Device and method for measuring an energy particle beam
The present invention relates to a dosimetry device for an energy particle beam from a source and including at least two ionization chambers, each of which includes a collector electrode and a polarization electrode, said electrodes in each ionization chamber being separated by a gap including a fluid, an energy beam from a single source passing through said ionization chambers, the device being characterized in that said ionization chambers have different charge collection efficiency factors. Said calculation algorithm for the dose rate deposited by said beam is based on the measurement of an output signal in each ionization chamber of the device and on a □gain□ factor related to a first ionization chamber, said □gain□ factor being theoretically predetermined on the basis of said intrinsic and/or extrinsic parameters of said ionization chambers.
1. A dosimetry device for an energy particle beam coming from a source and comprising at least first and second ionization chambers each comprising a collector electrode and a polarization electrode, said electrodes of each ionization chamber being separated by a gap or space comprising a fluid, said ionization chambers being configured to be passed through by an energy particle beam coming from a same source, wherein the ionization chambers have different charge collection efficiency factors, and the device comprises an acquisition device connected to a computer configured to perform an algorithm for calculating the dose rate deposited by said particle beam based on:
measuring an output signal in each considered ionization chamber; and
a gain factor relative to a first ionization chamber, given by the equation
G
=
1
-
(
i
1
i
2
)
norm
1
-
f
1
where
G is the gain factor relative to the first ionization chamber,
(
i
1
i
2
)
norm
is the ratio of theoretical values of output signals (i 1 and i 2 ) in the two considered ionization chambers normalized by the ratio of their respective amplification factors (R 1 /R 2 ), each amplification factor depending on said fluid, the width of the gap and the penetration power of the particle beam in said fluid for a considered ionization chamber,
f 1 is the theoretical value of the charge collection efficiency factor in the first ionization chamber,
said theoretical output signal values (i 1 , i 2 ) and f 1 being calculated as a function of intrinsic and/or extrinsic parameters of said considered chambers, and as a function of one or more pre-defined values of the current intensity of the particle beam, said gain factor G being independent of the value of said particle beam current intensity.
2. The device according to claim 1 , wherein said computation algorithm is configured to perform the following steps:
measuring the output signals (i 1 and i 2 ) in the two considered ionization chambers, and establishing a normalized ratio
(
i
1
i
2
)
norm
between said output signal values (i 1 and i 2 );
calculating the charge recombination rate (1−f) and deducing the charge collection efficiency factor (f) in a considered ionization chamber based on the results of the first step of the algorithm and based on the knowledge of the gain factor of the considered ionization chamber; and
computing the dose rate deposited by the beam based on a charge collection efficiency factor in the considered ionization chamber.
3. The device according to claim 1 , wherein said considered ionization chambers have a difference in terms of the gap thicknesses comprised between said collector and polarization electrodes, imparting a charge collection efficiency factor difference.
4. The device according to claim 1 , wherein said considered ionization chambers have a difference at the electric fields created between said collector and polarization electrodes, imparting a charge collection efficiency factor difference.
5. The device according to claim 1 , wherein said considered ionization chambers have a difference in terms of the nature of the fluids present in the gap comprised between said collector and polarization electrodes for each ionization chamber, imparting a charge collection efficiency factor difference.
6. The device according to claim 1 , wherein said considered ionization chambers have a difference in terms of the pressure and/or temperature of the fluids present in the gap comprised between said collector and polarization electrodes for each ionization chamber, imparting a charge collection efficiency factor difference.
7. The device according to claim 1 , wherein said considered ionization chambers have a difference in terms of their geometry and/or localization relative to the position of the source, so that the beam field entering each of the ionization chambers differs from one ionization chamber to the other, imparting a charge collection efficiency factor difference.
8. The device according to claim 1 , wherein said considered ionization chambers are separated by one or more energy absorbers, so that the energy beam entering each of the ionization chambers differs from one ionization chamber to the other, imparting a charge collection efficiency factor difference.
9. A method for measuring the dose rate deposited by a particle beam coming from a source, the method comprising:
(i) utilizing the device according to claim 1 , for which one chooses the intrinsic and/or extrinsic parameters of each of the considered ionization chambers;
(ii) establishing a gain factor, given by the equation
G
=
1
-
(
i
1
i
2
)
norm
1
-
f
1
where
G is the gain factor relative to the first ionization chamber,
(
i
1
i
2
)
norm
is the ratio of theoretical output signal values (i 1 and i 2 ) in the two considered ionization chambers normalized by the ratio of their respective amplification factors (R 1 /R 2 ), each amplification factor depending on said fluid, the width of the gap and the penetration power of the particle beam in said fluid for a considered ionization chamber, and
f 1 is the theoretical value of the charge collection efficiency factor in the first ionization chamber,
said theoretical output signal values (i 1 , i 2 ) and f 1 being calculated as a function of the intrinsic and/or extrinsic factors of said considered chambers chosen in the first step of the method and as a function of one or more pre-defined values of the current intensity of the particle beam, said gain factor G being independent of the value of said particle beam current intensity,
(iii) measuring the output signal for said two considered ionization chambers; and
(iv) processing said signals using a computer configured to carry out an algorithm for determining the charge collection efficiency factor of said first chamber, based on the gain factor, followed by calculating the dose rate deposited by said beam.
10. The method for measuring the dose rate deposited by an energy particle beam according to claim 9 , wherein said calculation of the dose rate deposited by said particle beam is done using the following steps:
(i) calculating the normalized ratio
(
i
1
i
2
)
norm
of the output signals (i 1 and i 2 ) measured in the two considered ionization chambers;
(ii) calculating the charge collection efficiency factor f 1 in a first ionization chamber based on said gain factor relative to that same ionization chamber and based on the value of the normalized ratio
(
i
1
i
2
)
norm
of the output signals (i 1 and i 2 ) measured in said two considered ionization chambers; and
(iii) calculating the dose rate deposited by said beam based on the measurement of the current in said first ionization chamber and the charge collection efficiency factor relative to said first ionization chamber.
11. A use of measuring method according to claim 9 to perform a dose rate measurement of an energy particle beam whereof the current intensity is greater than 1 nA.