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Experimental investigation of bubble occurrence and locality distribution of bubble detectors bombarded with high-energy helium ions
Radiation Measurements 50 (2013) 31e37
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Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Experimental investigation of bubble occurrence and locality distribution of bubble detectors bombarded with high-energy helium ions S.-L. Guo a, *, T. Doke b, D.-H. Zhang c, L. Li a, B.-L. Chen a, J. Kikuchi b, N. Hasebe b, K. Terasawa b, K. Hara b, T. Fuse b, N. Yasuda d, T. Murakami d a
China Institute of Atomic Energy, P.O. Box 275 (96), Beijing 102413, PR China Advanced Institute for Science and Engineering, Waseda University, Tokyo, Japan Institute of Modern Physics, Shanxi Normal University, Linfen, Shanxi, PR China d National Institute of Radiological Science, Chiba, Japan b c
h i g h l i g h t s < Four types of 23-cm long bubble detectors (BD) were irradiated to 150 MeV/amu He ions. < Direct bubble, recoil bubble, scattering bubble are formed, recognized and classified. < Detection efficiency of BD for He ions is 103e104 (bubble cm3)/(He cm2). < It is at the same level of efficiency as detecting fast neutrons. < Space neutron detection by BD must take the effect of cosmic ray He ions into account.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 November 2011 Received in revised form 10 October 2012 Accepted 24 October 2012
Large-sized bubble detectors with microscopic droplets of superheated liquids of dichlorodifluoromethane (Freon-12), dichlorotetrafluoroethane (Freon-114), tetrafluoroethane (Freon-134a), and mixture of Freon-12 and Freon-114, respectively, were irradiated with 150 MeV/amu helium ions at the HIMAC accelerator in NIRS, Chiba, Japan. Distributions of bubbles produced by the helium ions have been studied in each type of the detectors. The origin of the bubbles has been investigated. The detection efficiency of each type of the bubble detectors for helium ions with respect to the energy of the ions has been obtained. The phenomenon of bubble occurrence and its possible applications to the determination of He intensity from accelerators, research of track formation mechanism, energy loss straggling and neutron detection in the space and at higher altitude are discussed. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Bubble detectors High-energy helium ions Track formation mechanism Detection efficiency of bubble detector for helium ions Cosmic rays Space neutron detection
1. Introduction Bubble detector (BD) was first reported by Ing in Canada in 1984 (Ing and Birnboim, 1984). It is composed of superheated liquid droplets dispersed uniformly in elastic solid polymer. The size of the liquid droplets is about 10e20 mm. The ratio of the superheated liquid in the whole volume of the detector is from 104 to 20% by volume. Usually there are about 107 to 108 droplets in 1 cm3 of bubble detector.
* Corresponding author. Tel.: þ86 10 60386398; fax: þ86 10 60380995. E-mail address: [email protected] (S.-L. Guo). 1350-4487/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radmeas.2012.10.008
When a charged particle such as a heavy ion passes through a liquid droplet in the detector substances, the particle deposits energy in the droplet and causes evaporation of the liquid, forming a permanent bubble at the original position of the droplet in the detector. One or more bubbles constitute a track of the particle. Bubble detector has been widely used in neutron dosimetry and neutron spectrometry (Ing and Birnboim, 1984; Ing et al., 1997; Ing, 2001; Tommasino, 1993; Vanhavere et al., 1998, 2010; d’Errico, 2006; Benton et al., 2001; Lewis et al., 2001). In neutron detection, the bubbles are produced by charged recoil nuclei or reaction products induced by neutrons in the superheated liquid droplets. The recoil nucleus or reaction product deposits energy in the superheated liquid droplet, resulting in formation of a bubble in the
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detector. Usually, one neutron at most can produce one bubble in the detector. Therefore, the bubbles created by neutrons are randomly distributed in the whole detector (Ing and Birnboim, 1984). Since 1997, bubble detector has been studied to record tracks of high-energy heavy ions (Guo et al., 2001, 2003, 2005; Guo, 2006). It shows that: (1) The track of a high-energy heavy ion is a straight line consisting of a string of tiny bubbles; (2) Threshold property is the governing factor for formation of the tracks. The essence of the threshold is critical rate of energy loss (dE/dX)c rather than that of restricted energy loss (REL) (Guo et al., 2002a). The threshold value varies from (0.05 0.01) 103 to (6.04 0.80) 103 MeV g1 cm2 at (25 1) C for different types of self-made bubble detectors (Guo et al., 2003). Only when the rate of energy loss dE/dX of a heavy ion in the bubble detector is higher than the critical rate of energy loss (dE/dX)c can the heavy ion create bubbles by direct deposition of its energy in the bubble detector, and vice versa. If the path length on which dE/dX (dE/dX)c is larger than the distance between two liquid droplets, the heavy ion will create two or more than two bubbles, forming a linear array of bubbles along its trajectory. Experiments have shown that bubble detector, as a whole, can record linear tracks of high-energy heavy ions with Z 1. In other words, the heavy ions from the entire periodic table of element can be detected with bubble detectors (Guo et al., 2002b). A new question has been raised that if the rate of energy loss dE/ dX of heavy ions is just at the level of the threshold (dE/dX)c, or it is much lower than the threshold, what happens in the bubble detector after irradiation with the heavy ions. In a previous study with protons (Z ¼ 1), whose energy loss rate is much lower than the thresholds of the detectors, “scattering bubbles” and “recoil bubbles” have been observed in the bubble detectors (Guo et al., 2009). What happens in bubble detectors bombarded by other heavy ions with Z > 1 should be clarified through experiments and much more study needs to be done for this phenomenon. This paper presents the occurrence, origin, categories and distributions of bubbles in bubble detectors irradiated with highenergy helium ions, and the detection efficiencies of the bubble detectors for helium ions as well as the possible applications of this phenomenon in various fields.
temperature of irradiation room, the sensitization was made at least two and half hours before irradiation with heavy ions. The four types of bubble detectors were irradiated at the HIMAC accelerator in the National Institute of Radiological Science (NIRS), Chiba, Japan. A 150 MeV/amu He ion beam was accelerated and passed through beam window and an aperture of 25 cm in diameter. Behind the aperture there was a plastic scintillation detector in size of 10 10 cm2, which was used to count the number of He ions passed through the scintillation detector. The bubble detector was placed on the beam behind the scintillation detector and parallel to the He beam. The bottom of the glass tube was faced to the beam direction. The arrangement of irradiation is shown in Fig. 1. Before irradiation, the He beam was highly defocused so that the beam intensity in the whole area of the aperture was very uniform. The intensity of He beam from the accelerator was then reduced to very low so that one could stop irradiation at any expected number of He ions recorded by the scintillation detector. The room temperature was measured to be (25 1) C. The gap between sensitization and irradiation of the detector was 3.0, 2.5, 3.0 and 4.33 h for T-12, T-34, T-24 and T-14 type of detector, respectively. During irradiation, the recorded He ions by the scintillation detector was (3.23 0.32) 106, (3.27 0.33) 106, 7 7 (1.60 0.16) 10 and (1.60 0.16) 10 for T-12, T-34, T-24 and T14 bubble detector, respectively.
