APPLICATION OF HIGH ENERGY CHARGE TRANSFER TO BEAM DIAGNOSTICS
I. Katayama and S. Morinobu
Research Center for Nuclear Physics, Osaka University,
Charge transfer atomic reaction induced by high energy beams on solidmaterials is shown to be applicable to the beam diagnostics. The yield ofthe reaction is discussed in some detail.
The atomic collisions of fast heavy particles in solid materials are
known with their very small deflection angles and the presence of chargetransformation probability (charge transfer rection). There is an interesting calculationl) on the angular distribution of neutralized protons in theP+H + H+P reactions at energies of larger than 10 MeV, in which a local
maximum peak is predicted at 1.6 minutes. This means that the momentum
transfer in the reaction is of the order of lo-4. For heavier projectiles,
it is evident that the larger mass difference between electrons and projectiles works as a further reduction factor for the angular spread of thescattered particles. It is to be noted that these perturbations caused by
the reaction are small compared to the moemtnum widths and emittances of thebeams from the usual accelerators.
This feature of the atomic collision is of great interest since it
provides us a new tool for the beam diagnostics. An idea of its application
has been suggested in ref.2) and an example of the use in the calibration ofa magnetic spectrograph is seen in ref.3). In this short note we will
briefly discuss how large yield we can expect for charge transfer reaction,
which is of considerable importance from the practical view point.
It has been shown that the charge exchange collision in solid targets
at high energy can well be explained as a sequential process of electroncapture and electron stripping as usually seen in gas targets4). Let us
take the case of 3He2+ + 3Hel+ reaction in solid as an exapmle. The yieldof the 3Hel+ ions is given by the following expression,(1)

where No is the intensity of the primary beam, t the target thickness andos and oc, respectively, are electron capture (3He2+ + 3Hel+) and electron
stripping (3Hel+ + 3He2+) cross sections in the target. For large Ost
values, it is clear that the yield takes a constant value at Nooc/os· This
state is called charge equilibrium. Fig.l is our experimental data~> showing
the dependence of the yield on the target thickness for the 130 MeV 3He2+ions on carbon targets. The ocfos is seen to be of the order of lo-8 forthe present case. The figure also shows that the yield is reasonably large
to allow the use of the conventional nuclear radiation detectors for measuring the 'charge transformed particles'. Fig.2 shows the dependence of theyield on the atomic number of target materials, which was measured in thesame experiment2). It may be seen that the yield can be controlled within
the range of an order of magnitude by selecting the ~aterials for the target.
In the yield estimation for the arbitrary projectiles on the arbitrary
materials, one has to rely on the theoretical predictions. Born calculation
explains os quite well. The theory by GillespieS) shows the Os can be
written in the form os = 8nao2fV12(Il + I2/V12), where ao is Bohr radius, Vl
the projectile velocity in atomic unit and I's are functions of the atomic
numbers of the projectile (Zp) and target (Zt>· The os has a moderate
dependence on v1 and takes values around lo-17 ~ lo-18 cm2;atom for thecases shown in fig. 2 .,-117-

3H e2+ + C- 3H e1+ + C+~x10- 8c0"'ot;2.0d~ 10' 6dL-Q)

lL.;F.±-<lJ 1.0

E:r::::>

rn0+-.-.-.-.-,~~~,-,-~

040

80

200:::>

400 (~g/cm 2 )crw

Target Thickness
Fig. l.
. ld
Thickness dependence of 3He l+ yle
from carbon. The solid curve is aleast square fitting result using
eq. (l).

Fig. 2.
3 l+Equilibrium charge fraction of He
as a function of target atomic number
z at 68, 100 and 130 MeV. The solidllnes are to guide the eye.

The correct estimation of crc is still a challenging problem fortheoreticia ns6) • A convenient way to estimate it is to use the first order
Born calculation 7,8), Nikolaev8) introduced a semiempiric al correction
factor for the calculation to explain experiment al data. The more sophisticated theory including the higher order Born terms9) have been developed
only for the restricted cases of special electron transitions . For theprojectile velocities much larger than the target K-electron velocity, crc
behaves6) as ac "' zp5zt5vyl2, which is of the order of nuclear cToss section
for the cases shown in fig.2. For heavier ions, a semiempiric al scaling lawof crc is helpfullO),
The above estimations result in as and crc of the order of lo-17 andlo-25 cm2jatom, respectivel y, for the 130 MeV 3He ions on the carbon target.
The mean free path of the 3Hel+ ions in carbon, therefore, becomes so smallthat all the 3Hel+ ions emerging from the target may well be considered to
be produced at the exit face of the target. This means that the effects ofenergy straggling and multiple scattering are common for all the outgoingparticles irrespectiv e of their charge states, which also supports theapplicabili ty of charge transfer to the beam diagnostics .
References
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