GSI SCIENTIFIC REPORT 2010
PNI-MR-25
New measurements of W-values in argon, nitrogen and air for protons, helium
and carbon ions
J. Beck1 , U. Giesen1 , M. Bender2 , D. Schardt2 , and D. Severin2
1
Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany; 2 GSI, Darmstadt, Germany
Introduction
In particle therapy for cancer the dosimetry of the
charged-particle radiation is mostly performed by measuring the ionization produced in air-filled ionization chambers. The conversion of the reading of an ionization chamber into absorbed dose requires W-values, which are defined as the average energy needed to produce an ion pair
by a particle in a gas. Because of the increasing importance of ion therapy and the lack of experimental W-values
for charged particles in air, new measurements of W-values
are being carried out at PTB and GSI. Existing measurements for protons in air indicate an uncertainty of 4 % [1,2]
and the main goal of the present studies is to achieve an accuracy of about 1 %. A new experimental set-up has been
developed at PTB for the measurement of W-values in argon, nitrogen and air for protons and helium ions at energies from 0.5 to 3.5 MeV/u at PTB and for carbon ions
between 3.6 and 7.0 MeV/u at GSI.
Experimental Methods
A schematic overview of the PTB set-up is shown in Figure 1. The ionization chamber is filled with gases, usually
at atmospheric pressure. The active volume has a length of
25 cm, which is sufficient to stop protons and helium ions
up to about 4 MeV/u and carbon ions up to about 7 MeV/u.
The electric field direction in the ionization chamber is orthogonal to the beam direction, which minimizes the corrections due to recombination effects in the gas. A low
rate of 100 to 500 incoming ions per second is produced by
scattering of the primary beam from the accelerators on a
thin gold foil at the center of a scattering chamber. At 45◦
on either side relative to the beam direction, the scattered
ions are detected in the ionization chamber and a monitor
detector. The determination of the number of particles entering the active volume of the ionization chamber is measured using two independent methods. First, the fast pulse
of electrons inside the ionization chamber is measured behind a Frisch grid at the anode. This signal provides the
number and energy spectrum of ions for most gases. However, this method does not work with air because of the
electronegativity of oxygen. Secondly, the spectrum and
number of particles is simultaneously detected in a silicon
particle detector using a similar geometry. This rate of particles in the silicon detector is used for measurements with
air, after additional calibrations against the count rate inside the ionization chamber using the anode measurements
in argon or using a silicon detector.
Figure 1: Set-up for W-value measurements at PTB and
GSI. The ion beam enters the scattering chamber from
the lower right, passes a thin Au foil in the centre and is
stopped in the Faraday cup. The scattered beam is monitored at +45◦ by a particle detector and at -45◦ stopped in
the gas volume of the ionization chamber (left).
The number of ion pairs produced in the gas volume is determined from the simultaneous measurement of the ionization current collected at the cathode of the chamber using a calibrated Keithley electrometer.
Measurements
Up to now, measurements with 2 MeV/u protons and
helium ions at energies from 0.5 to 3.5 MeV/u were performed at PTB. Furthermore, measurements with carbon
ions of 3.6 MeV/u and 5.9 MeV/u were carried out at GSI.
Corrections of recombination effects were determined from
series of measurements with different voltages at the ionization chamber for each energy. The background from
beam-induced γ-radiation was measured without particles in the ionization chamber. Detailed analyses are in
progress and further measurements with carbon ions up to
7.0 MeV/u are planned.
References
[1] G.H. Hartmann et al., Determination of water absorbed dose
in a carbon ion beam using thimble ionization chambers,
Phys. Med. Biol. 44,1193(1999).
[2] B. Grosswendt and W.Y. Baek, W-values and radial dose distributions for protons in TE-gas and air at energies up to 500
MeV, Phys. Med. Biol. 43,325(1998).
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