3. Bubble distribution and categories After irradiation, small bubbles immediately occurred in the bubble detectors, which can be seen by naked eyes without any instruments. The size of the bubbles grew slowly with time until several hundred mm in diameter. Fig. 2 shows the photographs of the four types of bubble detectors after irradiation by He ions. The photographs were taken directly by a camera without any enlargement. The direction of the He ions impinging on the bubble detectors was from the left to the right as shown by the arrow. The incident beam stopped in the bubble detectors at the position as shown by R in Fig. 2. The ranges of the He ions in bubble detectors are consistent with that calculated by the computer code SRIM 2000 (Ziegler and Biersack, 2000). The distributions of bubbles in the four types of bubble detectors can be seen clearly in the photographs. The following features of bubble distributions in the detectors can be recognized from Fig. 2:
2. Bubble detector and irradiation Four types of large-sized bubble detectors with different thresholds were prepared: T-12, T-34, T-24 and T-14. The elastic solid media in all these detectors are mainly polyacrylamide. The superheated liquids in the microscopic droplets in the detectors are dichlorodifluoromethane (Freon-12) in T-12, tetrafluoroethane (Freon-134a) in T-34, dichlorotetrafluoroethane (Freon-114) in T-14 and Freon-12 and Freon-114 mixture (1:1 by volume) in T-24. The sensitive volume of each detector was 23 cm in length and 1.8 cm in diameter. The percentage of Freon in the total volume of the detectors was 2%. The bubble detectors were prepared in glass tubes with screw caps. About one gram of overlay was added in the glass tube near the screw cap in each detector. The composition of overlay was Freon-12 in T-12, T-24 and T-14, and Freon-134a in T34. The bubble detectors with overlay are insensitive to heavy ions. By pouring out the overlay the bubble detectors were sensitized to heavy ions. In order to have the temperature of the whole volume of the detectors reach equilibrium with the environment
Fig. 1. Photograph of the arrangement of irradiation of bubble detector with He ions at HIMAC accelerator.
S.-L. Guo et al. / Radiation Measurements 50 (2013) 31e37
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products is higher than the threshold of the detector, bubbles (recoil bubbles for short) occurred on the beam before stopping. The bubbles randomly distributed on the left-hand side of the abrupt change are mainly recoil bubbles mixed with scattering bubbles. 3.3. Direct bubbles
Fig. 2. Photograph of four types of bubble detectors after irradiation to high-energy He ions. The outer diameter of the detectors was 2 cm. The incident beam direction was from the left to the right in the figure. See detailed explanations in the text.
(1)All the bubbles are randomly distributed in the detectors. No linear arrays of bubbles can be found along the He trajectories. (2)An abrupt change of bubble density occurred in each of the bubble detectors. The density of bubbles was higher on the left side of the abrupt change than that on the right. (3) Theoretical calculation by computer code SRIM 2000 shows that the distance from the glass bottom to the points of abrupt change of bubble densities are equal to the range of the He ions in the bubble detectors. (4) There is a much denser region of bubbles near the end of the range in bubble detector T-12 and T-34, a less dense region in T24, and no dense region in T-14. From the arrangement of irradiation and the above features of bubble distributions one can realize that the random bubbles can be classified into four categories. 3.1. Scattering bubbles After a large area of beam window and aperture, He beam passed through the large scintillation detector (10 10 cm2) and its wrapping materials as well as in total more than 6.82 m air before reaching the bubble detector, some He ions were scattered by the nuclei in the scintillation detector and the other materials and deviated from the original beam direction. Some scattered He ions entered the bubble detector from the side surface of the detector and created bubbles in the detector. This kind of bubbles was similar to that observed before in proton irradiation (Guo et al., 2009) and called “scattering bubbles” for short, which spread quite uniformly along the whole length of the detector. The bubbles distributed randomly on the right-hand side of the abrupt change in the detectors are basically scattering bubbles.
In the much more dense region of bubbles (see Fig. 2aec) there should be some extra bubbles besides the recoil bubbles and scattering bubbles. The extra bubbles might be produced by direct deposition of energy of the He ions in the superheated liquid droplets. In previous study, it has been shown that the maximum rate of energy loss (Bragg peak) of He ions in bubble detectors T-12 and T-34 is nearly equal to the level of the thresholds of the detectors (Guo et al., 2003). In this case, the deposited energy by a He ion in a droplet might be enough to trigger a bubble, but not enough to trigger more than one bubble. This conjecture is supported by the bubble detectors T-24 and T-14. In T-24, the Bragg peak of He ions is obviously lower than the threshold (Guo et al., 2003), but the gap between the Bragg peak and the threshold is not so large. By fluctuation of energy deposition, a few of energy deposition events might incidentally reach the height of the threshold, resulting in formation of bubbles. The few but obvious bubbles near the end of the range in the detector T-24 (see Fig. 2c) are in agreement with the conjecture. The gap between the Bragg peak and the threshold of T-14 is very large (Guo et al., 2003). It is not possible for the fluctuation of energy deposition to jump over the big gap. Therefore, one cannot see any extra bubbles in the detector of T-14 in the region near the end of the He range shown in Fig. 2d. 3.4. Spurious bubbles In preparation of bubble detectors, some spurious bubbles might be formed within the detectors or on the inner wall of the glass tubes by poor management. Efforts had been made in preparation of the detectors to reduce the number of spurious bubbles. Most of the bubble detectors used in this work was nearly free from spurious bubbles. The spurious bubbles were distinguishable from that formed by charged particles by their sizes and positions in the detectors. In case the spurious bubbles were wrongly counted in by carelessness, the subtraction (see below) of the number of bubbles on the right-hand side from that on the left-hand side of the abrupt change would reduce the influence of spurious bubbles to a minimum. 4. Bubble density distributions The distributions of bubbles along the axis of T-12, T-34, T-24 and T-14 types of bubble detectors are shown by histogram in Fig. 3. The abscissa X in Fig. 3 is the distance along the axis of the glass tube from the outer surface of the bottom of the tube to the measured point. The thickness of the bottom of the glass tube is 1 mm. The ordinate N is the number of bubbles counted in the interval of X in length 10 mm. The width of counting is the whole circle of the glass tube for T-12, T-34 and T-24, but the width is 23.5 mm for T-24. The distributions of bubbles are similar for all the detectors (with some difference in T-14):
3.2. Recoil bubbles The beam ions of He might collide with the atoms in the superheated liquid droplets and produce recoil nuclei or reaction products. If the energy loss rate of the recoil nuclei or reaction
(1) All the distributions have two terraces. On the first terrace (X ¼ 10 to about 110 mm) the bubble densities are high. On the second terrace (X z 110 to about 200 mm) the bubble densities are low.
