A proportional counter for the measurement of energy deposition distributions of charged particles

NUCLEAR

INSTRUMENTS

AND

METHODS

I2I

(1974) 483-489;

©

NORTH-HOLLAND

PUBLISHING

CO.

A P R O P O R T I O N A L C O U N T E R F O R T H E M E A S U R E M E N T OF E N E R G Y D E P O S I T I O N D I S T R I B U T I O N S OF C H A R G E D P A R T I C L E S G E O R G E S B E R T H O U D , S T A N L E Y R. B U L L * , J A C Q U E S G I R O U X , Y V O N N E H E R B A U T and L A W R E N C E A. S M I T H * Service de Protection et des Etudes d'Environnement, Centre d'Etudes Nucldaires de Grenoble, Grenoble, France Received 1 July 1974 A proportional counter was constructed to m e a s u r e energy deposition frequency distributions o f charged particle b e a m s in tissue-equivalent gas. Site sizes down to 0.37/~m can be simulated.

T h e operating characteristics a n d calibration o f the counter, including a c o m p a r i s o n with the Vavilov distribution, are described.

1. Introduction

18.35 cm and a wall thickness of 3.0 ram. The ends are closed with stainless steel plates. The 50/~m diameter anode wire is mounted axially along the center of the counter. Surrounding the anode is a grid shield which is a nickel tube 100/~m thick. A slot or window 4.0 cm long in the grid shield defines the sensitive volume of the counter. The width of the window may be varied by sliding a sleeve over a portion of the window. A simulated size is achieved by selecting an appropriate combination of counter gas pressure and window width. To provide parallel equipotential lines and to thus insure that the boundaries of the sensitive volume are straight lines, circular wires at each end of the counter were maintained at the potential corresponding to an infinite cylinder potential. It is also possible to adjust the anode and grid voltages independently. A continuous flow of tissue-equivalent gas is provided by a system described below through the gas inlet and outlet ports on the counter. Charged particles can enter the counter along an axis parallel to and 3.5 cm from the anode. There is provision (see fig. 1) for mounting a collimator for use with an external particle beam or for mounting an alpha source for calibration purposes. Four collimators were constructed consisting of two stainless steel discs 3.0 m m thick separated by 5.0 m m with holes of 1.0, 1.5, 2.0, and 2.7 m m diameter. The different collimator sizes provided the capability to operate at acceptable count rates without pulse pile-up. Stainless steel was chosen to obtain a suitable combination of collimator effectiveness and minimization of neutron production from the (p,n) reaction. Counter pressure was maintained by placing a 6/~m mylar cover over the collimator entrance. An auxiliary source position is located at the radial extreme of the sensitive volume. At the end of the particle axis, opposite the source opening, a silicon surface barrier detector (diode) was

The biological effect of a radiation depends on the spatial distribution of energy deposition as well as on the total dose. As a result, techniques to measure the microscopic energy deposition of ionizing particles in small tissue volumes have been developed. Rossi and Rosenzweig 1) suggested the use of a spherical proportional counter filled at low pressure with tissue-equivalent gas to simulate small tissue sites ( ~ 1 /~m). These counters, usually referred to as Rossi spheres, present the problem of a density change at the counter wall2), which is important for charged particle beams. Glass and Samsky 3) designed and constructed a counter which functions essentially as a wall-less counter for charged particle beams and was operated with protons of less than 2 MeV. The particle beam passes through a collimator and then directly into the counter gas. ttilbert et al. 4) have described a proportional counter with entrance and exit windows of 0.00025" aluminized raylar for the measurement of energy deposition spectra of charged particle beams. A counter similar in design to that of Glass and Samsky, but with a large diameter for use with higher energy protons, has been constructed at the Centre d'Etudes Nucl6aires de Grenoble. This paper describes this counter, the associated equipment, and the operating characteristics of the system. 2. Equipment 2 . 1 . DESCRIPTION OF THE COUNTER

A cross-sectional view of the tissue-equivalent proportional counter is shown in fig. 1. The counter is a copper cylinder 25.0 cm long with an inside diameter of * On leave f r o m Nuclear Engineering, University o f M i s s o u r i C o l u m b i a , C o l u m b i a , Missouri 65201, U.S.A.

