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Spatial patterns of ionization in charged-particle tracks
Nuclear Instruments and Methods in Physics Research B40/41 North-Holland, Amsterdam
SPATIAL
PATTERNS
L.H. TOBUREN,
OF IONIZATION
N.F. METTING
Pacific Northwest Laboratory,
Richland
1275
(1989) 1275-1278
IN CHARGED-PARTICLE
TRACKS
*
and L.A. BRABY
WA 99352, USA
The physical structure of charged-particle tracks plays an important role in determinin g the response of a stopping material to the absorption of energy following irradiation by charged particles and neutrons. For biological systems, and for microelectronic circuits, the response is sensitive to the energy deposited in submicron-size volume elements. Because of the stochastic nature of energy deposition in and near charged particle tracks, there may be wide variations in the actual quantities of energy deposited in critical volume elements. Since direct measurements of energy deposition in condensed material (solids or liquids) are not technologically feasible, the effects of charged-particle track structure are usually estimated from one of several model calculations; the most commonly used are homogeneous track structure models. Recent interest in the radiation biology of high Linear-Energy-Transfer (LET) radiation has spurred interest in testing these model descriptions of the structure of high-energy heavy-ion tracks. As one means of providing such tests, experiments were recently conducted at the GSI-Darmstadt, UNILAC accelerator, to measure energy deposition in small volumes as a function of the radial distance from the path of fast, heavy ions. These measurements, conducted in collaboration with research groups of Dr. G. Kraft at GSI and Prof. Schmidt-Bijcking of the University of Frankfurt, were made for simulated tissue volumes 0.5 and 1.0 pm in diameter, located from 0 to 10 pm from the path of Ge ions having energies from 13.0- to 17.2-MeV/amu. Excellent agreement was observed between model calculations and measured dose distributions for radial distances up to a few micrometers in simulated tissue. At greater distances the actual measured dose in irradiated volumes was much more than the calculated average value. These differences reflect the stochastic nature of energy deposition in which a large fraction of the volume elements receive no dose from a given particle traversal, but elements which do receive energy receive relatively large amounts. This may have important consequences for effects which occur with a nonlinear or threshold energy response.
1. Introduction The initial spatial pattern of ionization produced as charged particles slow down in matter plays a significant role in the subsequent evolution of radiation damage. In biological systems, the local density of energy deposition in volumes of subcellular dimensions may be the dominant factor in determining the relative biological effectiveness (RBE) of different radiation types. Traditionally, radiation has been characterized by its linear energy transfer (LET) when searching for correlation between the observed RBE and type of radiation. However, high-energy heavy-ions of widely varying energy and mass may have the same LET, but exhibit significantly different RBE. The varied RBE are attributed to the different local energy densities, as determined by the range and energies of secondary electrons produced in the slowing down of these particles. Several models [l-5] have been developed to describe the structure of charged particle tracks in order to assess the local energy density in and near their path in tissue. In high LET radiobiology homogeneous track
* Work Supported by the Office of Health and Environmental Research (OHER), US Department of Energy under Contract DE-ACO6-76RL0 1830.
0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
structure models have been the most widely used. These models predict that the energy density should decrease as approximately l/b*, where b is the radial distance from the path of the charged particle. However, recent measurements of the ionization distributions in small volumes as a function of the radial distance from the path of very fast heavy ions, have shown marked differences between the calculated average and the measured local ionization density [6]. The present work is an extension of these measurements to somewhat slower ions in which the projectile is not completely bare, as was the case in the earlier work with iron ions. Ions which possess bound electrons present an even greater challenge to the theorist, in that screening of the projectile charge by bound electrons may have significant influence on the spectra of secondary electrons produced along the particle track.
2. Experimental techniques A new apparatus was designed and built to study the radial distribution of ionization produced by intermediate velocity (l-50 MeV/amu) ions. A schematic of the apparatus is shown in fig. 1. The target chamber was a cylinder 72 cm in diameter by 72 cm long. It incorporated an entrance collimation system for precisely locatXI. DETECTORS/CALIBRATION
1276
LH.
Toburen ei al. / Structure of charged-particle
tracks
Charged Particle Avalanche Detector (X-Y Positiin-Sensitive)
72 cm
*...............