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Fig. 3. Distributions of the bubbles along the axis of bubble detectors: (a) T-12, (b) T-34, (c) T-24 and (d) T-14, respectively.
(2) Between the two terraces there is a steep drop of the bubble density. The point of the steep drop corresponds to the end of the range of the He beam ions in the bubble detector. (3) There is a peak of bubble density followed by the steep drop for bubble detector T-12, T-34 and T-24, but no peak for T-14. (4) The bubble density on the first terrace increases as x increases until the steep drop of the bubble density. The above histograms of bubble distributions provide a solution to obtain the number (or density) of bubbles of different categories in the bubble detectors:
bubbles and direct bubbles can be deduced from the histogram in Fig. 3.
5. Detection efficiency of bubble detectors for He ions by recoil bubbles For convenience, one can define the detection efficiency of bubble detectors ε for heavy ions as follows:
ε ¼ (1) The bubbles on the lower terrace (L) correspond to scattering bubbles (S) plus spurious bubbles (Spu) (if they were wrongly counted in): L ¼ S þ Spu. The bubbles on the higher terrace (H) (except the peak) correspond to the sum of recoil bubbles (R), scattering bubbles (S) as well as spurious bubbles (Spu): H ¼ R þ S þ Spu. The bubbles on the peak (P) correspond to the sum of direct bubbles (D), recoil bubbles (R), scattering bubbles (S) and spurious bubbles (Spu): P ¼ D þ R þ S þ Spu. (2) The density of recoil bubbles (R) in each interval can be obtained by subtraction of average density on the lower terrace (L) from the density in the interval on the higher terrace (H): R ¼ H L. The density of direct bubbles (D) can be obtained by subtraction of the extrapolation value (Hex) of preceding densities on the higher terrace from the peak densities (P): D ¼ P Hex. Therefore, the numbers (or densities) of recoil
r F
(1)
where r is the density of bubbles (number of bubbles in unit volume of bubble detector) formed through a specified mechanism (recoil bubbles in this work); F is the fluence of incident heavy ions (He). ε can be further represented by equation:
ε ¼
N S lwh NHe
(2)
where N is the number of bubbles counted for the specified bubbles; l, w, and h are length, width and depth of the volume containing the counted bubbles in bubble detector; NHe is the number of He ions counted by the scintillation detector; S is the area of the scintillation detector (10 10 cm2)
S.-L. Guo et al. / Radiation Measurements 50 (2013) 31e37
Fig. 4 shows the results of detection efficiencies of bubble detectors T-12, T-34, T-24 and T-14. The detection efficiencies ε in Fig. 4 are calculated from Eq. (2), in which N being in each interval of 10 mm in length is calculated by deduction of the average number of scattering bubbles and spurious bubbles on the second (right) terrace from the number of bubbles counted in each interval on the first (left) terrace (see in Fig. 3). The energy Ea of He ions at the middle point of each interval was calculated by the computer code SRIM 2000. Before calculation of the detection efficiencies, the effect of bubble expansion had been corrected on the position of each bubble along X axis. The total shift of the point of steep drop was 9.0 mm towards the right direction along X axis for bubble detector T-12. No correction had been made for detectors T-34, T-24 and T-14, because the shifts of bubbles at the moment of bubble counting in these detectors were not very large (the photograph of T-34 in Fig. 2b was taken later than the bubble counting). No corrections were made for the reduction of the fluence F of He ions within the range R caused by the interactions of He ions with atoms of the detectors. The uncertainty for each result of measurement of the detection efficiency ε was composed of mainly statistical uncertainty of bubble counting and the uncertainties of the measurements of length l (0.5 mm), width w (1 mm) and visible depth h (1 mm) of each counting volume of the detector. The visible depth h was taken to be 3 mm for detector type T-12 and T-34, 4 mm for T-24 and T-14. The uncertainties of the measurements of He fluence F were ignored because the measurements of area S (10 10 cm2) of the scintillation detector and the number NHe of He ions were relatively more accurate than the measurements of the other
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parameters in Eq. (2). The effect of temperature variation during irradiation of the bubble detectors in the irradiation room on the detection efficiencies had not been taken into consideration in the uncertainties. The uncertainties by ignoring the reduction of He fluence F within the range and ignoring the effects of bubble expansions in detectors T-34, T-24 and T-14 were not included in the final uncertainties of the detection efficiencies. From Fig. 4 one can see that: (1) The detection efficiencies ε of detectors T-12 and T-34 for He ions with energy less than 530 MeV by recoil bubbles were in the range from about 103 to 2 104 (bubble cm3)/ (He cm2). ε decreases as Ea increases. (2) The detection efficiency ε of detector T-24 was in the range from about 3.3 104 to 1.7 104 (bubble cm3)/(He cm2), which was lower than that of T-12 and T-34. The detection efficiency ε of detector T-14 ranged from about 1.6 104 to 1.4 104 (bubble cm3)/(He cm2), which was still lower than the former. These changes of detection efficiencies might be caused by the differences of the thresholds of the bubble detectors. The thresholds of T-12 and T-34 are lower than that of T-24 and much lower than T-14 (Guo et al., 2003). The lower the threshold (dE/dX)c, the higher the detection efficiency ε. (3) The detection efficiencies ε of bubble detectors T-12, T-34 and T-24 at very low energy region Ea < 100 MeV were obtained to be much higherthanthatathigherenergyregion(seeFig.4aec).Thereason was that at this energy region, a number of direct bubbles were produced by direct deposition of energy of He ions in the superheated liquid droplets which then exploded and formed the direct
Fig. 4. Detection efficiencies of bubble detectors for He ions by recoil bubbles: (a) T-12, (b) T-34, (c) T-24 and (d) T-14, respectively.