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mounted. The active surface of the detector is 25 mm z and an applied bias of 700 V provided a depletion zone sufficient to stop protons of 16 MeV energy. The diode can be used both to measure energy spectra of the incident particle beam by creating a vacuum within the counter or to provide coincidence circuitry for the energy deposition measurements with the counter. The diode was calibrated by using calibrated 241Am and 2 ' ° p o alpha sources.

2.3. GAS FLOW SYSTEM A continuous flow of tissue-equivalent gas (64.9% CH4, 32.3% CO2, and 2.8% N2) was maintained in the counter by the system shown in fig. 3. When the counter is in operation, a gas flow of 1.5-2.0 l/h is maintained to remove the decomposed counting gas to insure stable operation. The gas flow is regulated by two microvalves and the gas flow exits the counter and flows into a vacuum reserve. It is possible to obtain stable operation with the system down to 10 torr.

2.2. ELECTRONICS

A block diagram of the electronics for operation of the counter is shown in fig. 2. The counter was connected to a low-noise, voltage-sensitive preamplifier with an electronic resolution of 0.65 keV. The signal passes to the amplifier, which incorporates pulse shaping and pole-zero cancellation, to the coincidence circuit, and finally to an 800-channel pulse height analyzer. The coincidence circuit was triggered by pulses from the surface barrier detector electronics, which were the result of protons that had passed through the sensitive volume in the counter and then into the diode. The diode signal was passed through a preamplifier, amplifier, a pulse height discriminator, and a time-delay/wave-shape unit before entering the coincidence circuit. For particle energy spectrum measurements, the signal from the diode preamplifier was diverted into the amplifier of the counter and the spectrum was analysed directly by the pulse height analyser.

3. Tests and calibration 3.1.

S E L E C T I O N OF O P E R A T I N G PARAMETERS

The grid and anode voltages for the counter were selected to avoid distortion of the field lines defining the sensitive volume and to insure operation in the proportional range. The relative resolution (fwhm/peak value) was determined as a function of grid voltage with the anode voltage as a parameter, using an 241Am alpha source. Figs. 4 and 5 show the curves obtained at pressures of 57.8 and 28.9 tort. For a fixed anode voltage, the resolution remains nearly constant for grid voltages between 12% to 30% of the anode voltage. A grid voltage of 25% of the anode voltage was selected as the operating voltage. For fixed voltage settings, the resolution is poorer at lower gas pressures. This is because of the increased statistical fluctuations in the avalanche production at low pressures.

MEASUREMENT OF ENERGY DEPOSITION DISTRIBUTIONS The proportionality range of the counter was determined by exposing the sensitive volume to radiations giving different energy deposition, the 241Am alpha source ( L E T ~ 1 0 0 k e V / / ~ m ) and a proton beam through a collimator (LEToo ~4.5 keV/pm). The pulse size from the counter was measured as a function of anode voltage (grid voltage fixed at 25% of the anode voltage) at 57.8 torr and 28.9 torr and is shown in fig. 6. Below 250 V, corresponding to the ionization region, the gas multiplication is 1 and at the higher pressure the pulse sizes are greater since more energy is deposited in the sensitive volume of the counter. The

485

proportional region, i.e. the region where the ratio of the pulse sizes for protons and alphas is cor~stant as a function of voltage, lies from 400 to above 800 V. In the proportional region, however, the gas gain is greater at lower pressure so the pulse sizes for the lower pressures are greater above 500 V. Based on the measurements described above, operating voltages of 800 V for the anode and 200 V for the grid were selected. At a pressure of 57.8 torr these voltages result in a relative resolution of approximately 15% with an 24tAm alpha source, which corresponds to the expected value.