B. _. __. . . . . . . . I,: I,
Exit 1 Window
i
4’ Fig. 1. Schematic drawing of the apparatus used to study the radial distribution of ionization produced by intermediate velocity ions.
ing the incident beam; a grid walled proportional counter and mounting system capable of two-dimensional motion for measurement of ionization in or near the charged-particle track; and a position-sensitive avalanche detector for ion beam detection. The proportional counter was defined by a cylinder of thin (- 0.02 mm diameter) wires, arranged on the circumference of a OS-cm-diameter circle. The sensitive volume of the proportional counter was a cylinder 0.5 cm in diameter by 0.5 cm long. The position sensitive avalanche detector, designed and constructed in Dr. H. Schmidt-Bijcking’s laboratory at the University of Frankfurt, West Germany, had a position resolution of a few tenths of a millimeter. The apparatus was installed on the biophysics beam port of the UNILAC accelerator at Darmstadt. Since the system was designed to detect individual ions the demand for beam intensity was very low. This made it feasible for our measurements to be performed as a parasite user of the accelerator beam and enabled data accumulation for reasonably long periods of time. Two runs, each approximately one week long, were conducted during the summer of 1987, one with a 5.9MeV/amu uranium beam, the second with 13.0- to 17.2-MeV/amu germanium beams. The results of the germanium measurements will be discussed here. For measurements of energy deposition profiles near the charged-particle track the entire chamber was filled with propane gas to a pressure that would enable the grid wall detector to operate as a proportional counter. Propane gas was used for its “tissue equivalent” properties and its reliable operation as a counter gas. The simulated
tissue
volume
represented
by the gas-filled
detector was then determined from a measure of the gas pressure and the dimensions of the proportional counter. For the measurements reported here, two simulated site diameters were used: 0.5 urn and 1.0 urn. The amount of energy deposited in the grid walled proportional counter, either by direct interactions of the incident ion or those of its secondary electrons was determined by pulse height analysis of the proportional counter output. The pulse height was calibrated to energy deposited by using alpha particles of known stopping power from an 24?4m source. An example of the pulse height distributions recorded for different
Fig. 2. Pulse height distributions observed for different positions of the proportional counter relative to the path of the germanium ion.
L. H. Toburen et al. / Structure of charged-particle
radial distances of the proportional counter from the ion beam is shown in fig. 2. Scattered ion or electronic noise pulses were rejected in these spectra by requiring coincidence between the detected ion and a proportional counter pulse. As the proportional counter was moved to greater distances from the beam, the pulse height decreased reflecting the smaller number of energy deposition events that occurred within the counter volume. In order to determine the mean energy deposited at each position of the detector, it was necessary to extrapolate these distributions to zero energy. This was accomplished by a simple horizontal line fitted to the low energy end of the spectra. This is consistent with the expected shape of the proportional counter pulse height spectra for energy deposition resulting in production of 50 to 100 ion pairs in the counter volume; approximately 80 ion pairs would be expected at the threshold setting. In addition, wide variations in the form of this extrapolation do not markedly affect the mean value derived from the distributions.
3. Results Measurement of the radial distribution of absorbed energy, obtained from the mean of distributions such as those shown in fig. 2, are shown in fig. 3 for 13.8MeV/amu germanium ions. Data are shown for two different simulated tissue volumes. The flattening of the measured distributions at the larger radial distances is representative of sites that receive energy from passage of a single electron, or photon, i.e., the minimum amount
Fig. 3. Radial distributions of the mean energy deposited in 0.5- and 1.0~pm-diameter tissue equivalent sites by 13.8 MeV/amu Ge ions. The theoretical calculations of the average energy deposited are from the work of Zhang et al. [l], Chatterjee and Schaefer [2] and Varma et al. [3].
tracks
1217
Fig. 4. Radial distributions of the mean energy deposited in a 0.5-pm-diameter tissue-equivalent site by 13.8- and 17.2MeV/amu Ge ions. Also shown are previously published distributions of the mean energy deposited in a 1.3~pm-diameter site by 600-MeV/amu Fe ions [6].