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bubbles. The detection efficiencies ε at the peaks of detectors T-12, T-34 and T-24 are the sum of two detection efficiencies (after deduction of scattering bubbles and spurious bubbles): The first detection efficiency is that by recoil bubbles, another is by direct bubbles. The difference between the peak value of ε and the value obtained by extrapolation of the ε values in the higher energy region will be equal to the detection efficiency of the bubble detector for He ion by direct bubbles. In Fig. 4, the uncertainties of the peak values are not marked out. The reason is that the volume density r of bubbles in this interval of X (10 mm) may be changed greatly by a small shift of the interval along X axis. It means that the height of the peak value can be changed by shifting the X coordinate. The reason is that the peak occurred in a very narrow region (1e2 mm) of X value. The width of the peak is much narrower than the interval 10 mm. The average density of the bubbles in 10 mm interval will be lower than the true value at the real peak of 1e2 mm in width. The average is related to the positions of the upper and lower margins of the X interval. But anyway, the direct bubbles do exist and the total number of them is fixed for a certain type of bubble detector. The density value is changed with selection of the position of the interval. (4) In bubble detector T-14 one cannot see the peak of ε at the lower energy region. The reason might be that at temperature of (25 1) C the direct energy deposition by He ion in the droplet in T-14 detector, even though with fluctuation, could not be enough to trigger the droplet to explode. The height of the peaks decreases in the sequence from T-12, T34 to T-24 and then to T-14. The systematic change might be related to the height of the thresholds of the detectors. The height of the peak decreases as the threshold of the detector increases. The formation of direct bubbles is reasonable in bubble detector types T-12 and T-34. The Bragg peaks of energy loss of He ions in these detectors are just about equal to the levels of the thresholds. The detection efficiency of T-12 and T-34 for He ion by direct bubbles is much lower than one. That is because of the distance between two droplets in the detectors is much far away compared with the path length of He ion over which the energy loss rate may be reach or higher than the threshold, the He ion can seldom to hit on a droplet to form a bubble. The formation of direct bubbles in detector T-24 by He ions would be more interesting. The threshold of T-24 is much higher than the Bragg peak of energy loss of He ions. If the threshold property of bubble detector is absolute strict (critical), the direct bubbles cannot be formed in T-24 by He ions. But one sees definitely the direct bubbles in bubble detector T-24 as shown in Fig. 4c as well as in Fig. 2c and Fig. 3c. The explanation can only be that the fluctuation (straggling) of energy deposition in a microscopic volume of the droplet (several tens of nm) results in the formation of direct bubbles. The straggling of energy loss can occasionally reach the level of threshold and trigger the droplets to explode to form bubbles. In this respect, one can see that bubble detectors, as a family, are very sensitive detectors to study straggling of energy deposition of heavy ions in matters. At the same time, one can see that the threshold property of bubble detectors is not classically critical. It will be affected by fluctuation. 6. Conclusions and discussions The following conclusions can be drawn from this study: (1) High-energy He ions can produce recoil bubbles in all the four types of bubble detectors T-12, T-34, T-24 and T-14 at the
temperature of (25 1) C. The detection efficiency of the bubble detector for He ions by recoil bubbles is in the range from about 104 to 103 in the energy region up to 500 MeV. The recoil bubbles may be produced in the whole energy region of the trajectories of He ions. Direct bubbles can be produced in the detectors T-12, T-34 and T-24 at the temperature of (25 1) C, but cannot be produced in detector T-14. Direct bubbles can only be produced in the last part (very low energy region) of He trajectories in bubble detector T-12, T-34 and T-24, but cannot be produced in other energy region of the trajectories. (2) The thresholds of bubble detectors do exist and obviously affect the formation (production) of recoil bubbles and direct bubbles in the detectors, but the threshold of each type of bubble detector cannot be treated as a strict limit to restrict the formation of direct bubbles. (3) The deviation or straggling of energy loss rate of heavy ions in bubble detectors could cause formation of bubbles in the case where the threshold of the detector is obviously higher than the Bragg peak of energy loss of the heavy ions in the detector. In this respect, bubble detector as a family, is a sensitive detector to study quantitatively the energy straggling of heavy ions in matter. (4) The phenomenon of occurrence of recoil bubbles as well as direct bubbles in bubble detectors under bombardment of high-energy He ions can be applied to determination of the intensity (or fluence) of He beam from accelerator if the energy of He ions is known. (5) This study shows that if bubble detectors are used in space or at higher altitude, where high-energy He ions exist in primary and secondary cosmic rays, the He ions would produce recoil bubbles as well as direct bubbles in some detectors. These bubbles cannot be distinguished from other bubbles, such as the bubbles produce by space neutrons. Therefore, in space neutron studies with bubble detectors, the influence of coexisted He ions (cosmic rays) must be taken into account. Acknowledgements The authors wish to thank the National Institute of Radiological Science (NIRS) and the operators of HIMAC accelerator for supplying the helium beam. This work is supported by National Natural Science Foundation of China (NNSFC) under project No. 19975071. References Benton, E.R., Benton, E.V., Frank, A.L., 2001. Neutron dosimetry in low-earth orbit using passive detectors. Radiation Measurements 33, 255e263. d’Errico, F., 2006. Status of radiation detection with superheated emulsions. Radiation Protection Dosimetry 120, 475e479. Guo, S.-L., 2006. Bubble detector investigations in China. Radiation Protection Dosimetry 120, 491e494. Guo, S.-L., Doke, T., Li, L., Chen, B.-L., Zhang, D.-H., Kikuchi, J., Terasawa, K., Komiyama, M., Hara, K., Fuse, T., Yasuda, N., Murakami, T., 2005. Comparison between theoretical model and experimental calibrations and its inference for track formation in bubble detectors. Radiation Measurements 40, 229e233. Guo, S.-L., Doke, T., Zhang, D.-H., Li, L., Chen, B.-L., Kikuchi, J., Terasawa, K., Komiyama, M., Hara, K., Fuse, T., Yasuda, N., Murakami, T., 2009. Study of bubble distributions by high-energy protons in bubble detectors and its hints in neutron detection at higher altitude and in space. Radiation Measurements 44, 885e888. Guo, S.-L., Li, L., Chen, B.-L., Doke, T., Kikuchi, J., Terasawa, K., Komiyarna, M., Hara, K., Fuse, T., 2002. Proton tracks in bubble detector. Nuclear Instruments and Methods B198, 135e141. Guo, S.-L., Li, L., Chen, B.-L., Doke, T., Kikuchi, J., Terasawa, K., Komiyama, M., Hara, K., Fuse, T., Murakami, T., 2003. Status of bubble detectors for high-energy heavy ions. Radiation Measurements 36, 183e187.
S.-L. Guo et al. / Radiation Measurements 50 (2013) 31e37 Guo, S.-L., Li, L., Doke, T., Kikuchi, J., Kyan, A., Yoshihira, E., Kato, T., Murakami, T., 2001. Characteristics of heavy ion tracks in bubble detectors. Radiation Measurements 34, 269e272. Guo, S.-L., Li, L., Doke, T., Kikuchi, J., Terasawa, K., Hara, K., Sakuma, N., Yasuda, N., Murakami, T., 2002. Experimental judgment of the origin of threshold of bubble detectors. Nuclear Techniques 25, 506e510. Ing, H., 2001. Neutron measurements using bubble detectors e terrestrial and space. Radiation Measurements 33, 275e286. Ing, H., Birnboim, H.C., 1984. A bubble-damage polymer detector for neutrons. Nuclear Tracks and Radiation Measurements 8, 285e288. Ing, H., Noulty, R.A., McLean, T.D., 1997. Bubble detectors e a maturing technology. Radiation Measurements 27 (1), 1e11.