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486

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3.2. RESOLUTION

The relative resolution as a function of anode voltage, always maintaining the grid voltage at 25% of the anode voltage, at pressures of 95, 40, 20, 15, and 10 torr was measured for the 241Am alpha source. With the adjustable window of the grid set at a length of 1.33 cm this simulates sensitive volume thicknesses of 1.76, 0.74, 0.37, 0.28 and 0.185 pro, respectively, for the above pressures. Fig. 7 shows the results of the measurement. It is possible to operate the counter at 10 torr with an anode voltage between 500 and 700 V, but at this pressure the decrease in resolution is substantial. Therefore, it is preferable to operate the counter at greater than 20 torr which corresponds to a simulated size of 0.37 pm.

3.3. CALIBRATION The energy deposition spectra as recorded in the pulse height analyzer were calibrated by the use of calibrated 24~Am (5.265 MeV) and Zl°Po (4.27 MeV) alpha sources. The surface barrier detector was first calibrated by evacuation of the counter. Then the counter was filled to a pressure such that the distance between the alpha source and the surface barrier detector simulated the distance between the alpha source and the face of the sensitive volume. The resulting alpha energy was measured with the surface barrier detector and the corresponding LET was taken from the tables of Barkas and BergerS). For example, for calibration at a pressure of 57.8 torr, the counter is filled to a pressure of 19.5 torr. The 5.265 MeV 24~Am

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MEASUREMENT

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DISTRIBUTIONS

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G. B E R T H O U D

alphas were found to have an energy of 4.49 MeV at the entrance to the sensitive volume when operating at 57.8 torr. Conversion from channel number in the analyzer to energy deposition was then made based on the standardized alpha sources. The W value is nearly constant for all types of radiation except heavy ions 6) and it is not affected by gas pressure. Changes in the amplifier gain, preamplifier gains, or other electronic settings were accounted for by determining the true gain ratios with a standard test signal. It was found that the presence of air in the counter caused significant shifts in the energy deposition spectra. If the counter had been opened for a period of time, it was necessary to evacuate the counter for 4 h and preferably up to 24 h to obtain counter stability.

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3.4. ENERGYDEPOSITIONSPECTRA Booz et al. 7) have shown the energy deposition spectrum for alphas can be well represented by the Vavilov 8) distribution for values of the Vavilov parameter, ~c, greater than 1.0. Measurements of the energy deposition spectra were made with the counter for 24tAm alphas of 5.265 MeV initial energy. Fig. 8 shows such a spectrum in a sensitive site of 1.07 pro. This corresponds to an operating pressure of 57.8 torr and an adjustable window length of 1.33 cm for which the average energy of the alphas was 4.49 MeV. The calculated Vavilov (n = 6.11) distribution is also shown in fig. 8 for comparison. The agreement between the Vavilov distribution and the experimental distribution serves as verification of proper counter operation. 4. Conclusions

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A proportional chamber for measuring energy deposition spectra in tissue-equivalent site thicknesses as small as 0.37 ~m has been built. The chamber will be used for operation in charged particle beams since there is no change of density at the boundary of the sensitive region.

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Fig. 6. Proportional counter pulse height.

The authors would like to acknowledge the assistance of J. Rouillon.

MEASUREMENT

OF E N E R G Y

DEPOSITION

DISTRIBUTIONS

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References 1) H.

Rossi a n d W. Rozenzweig, Radiology 64 (1955) 404. 2) A. M. Kellerer, Rad. Res. 47 (1971) 377. 3) W. A. Glass a n d D. N. Samsky, Rad. Res. 32 (1967) 138. 4) j. W. Hilbert, N. A. Bailey a n d R. G. Lane, Phys. Rev. 168 (1968) 290.

5) W. H. Barkas a n d M. J. Berger, Natl. Acad. Sci.-Natl. Res. Council Publ. 1133 (1964) 103. 0) j. A. Dennis, Rad. Effects 8 (1971) 87. 7) j. Booz, C. Giglio, A. Walker a n d G. Gaggero, 3rd Symp. o n Microdosimetry, Stresa (Italy) (1971) p. 833. 8) S. M. Seltzer a n d M. J. Berger, Natl. Acad. Sci.-Natl. Res. Council Publ. 1133 (1964) 187.