of energy a site of this size will receive if it is hit. Note that the plotted value, _??(or D(b)) is the energy absorbed per mass of the simulated site. Thus comparable energy absorbed in a larger site would result in a smaller 2 because of the larger mass of material in that volume. If the measured values are converted to energy loss per unit length in traversing the proportional counter (not shown), the two site sizes give the same values, as expected. The model calculations shown in fig. 3 all represent the average amount of energy deposition expected at a specified distance from a charged-particle track. The difference between the measured and calculated values at large b represents the decreasing probability that any particular ion will deposit energy in a given small volume. The ratio of energy deposition events per particle is also determined in these measurements. When this ratio is used to calculate the average energy deposited per ion, the results are in excellent agreement with the calculated averages. Consider, for example, that at 3.2 pm from the particle path approximately l/3 of the ions deposit energy in a l-pmdiameter site. Data for germanium beams of two different energies are shown in fig. 4, along with previous measurements [6] for much higher energy iron ions. Note that the same trend is observed for the radial distribution of energy deposited by each ion although the magnitude of the energy deposited at the greater distances from the ion path decreases with increasing ion velocity. It should also be noted that the iron data represent energy deposited in a 1.3~pm-diameter site; thus the plotted quantity, 2, is somewhat reduced relative to the 0.5~pm Ge data, owing to the energy-per-unit-mass definition XI. DETECTORS/CALIBRATION
1278
L. H. Toburen et al. / Siructure of charged-particle
of 2. If one compares the actual energy deposited for these three ions, rather than comparing the respective 2, the ratio of values obtained in the plateau region approximately reflects the stopping power of the maximum-energy electrons ejected by each energy ion. Although these data represent a preliminaty analysis of only part of the data obtained from the UNILAC measurements, there is clear evidence of the difference between the quantity calculated by commonly used models and the energy deposition experienced by cells. Critical volume elements are not exposed to the average dose, but are exposed and respond to energy deposited in much larger localized doses. The authors thank Dr. G. Kraft, M. Scholz, and F. Rraske of GSI-Darmstadt, and Professor H. SchmidtB&&ing, R. Dome,, and R. Seip of the University of Frankfurt for their valuable assistance during these measurements. Two of the authors (L.H.T. and N.F.M)
tracks
thank the Gesellschaft fur Schwerionenforschung for its hospitality and support while these measurements were conducted.
References [l] C. Zhang, D.E. Dunn and R. Katz, Radiat. Protect. Dosim. 13 (1985) 215. [2] A. Chattejee and H.J. Schaefer, Radiat. Environ. Biophys. 13 (1976) 215. [3] M.N. Varma, J.W. Baum and P. Kliauga, Proc. 6th Symp. on Microdosimetry, eds. J. Booz and H.G. Ebert (Harwood, Brussels, 1978) pp. 227-237. [4] M.P.R. Wahgorski, R.N. Hand R. Katz, Nucl. Tracks Radiat. Meas. 11 (1986) 309. [5] W.E. Wilson, L.H. Toburen and H.G. Paretzke, Proc. 6th Symp. on Microdosimetry, eds. J. Booz and H.G. Ebert (Hanwod, Brussels, 1978) pp. 239-250. [6] N.F. Metting, Nucl. Instr. and Meth. B24/25 (1987) 1050.
SPATIAL
PATTERNS
L.H. TOBUREN,
OF IONIZATION
N.F. METTING
Pacific Northwest Laboratory,
Richland
1275
(1989) 1275-1278
IN CHARGED-PARTICLE
TRACKS
*
and L.A. BRABY
WA 99352, USA
The physical structure of charged-particle tracks plays an important role in determinin g the response of a stopping material to the absorption of energy following irradiation by charged particles and neutrons. For biological systems, and for microelectronic circuits, the response is sensitive to the energy deposited in submicron-size volume elements. Because of the stochastic nature of energy deposition in and near charged particle tracks, there may be wide variations in the actual quantities of energy deposited in critical volume elements. Since direct measurements of energy deposition in condensed material (solids or liquids) are not technologically feasible, the effects of charged-particle track structure are usually estimated from one of several model calculations; the most commonly used are homogeneous track structure models. Recent interest in the radiation biology of high Linear-Energy-Transfer (LET) radiation has spurred interest in testing these model descriptions of the structure of high-energy heavy-ion tracks. As one means of providing such tests, experiments were recently conducted at the GSI-Darmstadt, UNILAC accelerator, to measure energy deposition in small volumes as a function of the radial distance from the path of fast, heavy ions. These measurements, conducted in collaboration with research groups of Dr. G. Kraft at GSI and Prof. Schmidt-Bijcking of the University of Frankfurt, were made for simulated tissue volumes 0.5 and 1.0 pm in diameter, located from 0 to 10 pm from the path of Ge ions having energies from 13.0- to 17.2-MeV/amu. Excellent agreement was observed between model calculations and measured dose distributions for radial distances up to a few micrometers in simulated tissue. At greater distances the actual measured dose in irradiated volumes was much more than the calculated average value. These differences reflect the stochastic nature of energy deposition in which a large fraction of the volume elements receive no dose from a given particle traversal, but elements which do receive energy receive relatively large amounts. This may have important consequences for effects which occur with a nonlinear or threshold energy response.