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Lewis, B.J., McCal, M.J., Gren, A.R., Bennett, L.G.I., Pierre, M., Schrewe, U.J., O’Brien, K., Felsberger, E., 2001. Aircrew exposure from Cosmic radiation on commercial airline flights. Radiation Protection Dosimetry 93 (4), 293e314. Tommasino, L., 1993. Importance of track detectors in space dosimetry. Nuclear Tracks and Radiation Measurements 22, 707e717. Vanhavere, F., Marlein, B., Nascimento, L.F., Lovestam, G., 2010. Testing three types of active personal neutron dosemeters for application in a nuclear research centre. Radiation Measurements 45, 1593e1596. Vanhavere, F., Loss, M., Plompen, A.J.M., Wattecamps, E., Thierens, H., 1998. A combined use of BD-PND and BDT bubble detectors in neutron dosimetry. Radiation Measurements 29, 573e577. Ziegler, J.F., Biersack, J.P., 2000. SRIM Version 2000.
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Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Experimental investigation of bubble occurrence and locality distribution of bubble detectors bombarded with high-energy helium ions S.-L. Guo a, *, T. Doke b, D.-H. Zhang c, L. Li a, B.-L. Chen a, J. Kikuchi b, N. Hasebe b, K. Terasawa b, K. Hara b, T. Fuse b, N. Yasuda d, T. Murakami d a
China Institute of Atomic Energy, P.O. Box 275 (96), Beijing 102413, PR China Advanced Institute for Science and Engineering, Waseda University, Tokyo, Japan Institute of Modern Physics, Shanxi Normal University, Linfen, Shanxi, PR China d National Institute of Radiological Science, Chiba, Japan b c
h i g h l i g h t s < Four types of 23-cm long bubble detectors (BD) were irradiated to 150 MeV/amu He ions. < Direct bubble, recoil bubble, scattering bubble are formed, recognized and classified. < Detection efficiency of BD for He ions is 103e104 (bubble cm3)/(He cm2). < It is at the same level of efficiency as detecting fast neutrons. < Space neutron detection by BD must take the effect of cosmic ray He ions into account.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 November 2011 Received in revised form 10 October 2012 Accepted 24 October 2012
Large-sized bubble detectors with microscopic droplets of superheated liquids of dichlorodifluoromethane (Freon-12), dichlorotetrafluoroethane (Freon-114), tetrafluoroethane (Freon-134a), and mixture of Freon-12 and Freon-114, respectively, were irradiated with 150 MeV/amu helium ions at the HIMAC accelerator in NIRS, Chiba, Japan. Distributions of bubbles produced by the helium ions have been studied in each type of the detectors. The origin of the bubbles has been investigated. The detection efficiency of each type of the bubble detectors for helium ions with respect to the energy of the ions has been obtained. The phenomenon of bubble occurrence and its possible applications to the determination of He intensity from accelerators, research of track formation mechanism, energy loss straggling and neutron detection in the space and at higher altitude are discussed. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Bubble detectors High-energy helium ions Track formation mechanism Detection efficiency of bubble detector for helium ions Cosmic rays Space neutron detection
1. Introduction Bubble detector (BD) was first reported by Ing in Canada in 1984 (Ing and Birnboim, 1984). It is composed of superheated liquid droplets dispersed uniformly in elastic solid polymer. The size of the liquid droplets is about 10e20 mm. The ratio of the superheated liquid in the whole volume of the detector is from 104 to 20% by volume. Usually there are about 107 to 108 droplets in 1 cm3 of bubble detector.
* Corresponding author. Tel.: þ86 10 60386398; fax: þ86 10 60380995. E-mail address: [email protected] (S.-L. Guo). 1350-4487/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radmeas.2012.10.008
When a charged particle such as a heavy ion passes through a liquid droplet in the detector substances, the particle deposits energy in the droplet and causes evaporation of the liquid, forming a permanent bubble at the original position of the droplet in the detector. One or more bubbles constitute a track of the particle. Bubble detector has been widely used in neutron dosimetry and neutron spectrometry (Ing and Birnboim, 1984; Ing et al., 1997; Ing, 2001; Tommasino, 1993; Vanhavere et al., 1998, 2010; d’Errico, 2006; Benton et al., 2001; Lewis et al., 2001). In neutron detection, the bubbles are produced by charged recoil nuclei or reaction products induced by neutrons in the superheated liquid droplets. The recoil nucleus or reaction product deposits energy in the superheated liquid droplet, resulting in formation of a bubble in the
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detector. Usually, one neutron at most can produce one bubble in the detector. Therefore, the bubbles created by neutrons are randomly distributed in the whole detector (Ing and Birnboim, 1984). Since 1997, bubble detector has been studied to record tracks of high-energy heavy ions (Guo et al., 2001, 2003, 2005; Guo, 2006). It shows that: (1) The track of a high-energy heavy ion is a straight line consisting of a string of tiny bubbles; (2) Threshold property is the governing factor for formation of the tracks. The essence of the threshold is critical rate of energy loss (dE/dX)c rather than that of restricted energy loss (REL) (Guo et al., 2002a). The threshold value varies from (0.05 0.01) 103 to (6.04 0.80) 103 MeV g1 cm2 at (25 1) C for different types of self-made bubble detectors (Guo et al., 2003). Only when the rate of energy loss dE/dX of a heavy ion in the bubble detector is higher than the critical rate of energy loss (dE/dX)c can the heavy ion create bubbles by direct deposition of its energy in the bubble detector, and vice versa. If the path length on which dE/dX (dE/dX)c is larger than the distance between two liquid droplets, the heavy ion will create two or more than two bubbles, forming a linear array of bubbles along its trajectory. Experiments have shown that bubble detector, as a whole, can record linear tracks of high-energy heavy ions with Z 1. In other words, the heavy ions from the entire periodic table of element can be detected with bubble detectors (Guo et al., 2002b). A new question has been raised that if the rate of energy loss dE/ dX of heavy ions is just at the level of the threshold (dE/dX)c, or it is much lower than the threshold, what happens in the bubble detector after irradiation with the heavy ions. In a previous study with protons (Z ¼ 1), whose energy loss rate is much lower than the thresholds of the detectors, “scattering bubbles” and “recoil bubbles” have been observed in the bubble detectors (Guo et al., 2009). What happens in bubble detectors bombarded by other heavy ions with Z > 1 should be clarified through experiments and much more study needs to be done for this phenomenon. This paper presents the occurrence, origin, categories and distributions of bubbles in bubble detectors irradiated with highenergy helium ions, and the detection efficiencies of the bubble detectors for helium ions as well as the possible applications of this phenomenon in various fields.
temperature of irradiation room, the sensitization was made at least two and half hours before irradiation with heavy ions. The four types of bubble detectors were irradiated at the HIMAC accelerator in the National Institute of Radiological Science (NIRS), Chiba, Japan. A 150 MeV/amu He ion beam was accelerated and passed through beam window and an aperture of 25 cm in diameter. Behind the aperture there was a plastic scintillation detector in size of 10 10 cm2, which was used to count the number of He ions passed through the scintillation detector. The bubble detector was placed on the beam behind the scintillation detector and parallel to the He beam. The bottom of the glass tube was faced to the beam direction. The arrangement of irradiation is shown in Fig. 1. Before irradiation, the He beam was highly defocused so that the beam intensity in the whole area of the aperture was very uniform. The intensity of He beam from the accelerator was then reduced to very low so that one could stop irradiation at any expected number of He ions recorded by the scintillation detector. The room temperature was measured to be (25 1) C. The gap between sensitization and irradiation of the detector was 3.0, 2.5, 3.0 and 4.33 h for T-12, T-34, T-24 and T-14 type of detector, respectively. During irradiation, the recorded He ions by the scintillation detector was (3.23 0.32) 106, (3.27 0.33) 106, 7 7 (1.60 0.16) 10 and (1.60 0.16) 10 for T-12, T-34, T-24 and T14 bubble detector, respectively.
