Experimental determination of the effective point of measurement of the PTW-31010 ionization chamber in proton and carbon ion beams

PURPOSE

The accurate knowledge of the effective point of measurement (P_eff ) is particularly important for measurements in proximity to high dose gradients such as in the distal fall-off of particle beams. For plane-parallel ionization chambers (ICs), P_eff is well known and located at the center of the inner surface of the entrance window. For cylindrical ICs, P_eff is shifted from the chamber's center toward the beam source. According to IAEA TRS-398, this shift can be calculated as 0.75·r_cyl for light ions with r_cyl being the radius of the cavity. For proton beams and in absence of a dose gradient, no shift is recommended. We have experimentally determined P_eff for the 0.125 cc Semiflex IC in both proton and carbon ion beams.

METHODS

The first method consisted of simultaneous irradiation of a plane-parallel IC and the Semiflex in a 4-cm wide spread-out Bragg peak. In the second method, a single-energy beam was used, and both ICs were positioned successively at the same measurement depths. For both approaches, the shift of the distal edge of the depth ionization distributions recorded by the two chambers at different reference points was used to calculate P_eff of the Semiflex. Both methods were applied in carbon ion beams, and only the latter was applied in proton beams.

RESULTS

Both methods yielded a similar P_eff for carbon ions, 0.88·r_cyl , and 0.84·r_cyl , which results in a difference of only 0.1 mm. The difference to the recommended value of 0.75·r_cyl is 0.4 and 0.3 mm, respectively, which is larger than the positioning uncertainty. In the proton beam, a P_eff of 0.92·r_cyl was obtained.

CONCLUSIONS

The P_eff for the 0.125 cc Semiflex IC is shifted further from the cavity center as recommended by IAEA TRS-398 for light ions, with the shift for proton beams being even larger than for carbon ion beams.

Betatron radiation and emittance growth in plasma wakefield accelerators

Beam-driven plasma wakefield acceleration (PWFA) has demonstrated significant progress during the past two decades of research. The new Facility for Advanced Accelerator Experimental Tests (FACET) II, currently under construction, will provide 10 GeV electron beams with unprecedented parameters for the next generation of PWFA experiments. In the context of the FACET II facility, we present simulation results on expected betatron radiation and its potential application to diagnose emittance preservation and hosing instability in the upcoming PWFA experiments. This article is part of the Theo Murphy meeting issue 'Directions in particle beam-driven plasma wakefield acceleration'.

Producing multi-coloured bunches through beam-induced ionization injection in plasma wakefield accelerator

This paper discusses the properties of electron beams formed in plasma wakefield accelerators through ionization injection. In particular, the potential for generating a beam composed of co-located multi-colour beamlets is demonstrated in the case where the ionization is initiated by the evolving charge field of the drive beam itself. The physics of the processes of ionization and injection are explored through OSIRIS simulations. Experimental evidence showing similar features are presented from the data obtained in the E217 experiment at the FACET facility of the SLAC National Laboratory. This article is part of the Theo Murphy meeting issue 'Directions in particle beam-driven plasma wakefield acceleration'.

Response of synthetic diamond detectors in proton, carbon, and oxygen ion beams

PURPOSE

In this work, the LET-dependence of the response of synthetic diamond detectors is investigated in different particle beams.

METHOD

Measurements were performed in three nonmodulated particle beams (proton, carbon, and oxygen). The response of five synthetic diamond detectors was compared to the response of a Markus or an Advanced Markus ionization chamber. The synthetic diamond detectors were used with their axis parallel to the beam axis and without any bias voltage. A high bias voltage was applied to the ionization chambers, to minimize ion recombination, for which no correction is applied (+300 V and +400 V were applied to the Markus and Advanced Markus ionization chambers respectively).

RESULTS

The ratio between the normalized response of the synthetic diamond detectors and the normalized response of the ionization chamber shows an under-response of the synthetic diamond detectors in carbon and oxygen ion beams. No under-response of the synthetic diamond detectors is observed in protons. For each beam, combining results obtained for the five synthetic diamond detectors and considering the uncertainties, a linear fit of the ratio between the normalized response of the synthetic diamond detectors and the normalized response of the ionization chamber is determined. The response of the synthetic diamond detectors can be described as a function of LET as (-6.22E-4 ± 3.17E-3) • LET + (0.99 ± 0.01) in proton beam, (-2.51E-4 ± 1.18E-4) • LET + (1.01 ± 0.01) in carbon ion beam and (-2.77E-4 ± 0.56E-4) • LET + (1.03 ± 0.01) in oxygen ion beam. Combining results obtained in carbon and oxygen ion beams, a LET dependence of about 0.026% (±0.013%) per keV/μm is estimated.

CONCLUSIONS

Due to the high LET value, a LET dependence of the response of the synthetic diamond detector was observed in the case of carbon and oxygen beams. The effect was found to be negligible in proton beams, due to the low LET value. The under-response of the synthetic diamond detector may result from the recombination of electron/hole in the thin synthetic diamond layer, due to the high LET-values. More investigations are required to confirm this assumption.