1. Introduction The initial spatial pattern of ionization produced as charged particles slow down in matter plays a significant role in the subsequent evolution of radiation damage. In biological systems, the local density of energy deposition in volumes of subcellular dimensions may be the dominant factor in determining the relative biological effectiveness (RBE) of different radiation types. Traditionally, radiation has been characterized by its linear energy transfer (LET) when searching for correlation between the observed RBE and type of radiation. However, high-energy heavy-ions of widely varying energy and mass may have the same LET, but exhibit significantly different RBE. The varied RBE are attributed to the different local energy densities, as determined by the range and energies of secondary electrons produced in the slowing down of these particles. Several models [l-5] have been developed to describe the structure of charged particle tracks in order to assess the local energy density in and near their path in tissue. In high LET radiobiology homogeneous track
* Work Supported by the Office of Health and Environmental Research (OHER), US Department of Energy under Contract DE-ACO6-76RL0 1830.
0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
structure models have been the most widely used. These models predict that the energy density should decrease as approximately l/b*, where b is the radial distance from the path of the charged particle. However, recent measurements of the ionization distributions in small volumes as a function of the radial distance from the path of very fast heavy ions, have shown marked differences between the calculated average and the measured local ionization density [6]. The present work is an extension of these measurements to somewhat slower ions in which the projectile is not completely bare, as was the case in the earlier work with iron ions. Ions which possess bound electrons present an even greater challenge to the theorist, in that screening of the projectile charge by bound electrons may have significant influence on the spectra of secondary electrons produced along the particle track.
2. Experimental techniques A new apparatus was designed and built to study the radial distribution of ionization produced by intermediate velocity (l-50 MeV/amu) ions. A schematic of the apparatus is shown in fig. 1. The target chamber was a cylinder 72 cm in diameter by 72 cm long. It incorporated an entrance collimation system for precisely locatXI. DETECTORS/CALIBRATION
1276
LH.
Toburen ei al. / Structure of charged-particle
tracks
Charged Particle Avalanche Detector (X-Y Positiin-Sensitive)
72 cm
*...............
B. _. __. . . . . . . . I,: I,
Exit 1 Window
i
4’ Fig. 1. Schematic drawing of the apparatus used to study the radial distribution of ionization produced by intermediate velocity ions.
ing the incident beam; a grid walled proportional counter and mounting system capable of two-dimensional motion for measurement of ionization in or near the charged-particle track; and a position-sensitive avalanche detector for ion beam detection. The proportional counter was defined by a cylinder of thin (- 0.02 mm diameter) wires, arranged on the circumference of a OS-cm-diameter circle. The sensitive volume of the proportional counter was a cylinder 0.5 cm in diameter by 0.5 cm long. The position sensitive avalanche detector, designed and constructed in Dr. H. Schmidt-Bijcking’s laboratory at the University of Frankfurt, West Germany, had a position resolution of a few tenths of a millimeter. The apparatus was installed on the biophysics beam port of the UNILAC accelerator at Darmstadt. Since the system was designed to detect individual ions the demand for beam intensity was very low. This made it feasible for our measurements to be performed as a parasite user of the accelerator beam and enabled data accumulation for reasonably long periods of time. Two runs, each approximately one week long, were conducted during the summer of 1987, one with a 5.9MeV/amu uranium beam, the second with 13.0- to 17.2-MeV/amu germanium beams. The results of the germanium measurements will be discussed here. For measurements of energy deposition profiles near the charged-particle track the entire chamber was filled with propane gas to a pressure that would enable the grid wall detector to operate as a proportional counter. Propane gas was used for its “tissue equivalent” properties and its reliable operation as a counter gas. The simulated
tissue
volume
represented
by the gas-filled
detector was then determined from a measure of the gas pressure and the dimensions of the proportional counter. For the measurements reported here, two simulated site diameters were used: 0.5 urn and 1.0 urn. The amount of energy deposited in the grid walled proportional counter, either by direct interactions of the incident ion or those of its secondary electrons was determined by pulse height analysis of the proportional counter output. The pulse height was calibrated to energy deposited by using alpha particles of known stopping power from an 24?4m source. An example of the pulse height distributions recorded for different
Fig. 2. Pulse height distributions observed for different positions of the proportional counter relative to the path of the germanium ion.
L. H. Toburen et al. / Structure of charged-particle
radial distances of the proportional counter from the ion beam is shown in fig. 2. Scattered ion or electronic noise pulses were rejected in these spectra by requiring coincidence between the detected ion and a proportional counter pulse. As the proportional counter was moved to greater distances from the beam, the pulse height decreased reflecting the smaller number of energy deposition events that occurred within the counter volume. In order to determine the mean energy deposited at each position of the detector, it was necessary to extrapolate these distributions to zero energy. This was accomplished by a simple horizontal line fitted to the low energy end of the spectra. This is consistent with the expected shape of the proportional counter pulse height spectra for energy deposition resulting in production of 50 to 100 ion pairs in the counter volume; approximately 80 ion pairs would be expected at the threshold setting. In addition, wide variations in the form of this extrapolation do not markedly affect the mean value derived from the distributions.