3. Bubble distribution and categories After irradiation, small bubbles immediately occurred in the bubble detectors, which can be seen by naked eyes without any instruments. The size of the bubbles grew slowly with time until several hundred mm in diameter. Fig. 2 shows the photographs of the four types of bubble detectors after irradiation by He ions. The photographs were taken directly by a camera without any enlargement. The direction of the He ions impinging on the bubble detectors was from the left to the right as shown by the arrow. The incident beam stopped in the bubble detectors at the position as shown by R in Fig. 2. The ranges of the He ions in bubble detectors are consistent with that calculated by the computer code SRIM 2000 (Ziegler and Biersack, 2000). The distributions of bubbles in the four types of bubble detectors can be seen clearly in the photographs. The following features of bubble distributions in the detectors can be recognized from Fig. 2:
2. Bubble detector and irradiation Four types of large-sized bubble detectors with different thresholds were prepared: T-12, T-34, T-24 and T-14. The elastic solid media in all these detectors are mainly polyacrylamide. The superheated liquids in the microscopic droplets in the detectors are dichlorodifluoromethane (Freon-12) in T-12, tetrafluoroethane (Freon-134a) in T-34, dichlorotetrafluoroethane (Freon-114) in T-14 and Freon-12 and Freon-114 mixture (1:1 by volume) in T-24. The sensitive volume of each detector was 23 cm in length and 1.8 cm in diameter. The percentage of Freon in the total volume of the detectors was 2%. The bubble detectors were prepared in glass tubes with screw caps. About one gram of overlay was added in the glass tube near the screw cap in each detector. The composition of overlay was Freon-12 in T-12, T-24 and T-14, and Freon-134a in T34. The bubble detectors with overlay are insensitive to heavy ions. By pouring out the overlay the bubble detectors were sensitized to heavy ions. In order to have the temperature of the whole volume of the detectors reach equilibrium with the environment
Fig. 1. Photograph of the arrangement of irradiation of bubble detector with He ions at HIMAC accelerator.
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products is higher than the threshold of the detector, bubbles (recoil bubbles for short) occurred on the beam before stopping. The bubbles randomly distributed on the left-hand side of the abrupt change are mainly recoil bubbles mixed with scattering bubbles. 3.3. Direct bubbles
Fig. 2. Photograph of four types of bubble detectors after irradiation to high-energy He ions. The outer diameter of the detectors was 2 cm. The incident beam direction was from the left to the right in the figure. See detailed explanations in the text.
(1)All the bubbles are randomly distributed in the detectors. No linear arrays of bubbles can be found along the He trajectories. (2)An abrupt change of bubble density occurred in each of the bubble detectors. The density of bubbles was higher on the left side of the abrupt change than that on the right. (3) Theoretical calculation by computer code SRIM 2000 shows that the distance from the glass bottom to the points of abrupt change of bubble densities are equal to the range of the He ions in the bubble detectors. (4) There is a much denser region of bubbles near the end of the range in bubble detector T-12 and T-34, a less dense region in T24, and no dense region in T-14. From the arrangement of irradiation and the above features of bubble distributions one can realize that the random bubbles can be classified into four categories. 3.1. Scattering bubbles After a large area of beam window and aperture, He beam passed through the large scintillation detector (10 10 cm2) and its wrapping materials as well as in total more than 6.82 m air before reaching the bubble detector, some He ions were scattered by the nuclei in the scintillation detector and the other materials and deviated from the original beam direction. Some scattered He ions entered the bubble detector from the side surface of the detector and created bubbles in the detector. This kind of bubbles was similar to that observed before in proton irradiation (Guo et al., 2009) and called “scattering bubbles” for short, which spread quite uniformly along the whole length of the detector. The bubbles distributed randomly on the right-hand side of the abrupt change in the detectors are basically scattering bubbles.
In the much more dense region of bubbles (see Fig. 2aec) there should be some extra bubbles besides the recoil bubbles and scattering bubbles. The extra bubbles might be produced by direct deposition of energy of the He ions in the superheated liquid droplets. In previous study, it has been shown that the maximum rate of energy loss (Bragg peak) of He ions in bubble detectors T-12 and T-34 is nearly equal to the level of the thresholds of the detectors (Guo et al., 2003). In this case, the deposited energy by a He ion in a droplet might be enough to trigger a bubble, but not enough to trigger more than one bubble. This conjecture is supported by the bubble detectors T-24 and T-14. In T-24, the Bragg peak of He ions is obviously lower than the threshold (Guo et al., 2003), but the gap between the Bragg peak and the threshold is not so large. By fluctuation of energy deposition, a few of energy deposition events might incidentally reach the height of the threshold, resulting in formation of bubbles. The few but obvious bubbles near the end of the range in the detector T-24 (see Fig. 2c) are in agreement with the conjecture. The gap between the Bragg peak and the threshold of T-14 is very large (Guo et al., 2003). It is not possible for the fluctuation of energy deposition to jump over the big gap. Therefore, one cannot see any extra bubbles in the detector of T-14 in the region near the end of the He range shown in Fig. 2d. 3.4. Spurious bubbles In preparation of bubble detectors, some spurious bubbles might be formed within the detectors or on the inner wall of the glass tubes by poor management. Efforts had been made in preparation of the detectors to reduce the number of spurious bubbles. Most of the bubble detectors used in this work was nearly free from spurious bubbles. The spurious bubbles were distinguishable from that formed by charged particles by their sizes and positions in the detectors. In case the spurious bubbles were wrongly counted in by carelessness, the subtraction (see below) of the number of bubbles on the right-hand side from that on the left-hand side of the abrupt change would reduce the influence of spurious bubbles to a minimum. 4. Bubble density distributions The distributions of bubbles along the axis of T-12, T-34, T-24 and T-14 types of bubble detectors are shown by histogram in Fig. 3. The abscissa X in Fig. 3 is the distance along the axis of the glass tube from the outer surface of the bottom of the tube to the measured point. The thickness of the bottom of the glass tube is 1 mm. The ordinate N is the number of bubbles counted in the interval of X in length 10 mm. The width of counting is the whole circle of the glass tube for T-12, T-34 and T-24, but the width is 23.5 mm for T-24. The distributions of bubbles are similar for all the detectors (with some difference in T-14):
3.2. Recoil bubbles The beam ions of He might collide with the atoms in the superheated liquid droplets and produce recoil nuclei or reaction products. If the energy loss rate of the recoil nuclei or reaction
(1) All the distributions have two terraces. On the first terrace (X ¼ 10 to about 110 mm) the bubble densities are high. On the second terrace (X z 110 to about 200 mm) the bubble densities are low.
