigma }_{\perp }$.

Figure 6

Momentum peak shift of fragments produced in $\mathrm{Ar}+\mathrm{Be}$ (upper panel) and $\mathrm{Ar}+\mathrm{Ta}$ (lower panel). The primary beam energies are drawn with dotted lines, and the kinetic energies at the half-point of target thickness (projectile energies) are the solid lines. See the text for the labels and curves.

Figure 7

Peak value, $P_0$, of fragment momentum distribution for HE and LE components.

Figure 8

Momentum widths at high-momentum side (Be target) for (a) ${\sigma }_H$ and (b) reduced widths according to the Goldhaber model.

Figure 9

Momentum widths at high-momentum side (Ta target) for (a) ${\sigma }_H$ and (b) reduced widths according to the Goldhaber model.

Figure 10

Momentum widths at low-momentum side by using (a) the Be target and (b) the Ta target. The solid lines in both figures are the linear fits of the Ta target data. The experimental results (${\sigma }_L$) and the fitting lines are located far from a prediction of the Goldhaber model. The isotope chains of experimental data are also drawn as solid lines.

Figure 11

Momentum distribution for ${}^{36}\mathrm{Al}$. This isotope cannot be produced with only the nuclear fragmentation process. One neutron should be picked up from the target. The measured peak momentum corresponds to $83.9A$ MeV. The measured momentum peak shows a larger shift toward the low-momentum side, compared with that of the same mass number, $~0.9A$ MeV.

Figure 12

Measured cross sections presented as isotope distributions for $3⩽Z⩽14$ elements detected in ${}^{40}\mathrm{Ar}+{}^9\mathrm{Be}$ reactions at 90 MeV/nucleon. Experimental fragmentation data are shown as filled circles. EPAX predictions are shown as solid curves.

Figure 13

Measured cross sections presented as isotope distributions for $3⩽Z⩽19$ elements detected in ${}^{40}\mathrm{Ar}+{}^{181}\mathrm{Ta}$ reactions at 94 MeV/nucleon. Experimental fragmentation data are shown as filled circles. EPAX predictions are shown as solid curves.

Figure 14

Ratio of the production cross sections for each fragment produced with Be and Ta targets. The solid lines are drawn for the same mass number of nuclei (i.e., isobars). The $Z_{\beta }$ is the β-stable charge for each isobar. The cross section ratio of ${\sigma }_{\mathrm{Ta}}(A,Z)/{\sigma }_{\mathrm{Be}}(A,Z)$ is normalized with the mass-yield ratio of $Y_{\mathrm{Ta}}$(A)/$Y_{\mathrm{Be}}$(A) observed.

Figure 15

Charge distribution for $A_f=29$.

Figure 16

Deviation of the most probable charge from the EPAX parametrization to experimental data for the production targets of Be and Ta.

Figure 17

Peak of charge distributions as a function of fragment mass for the production targets of (a) Be and (b) Ta. The solid curves are the most probable charge $Z_{\mathrm{prob}}$ of EPAX. The dashed curves are the β-stable charge $Z_{\beta }$. The dashed lines are the $N/Z$ ratio of ${}^{40}\mathrm{Ar}$ projectile and compounds of ${}^{40}\mathrm{Ar}+{}^9\mathrm{Be}$ and ${}^{40}\mathrm{Ar}+{}^{181}\mathrm{Ta}$, respectively.

Figure 18

$U$ parameters as a function of fragment mass for the production targets of Be and Ta. On the assumption that the value of $U$ is the same for every group of four sets of $A_f$ data, the fitting results are shown with the systematic error. The dashed lines are the values of $U=1.65$ from the EPAX.

Figure 19

Predictive power of cross sections of the new parametrization. The solid box marks the data of ${}^{24}\mathrm{O}$ with a tantalum target.