3. Results Measurement of the radial distribution of absorbed energy, obtained from the mean of distributions such as those shown in fig. 2, are shown in fig. 3 for 13.8MeV/amu germanium ions. Data are shown for two different simulated tissue volumes. The flattening of the measured distributions at the larger radial distances is representative of sites that receive energy from passage of a single electron, or photon, i.e., the minimum amount
Fig. 3. Radial distributions of the mean energy deposited in 0.5- and 1.0~pm-diameter tissue equivalent sites by 13.8 MeV/amu Ge ions. The theoretical calculations of the average energy deposited are from the work of Zhang et al. [l], Chatterjee and Schaefer [2] and Varma et al. [3].
tracks
1217
Fig. 4. Radial distributions of the mean energy deposited in a 0.5-pm-diameter tissue-equivalent site by 13.8- and 17.2MeV/amu Ge ions. Also shown are previously published distributions of the mean energy deposited in a 1.3~pm-diameter site by 600-MeV/amu Fe ions [6].
of energy a site of this size will receive if it is hit. Note that the plotted value, _??(or D(b)) is the energy absorbed per mass of the simulated site. Thus comparable energy absorbed in a larger site would result in a smaller 2 because of the larger mass of material in that volume. If the measured values are converted to energy loss per unit length in traversing the proportional counter (not shown), the two site sizes give the same values, as expected. The model calculations shown in fig. 3 all represent the average amount of energy deposition expected at a specified distance from a charged-particle track. The difference between the measured and calculated values at large b represents the decreasing probability that any particular ion will deposit energy in a given small volume. The ratio of energy deposition events per particle is also determined in these measurements. When this ratio is used to calculate the average energy deposited per ion, the results are in excellent agreement with the calculated averages. Consider, for example, that at 3.2 pm from the particle path approximately l/3 of the ions deposit energy in a l-pmdiameter site. Data for germanium beams of two different energies are shown in fig. 4, along with previous measurements [6] for much higher energy iron ions. Note that the same trend is observed for the radial distribution of energy deposited by each ion although the magnitude of the energy deposited at the greater distances from the ion path decreases with increasing ion velocity. It should also be noted that the iron data represent energy deposited in a 1.3~pm-diameter site; thus the plotted quantity, 2, is somewhat reduced relative to the 0.5~pm Ge data, owing to the energy-per-unit-mass definition XI. DETECTORS/CALIBRATION
1278
L. H. Toburen et al. / Siructure of charged-particle
of 2. If one compares the actual energy deposited for these three ions, rather than comparing the respective 2, the ratio of values obtained in the plateau region approximately reflects the stopping power of the maximum-energy electrons ejected by each energy ion. Although these data represent a preliminaty analysis of only part of the data obtained from the UNILAC measurements, there is clear evidence of the difference between the quantity calculated by commonly used models and the energy deposition experienced by cells. Critical volume elements are not exposed to the average dose, but are exposed and respond to energy deposited in much larger localized doses. The authors thank Dr. G. Kraft, M. Scholz, and F. Rraske of GSI-Darmstadt, and Professor H. SchmidtB&&ing, R. Dome,, and R. Seip of the University of Frankfurt for their valuable assistance during these measurements. Two of the authors (L.H.T. and N.F.M)
tracks
thank the Gesellschaft fur Schwerionenforschung for its hospitality and support while these measurements were conducted.
References [l] C. Zhang, D.E. Dunn and R. Katz, Radiat. Protect. Dosim. 13 (1985) 215. [2] A. Chattejee and H.J. Schaefer, Radiat. Environ. Biophys. 13 (1976) 215. [3] M.N. Varma, J.W. Baum and P. Kliauga, Proc. 6th Symp. on Microdosimetry, eds. J. Booz and H.G. Ebert (Harwood, Brussels, 1978) pp. 227-237. [4] M.P.R. Wahgorski, R.N. Hand R. Katz, Nucl. Tracks Radiat. Meas. 11 (1986) 309. [5] W.E. Wilson, L.H. Toburen and H.G. Paretzke, Proc. 6th Symp. on Microdosimetry, eds. J. Booz and H.G. Ebert (Hanwod, Brussels, 1978) pp. 239-250. [6] N.F. Metting, Nucl. Instr. and Meth. B24/25 (1987) 1050.