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Fig. 3. Distributions of the bubbles along the axis of bubble detectors: (a) T-12, (b) T-34, (c) T-24 and (d) T-14, respectively.
(2) Between the two terraces there is a steep drop of the bubble density. The point of the steep drop corresponds to the end of the range of the He beam ions in the bubble detector. (3) There is a peak of bubble density followed by the steep drop for bubble detector T-12, T-34 and T-24, but no peak for T-14. (4) The bubble density on the first terrace increases as x increases until the steep drop of the bubble density. The above histograms of bubble distributions provide a solution to obtain the number (or density) of bubbles of different categories in the bubble detectors:
bubbles and direct bubbles can be deduced from the histogram in Fig. 3.
5. Detection efficiency of bubble detectors for He ions by recoil bubbles For convenience, one can define the detection efficiency of bubble detectors ε for heavy ions as follows:
ε ¼ (1) The bubbles on the lower terrace (L) correspond to scattering bubbles (S) plus spurious bubbles (Spu) (if they were wrongly counted in): L ¼ S þ Spu. The bubbles on the higher terrace (H) (except the peak) correspond to the sum of recoil bubbles (R), scattering bubbles (S) as well as spurious bubbles (Spu): H ¼ R þ S þ Spu. The bubbles on the peak (P) correspond to the sum of direct bubbles (D), recoil bubbles (R), scattering bubbles (S) and spurious bubbles (Spu): P ¼ D þ R þ S þ Spu. (2) The density of recoil bubbles (R) in each interval can be obtained by subtraction of average density on the lower terrace (L) from the density in the interval on the higher terrace (H): R ¼ H L. The density of direct bubbles (D) can be obtained by subtraction of the extrapolation value (Hex) of preceding densities on the higher terrace from the peak densities (P): D ¼ P Hex. Therefore, the numbers (or densities) of recoil
r F
(1)
where r is the density of bubbles (number of bubbles in unit volume of bubble detector) formed through a specified mechanism (recoil bubbles in this work); F is the fluence of incident heavy ions (He). ε can be further represented by equation:
ε ¼
N S lwh NHe
(2)
where N is the number of bubbles counted for the specified bubbles; l, w, and h are length, width and depth of the volume containing the counted bubbles in bubble detector; NHe is the number of He ions counted by the scintillation detector; S is the area of the scintillation detector (10 10 cm2)
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Fig. 4 shows the results of detection efficiencies of bubble detectors T-12, T-34, T-24 and T-14. The detection efficiencies ε in Fig. 4 are calculated from Eq. (2), in which N being in each interval of 10 mm in length is calculated by deduction of the average number of scattering bubbles and spurious bubbles on the second (right) terrace from the number of bubbles counted in each interval on the first (left) terrace (see in Fig. 3). The energy Ea of He ions at the middle point of each interval was calculated by the computer code SRIM 2000. Before calculation of the detection efficiencies, the effect of bubble expansion had been corrected on the position of each bubble along X axis. The total shift of the point of steep drop was 9.0 mm towards the right direction along X axis for bubble detector T-12. No correction had been made for detectors T-34, T-24 and T-14, because the shifts of bubbles at the moment of bubble counting in these detectors were not very large (the photograph of T-34 in Fig. 2b was taken later than the bubble counting). No corrections were made for the reduction of the fluence F of He ions within the range R caused by the interactions of He ions with atoms of the detectors. The uncertainty for each result of measurement of the detection efficiency ε was composed of mainly statistical uncertainty of bubble counting and the uncertainties of the measurements of length l (0.5 mm), width w (1 mm) and visible depth h (1 mm) of each counting volume of the detector. The visible depth h was taken to be 3 mm for detector type T-12 and T-34, 4 mm for T-24 and T-14. The uncertainties of the measurements of He fluence F were ignored because the measurements of area S (10 10 cm2) of the scintillation detector and the number NHe of He ions were relatively more accurate than the measurements of the other
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parameters in Eq. (2). The effect of temperature variation during irradiation of the bubble detectors in the irradiation room on the detection efficiencies had not been taken into consideration in the uncertainties. The uncertainties by ignoring the reduction of He fluence F within the range and ignoring the effects of bubble expansions in detectors T-34, T-24 and T-14 were not included in the final uncertainties of the detection efficiencies. From Fig. 4 one can see that: (1) The detection efficiencies ε of detectors T-12 and T-34 for He ions with energy less than 530 MeV by recoil bubbles were in the range from about 103 to 2 104 (bubble cm3)/ (He cm2). ε decreases as Ea increases. (2) The detection efficiency ε of detector T-24 was in the range from about 3.3 104 to 1.7 104 (bubble cm3)/(He cm2), which was lower than that of T-12 and T-34. The detection efficiency ε of detector T-14 ranged from about 1.6 104 to 1.4 104 (bubble cm3)/(He cm2), which was still lower than the former. These changes of detection efficiencies might be caused by the differences of the thresholds of the bubble detectors. The thresholds of T-12 and T-34 are lower than that of T-24 and much lower than T-14 (Guo et al., 2003). The lower the threshold (dE/dX)c, the higher the detection efficiency ε. (3) The detection efficiencies ε of bubble detectors T-12, T-34 and T-24 at very low energy region Ea < 100 MeV were obtained to be much higherthanthatathigherenergyregion(seeFig.4aec).Thereason was that at this energy region, a number of direct bubbles were produced by direct deposition of energy of He ions in the superheated liquid droplets which then exploded and formed the direct
Fig. 4. Detection efficiencies of bubble detectors for He ions by recoil bubbles: (a) T-12, (b) T-34, (c) T-24 and (d) T-14, respectively.
