Charge-state resolved laser acceleration of gold ions to beyond 7 MeV/u

Abstract

In the past years, the interest in the laser-driven acceleration of heavy ions in the mass range of [Formula: see text] has been increasing due to promising application ideas like the fission-fusion nuclear reaction mechanism, aiming at the production of neutron-rich isotopes relevant for the astrophysical r-process nucleosynthesis. In this paper, we report on the laser acceleration of gold ions to beyond 7 MeV/u, exceeding for the first time an important prerequisite for this nuclear reaction scheme. Moreover, the gold ion charge states have been detected with an unprecedented resolution, which enables the separation of individual charge states up to 4 MeV/u. The recorded charge-state distributions show a remarkable dependency on the target foil thickness and differ from simulations, lacking a straight-forward explanation by the established ionization models.

Figure 1

Experimental setup at the PHELIX laser.

Figure 2

(a) Microscope image of an irradiated CR-39 sheet. The black dots originate from gold and carbon impacts and have been registered by the software SAMAICA, which fitted ellipses around the dots (in green). Carbon and gold ion impacts can be discriminated by their size, which is confirmed by the inset showing lineouts through several pits of the respective ion species. The FWHM of the carbon and gold ion impacts in this specific example are about 32 and 38 pixels, respectively (not shown in the figure). (b) Scanned CR-39 image from a detector placed behind the TPS. Each point corresponds to an ion impact. The color map visualizes the ion pit size in form of the enclosed area (number of enclosed pixels). The visible structures can be assigned to C${}^{6+}$, C${}^{5+}$, C${}^{4+}$ and gold ions, respectively. This image shows the dependency of the ion pit size both on ion species (carbon ions in green and gold ions in red can be distinguished even in their overlapping region, see inset) and on the particles’ kinetic energies.

Figure 3

Illustration of the background discrimination using the raw data acquired with CR-39 track detectors. Only pits are accepted, whose ellipse parameters lie within certain boundaries, that can be attributed to ion signal. In total, three filtering stages have been applied to the CR-39 data. The accepted data points in (a) are drawn in black. In (b), the ion species can be separated by their pit sizes: orange refers to gold ions, violet to oxygens and yellow to carbons. (c) The last filtering step for the example of gold ions. The accepted data points are displayed in green. The two-dimensional gate conditions in each step have been defined by hand. The selected regions around the parameter correlations were chosen as small as possible in order to exclude a maximum number of background data points, but large enough that no correlated patterns related to the observed ion curves - especially for heavy ions—appeared in the discarded data.

Figure 4

Raw data recorded with CR-39 for shots on unheated foils with a thickness of 500 nm (top), which delivered the best charge-state resolution and 25 nm (bottom), which resulted in the highest gold ion energies. The blue data points correspond to light ions, while the orange ones visualize larger pits caused by heavier ions, in particular gold. The grey dashed lines show the calculated lines for the mass-to-charge ratios of 2, 2.5 and 3, respectively. Details of the gold traces are shown in the insets. As indicated, the magnetic (electric) field deflected the ions to the right (downwards).

Figure 5

CR-39 raw data after background removal for a laser shot onto a 100 nm thin gold foil that was laser-heated before the shot for surface cleaning. The iso-energy lines starting from 2 MeV/u up to 7 MeV/u are indicated by black dotted lines.

Figure 6

Gold ion energy spectra for shots on gold foils with varying thickness, integrated over all charge states. The grey error margin includes uncertainties due to the calibration of the magnetic field, the determination of the exact charge-state range and due to the intrinsic energy resolution of the TPS. Slight uncertainties of detector positioning and ion numbers have been included as well.

Figure 7

Left: CR-39 raw data after background discrimination for laser shots on (a) unheated and (b) heated 500 nm thick gold foils. Right: IP raw data for laser shots on (c) unheated and (d) heated 500 nm gold foils.

Figure 8

CR-39 raw data with reconstructed curves (dotted lines) in the charge state range of ${67}^{+}-{72}^{+}$ for the assignment of the manually traced data for heavy ion pits.

Figure 9

Comparison of the measured gold ion charge states from shots on gold foils with varying thickness to steps in the gold ionization energy indicated by the vertical orange lines (dashed lines correspond to larger, dotted lines to smaller steps). The blue distributions show the gold ion numbers for each charge state integrated over all energies. The green distributions display the number of gold ions integrated between 1.8 and 3.9 MeV/u, for which the individual charge states were resolvable for most of the shots. The yellow point depicts the mean charge state of the green distribution with an error bar showing the uncertainty of the total charge state range. The distributions are normalized to the respective maximum of the blue curves.

Figure 10

Energy-dependent gold ion charge-state distributions for a shot on a heated, 100 nm thick gold foil. The energies are increasing from the bottom to the top, which is also visualized by the face colors of the distributions (blue means low, red means high energy). The ion numbers of each distribution were normalized to their respective maximum, which is stated for each panel individually (‘norm = X’). The respective mean value has been marked by an asterisk (*) for each charge-state distribution. The error bars in the four topmost panels indicate the charge-state resolution within the shown distribution. The total charge state mean value, averaged over all kinetic energies, is indicated by the dotted, orange line. In the topmost plot, the uncertainty of the charge-state range itself is indicated by the orange error bar.