Stopping power of gases for heavy ions: Gas-solid effect: I. 2–13 MeV/u Ne and Ar projectiles

doi:10.1016/0168-583X(89)90683-6

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Volume 44, Issue 1, November 1989, Pages 1-18

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Abstract

The stopping powers of twelve gaseous media have been measured for Ne and Ar ions in the energy range 2–13 MeV/u. The experimental method consists in measuring the ion beam energy before and after its slowing down in a given thickness x of gas. This thickness is derived from the simultaneous measurements of the energy loss of 8.78 and 6.06 MeV alpha particles in the same gas cell.

The experimental data are compared with the calculated values of Northcliffe and Schilling, and of Ziegler. A fair agreement is found, except for the Ar + He experiments for which the calculated stopping powers are too low. By comparing the experimental values with the stopping powers calculated for solid media through the Hubert parameterization, no gas-solid effect is found for Ne ions, but a sizeable one (= 10%) appears for the stopping of Ar ions in Ne and Ar.

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The graph also shows that binary stopping theory [120], to be mentioned in the next section, describes the overall behavior quite well. Although Fig. 23 appears to deny a gas–solid difference in stopping, subsequent measurements [123–125] demonstrated that stopping cross sections indeed tend to be greater in solids than in gases, if differences in atomic number are taken into account properly. Stopping theory for swift heavy ions has been developed from the mid 1990s and can now be considered to be a mature field which does not any longer depend on empirical concepts such as the effective charge.
In response to an invitation by the organizers of the 27th international conference on atomic collisions in solids, a brief survey is presented, starting from the roots of the field in the 1950s and 1960s, of some major discoveries, longstanding problems, surprising findings and memorable controversies in topics covered by the conference. Considering the breadth of the field, the selection of topics is necessarily subjective, but with the emphasis on channeling, stopping and sputtering, three topical areas are discussed which have been active from the early 1960s until now.
Stopping powers and ranges for the heaviest atoms
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As we mentioned, the differences in the HI effective charges or SPs for solids and gases were firstly observed in [16], where it was shown that SPs of gases are up to 20% smaller than those of solids with the neighbouring atomic number of the stopping medium ZT. These observations were confirmed in the following works [19,20] for HI stopping in gases with ZT ⩽ 18. At present, the gas–solid effect is beyond question (see, for example, the review [21]).
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A new stopping power parameterization for 0.1-15 MeV/nucleon heavy and superheavy ions in solids and gases
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Electronic stopping powers of heavy ions in several media are deduced from the corresponding proton data by using new effective charge parameterizations. Separate sets of parameters were deduced for solid and gaseous materials using the available data for heavy ions in the energy range from 0.1 to 15 MeV/nucleon. Using these results, predictions are made for stopping powers of heavy and superheavy elements with Z = 100–130 in the energy range of 0.5–15 MeV/nucleon.
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The BigSol Superconducting Solenoid Beam Line at the Texas A&M Superconducting Cyclotron has been used to measure energy losses of ⁴⁰Ar, ⁸⁴Kr, ¹⁹⁷Au and ²³⁸U ions in mylar, aluminum and isobutane at energies ranging from the Bragg peak up to several MeV/nucleon. The experimental data are compared with predictions from the SRIM code. In general experimental data for ⁴⁰Ar and ⁸⁴Kr are in agreement with model predictions whereas differences on the order of 10% are evidenced in some cases for ¹⁹⁷Au and ²³⁸U ions especially at and around the Bragg peak energies.
The solid-gas difference in stopping powers, and statistical analysis of stopping power data
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For the purpose of treating the difference in stopping power between condensed and gaseous substances, we first discuss our method of statistical analysis of published experimental data. We distinguish between a positive effect where the difference between mass stopping powers S_solid − S_gas is positive and a negative effect where the opposite holds true. Experimentally, the positive effect has been found so far with heavy ions at high energy, and the negative effect with light ions at low energy. The positive effect is due to the difference in ionic charges and can be described by the CasP program by Grande and Schiwietz. An apparent persistence of the positive effect down to low energy is found to be mainly an artifact inherent in the stopping table of ICRU Report 73. The negative effect can be described by a difference in mean ionization potentials between gases and condensed matter. It is large for metals, much smaller for compounds. It should be possible to see both effects in one projectile – target combination.
On the gas-solid difference in stopping power for low energy ions
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In an earlier paper we claimed that the well-known gas–solid difference in high energy mass stopping powers persists down to low energy; this claim was essentially based on stopping powers for N ions. We now re-investigate this claim using many more experimental data, comparing them to the recent stopping power table of ICRU Report 73 which has no gas–solid difference built in, and by using our statistical analysis program. We find that, at a specific energy of 25 keV/n, the average gas–solid difference amounts to −40% for N ions and to −48% for all ions from ₃Li to ₁₈Ar taken together, the stopping power for gas being lower. It appears therefore that the agreement between experiment and theory could be improved by taking this difference into account.

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^∗: Present address: II. Physikalisches Institut, Universität Giessen, Giessen, FRG.

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Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Abstract

References (20)

Nucl. Instr. and Meth.

Nucl. Instr. and Meth.

Nucl. Instr. and Meth.

Nucl. Instr. and Meth.

Nucl. Instr. and Meth.

Phys. Lett.

Thesis Toulouse

Report IPNO-DRE-88-25

Phys. Rev.

Ann. Phys. (France)

K. Dan. Vidensk. Selsk. Mat. Fys. Medd.

Cited by (32)

Six decades of atomic collisions in solids

Stopping powers and ranges for the heaviest atoms

A new stopping power parameterization for 0.1-15 MeV/nucleon heavy and superheavy ions in solids and gases

Energy loss of energetic <sup>40</sup>Ar, <sup>84</sup>Kr, <sup>197</sup>Au and <sup>238</sup>U ions in mylar, aluminum and isobutane

The solid-gas difference in stopping powers, and statistical analysis of stopping power data

On the gas-solid difference in stopping power for low energy ions