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bubbles. The detection efficiencies ε at the peaks of detectors T-12, T-34 and T-24 are the sum of two detection efficiencies (after deduction of scattering bubbles and spurious bubbles): The first detection efficiency is that by recoil bubbles, another is by direct bubbles. The difference between the peak value of ε and the value obtained by extrapolation of the ε values in the higher energy region will be equal to the detection efficiency of the bubble detector for He ion by direct bubbles. In Fig. 4, the uncertainties of the peak values are not marked out. The reason is that the volume density r of bubbles in this interval of X (10 mm) may be changed greatly by a small shift of the interval along X axis. It means that the height of the peak value can be changed by shifting the X coordinate. The reason is that the peak occurred in a very narrow region (1e2 mm) of X value. The width of the peak is much narrower than the interval 10 mm. The average density of the bubbles in 10 mm interval will be lower than the true value at the real peak of 1e2 mm in width. The average is related to the positions of the upper and lower margins of the X interval. But anyway, the direct bubbles do exist and the total number of them is fixed for a certain type of bubble detector. The density value is changed with selection of the position of the interval. (4) In bubble detector T-14 one cannot see the peak of ε at the lower energy region. The reason might be that at temperature of (25 1) C the direct energy deposition by He ion in the droplet in T-14 detector, even though with fluctuation, could not be enough to trigger the droplet to explode. The height of the peaks decreases in the sequence from T-12, T34 to T-24 and then to T-14. The systematic change might be related to the height of the thresholds of the detectors. The height of the peak decreases as the threshold of the detector increases. The formation of direct bubbles is reasonable in bubble detector types T-12 and T-34. The Bragg peaks of energy loss of He ions in these detectors are just about equal to the levels of the thresholds. The detection efficiency of T-12 and T-34 for He ion by direct bubbles is much lower than one. That is because of the distance between two droplets in the detectors is much far away compared with the path length of He ion over which the energy loss rate may be reach or higher than the threshold, the He ion can seldom to hit on a droplet to form a bubble. The formation of direct bubbles in detector T-24 by He ions would be more interesting. The threshold of T-24 is much higher than the Bragg peak of energy loss of He ions. If the threshold property of bubble detector is absolute strict (critical), the direct bubbles cannot be formed in T-24 by He ions. But one sees definitely the direct bubbles in bubble detector T-24 as shown in Fig. 4c as well as in Fig. 2c and Fig. 3c. The explanation can only be that the fluctuation (straggling) of energy deposition in a microscopic volume of the droplet (several tens of nm) results in the formation of direct bubbles. The straggling of energy loss can occasionally reach the level of threshold and trigger the droplets to explode to form bubbles. In this respect, one can see that bubble detectors, as a family, are very sensitive detectors to study straggling of energy deposition of heavy ions in matters. At the same time, one can see that the threshold property of bubble detectors is not classically critical. It will be affected by fluctuation. 6. Conclusions and discussions The following conclusions can be drawn from this study: (1) High-energy He ions can produce recoil bubbles in all the four types of bubble detectors T-12, T-34, T-24 and T-14 at the
temperature of (25 1) C. The detection efficiency of the bubble detector for He ions by recoil bubbles is in the range from about 104 to 103 in the energy region up to 500 MeV. The recoil bubbles may be produced in the whole energy region of the trajectories of He ions. Direct bubbles can be produced in the detectors T-12, T-34 and T-24 at the temperature of (25 1) C, but cannot be produced in detector T-14. Direct bubbles can only be produced in the last part (very low energy region) of He trajectories in bubble detector T-12, T-34 and T-24, but cannot be produced in other energy region of the trajectories. (2) The thresholds of bubble detectors do exist and obviously affect the formation (production) of recoil bubbles and direct bubbles in the detectors, but the threshold of each type of bubble detector cannot be treated as a strict limit to restrict the formation of direct bubbles. (3) The deviation or straggling of energy loss rate of heavy ions in bubble detectors could cause formation of bubbles in the case where the threshold of the detector is obviously higher than the Bragg peak of energy loss of the heavy ions in the detector. In this respect, bubble detector as a family, is a sensitive detector to study quantitatively the energy straggling of heavy ions in matter. (4) The phenomenon of occurrence of recoil bubbles as well as direct bubbles in bubble detectors under bombardment of high-energy He ions can be applied to determination of the intensity (or fluence) of He beam from accelerator if the energy of He ions is known. (5) This study shows that if bubble detectors are used in space or at higher altitude, where high-energy He ions exist in primary and secondary cosmic rays, the He ions would produce recoil bubbles as well as direct bubbles in some detectors. These bubbles cannot be distinguished from other bubbles, such as the bubbles produce by space neutrons. Therefore, in space neutron studies with bubble detectors, the influence of coexisted He ions (cosmic rays) must be taken into account. Acknowledgements The authors wish to thank the National Institute of Radiological Science (NIRS) and the operators of HIMAC accelerator for supplying the helium beam. This work is supported by National Natural Science Foundation of China (NNSFC) under project No. 19975071. References Benton, E.R., Benton, E.V., Frank, A.L., 2001. Neutron dosimetry in low-earth orbit using passive detectors. Radiation Measurements 33, 255e263. d’Errico, F., 2006. Status of radiation detection with superheated emulsions. Radiation Protection Dosimetry 120, 475e479. Guo, S.-L., 2006. Bubble detector investigations in China. Radiation Protection Dosimetry 120, 491e494. Guo, S.-L., Doke, T., Li, L., Chen, B.-L., Zhang, D.-H., Kikuchi, J., Terasawa, K., Komiyama, M., Hara, K., Fuse, T., Yasuda, N., Murakami, T., 2005. Comparison between theoretical model and experimental calibrations and its inference for track formation in bubble detectors. Radiation Measurements 40, 229e233. Guo, S.-L., Doke, T., Zhang, D.-H., Li, L., Chen, B.-L., Kikuchi, J., Terasawa, K., Komiyama, M., Hara, K., Fuse, T., Yasuda, N., Murakami, T., 2009. Study of bubble distributions by high-energy protons in bubble detectors and its hints in neutron detection at higher altitude and in space. Radiation Measurements 44, 885e888. Guo, S.-L., Li, L., Chen, B.-L., Doke, T., Kikuchi, J., Terasawa, K., Komiyarna, M., Hara, K., Fuse, T., 2002. Proton tracks in bubble detector. Nuclear Instruments and Methods B198, 135e141. Guo, S.-L., Li, L., Chen, B.-L., Doke, T., Kikuchi, J., Terasawa, K., Komiyama, M., Hara, K., Fuse, T., Murakami, T., 2003. Status of bubble detectors for high-energy heavy ions. Radiation Measurements 36, 183e187.
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Lewis, B.J., McCal, M.J., Gren, A.R., Bennett, L.G.I., Pierre, M., Schrewe, U.J., O’Brien, K., Felsberger, E., 2001. Aircrew exposure from Cosmic radiation on commercial airline flights. Radiation Protection Dosimetry 93 (4), 293e314. Tommasino, L., 1993. Importance of track detectors in space dosimetry. Nuclear Tracks and Radiation Measurements 22, 707e717. Vanhavere, F., Marlein, B., Nascimento, L.F., Lovestam, G., 2010. Testing three types of active personal neutron dosemeters for application in a nuclear research centre. Radiation Measurements 45, 1593e1596. Vanhavere, F., Loss, M., Plompen, A.J.M., Wattecamps, E., Thierens, H., 1998. A combined use of BD-PND and BDT bubble detectors in neutron dosimetry. Radiation Measurements 29, 573e577. Ziegler, J.F., Biersack, J.P., 2000. SRIM Version 2000.