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The primary target model of energetic ions penetration in thin botanic samples
12 August 2002
Physics Letters A 300 (2002) 611–618 www.elsevier.com/locate/pla
The primary target model of energetic ions penetration in thin botanic samples Yugang Wang ∗ , Guanghua Du, Jianming Xue, Feng Liu, Sixue Wang, Sha Yan, Weijiang Zhao Ion Beam Group, Institute of Heavy Ion Physics, Peking University, Beijing 100871, PR China Received 2 January 2002; received in revised form 3 July 2002; accepted 4 July 2002 Communicated by B. Fricke
Abstract The ion transmission spectra of very low current MeV H+ ions through two kinds of botanic samples, kidney bean slices and onion endocuticle, were carried out. The experimental spectra confirmed the botanic sample is inhomogeneous in mass density. A target model with local density approximation was suggested to describe the penetration of the energetic ions in such kind of materials. From the fitting of proton transmission spectra of two-energies, this target model was verified primarily. Including the influence of surface roughness and irradiation damage, this target model could be improved to predict the profile of penetration depth and range distribution of the energetic ions in the botanic samples. 2002 Elsevier Science B.V. All rights reserved. PACS: 61.82.-d; 34.50.Bw; 87.50.-a Keywords: Energetic ion penetration; Transmission spectrum; Biological membrane; Target model
1. Introduction The mutation effect of low energy heavy ions on crop seeds has gained more and more attentions since it was found in 1980’s due to its prospective application in biological study and agriculture [1]. However, the mechanism of this new application has not been well studied. Based on LSS theory [2], the range of such low-energy ions in condensed organic materials is not more than 1 µm, which is far less than the actual distance between the embryo and the surface of the seeds (usually 100 µm). One of the * Corresponding author.
E-mail address: [email protected] (Y. Wang).
possible explanations of this new phenomenon is that the penetration depth of a small part of those lowenergy ions in the plant seed is much longer than that predicted by T RIM code assuming a homogeneous target material. In this case the DNA molecules in the embryo of the seed can be broken directly by the energetic incident ions. In order to understand the whole process happened during and after the lowenergy ion irradiation of plant seeds, the first step—the physical process of the penetration of energetic ions in these samples should be studied carefully. To investigate the penetration process and the depth of the energetic ions in the botanic samples, in the past many analytic methods have been applied like particle induced X-ray emission (PIXE), positron annihilation and secondary ion mass spectrometry
0375-9601/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 9 6 0 1 ( 0 2 ) 0 0 9 3 8 - 6
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Y. Wang et al. / Physics Letters A 300 (2002) 611–618
(SIMS) experiments have been used to detect the depth profile of implanted low-energy ions in some kinds of dry seeds. It was reported that a part of these ions was detected at a depth of more than 100 µm [3]. With STM (scanning tunneling microscope) observation, the penetration depth of some of 40 keV N+ ions in kidney bean slices was measured beyond 50 µm [4]. And in our previous work, the transmission measurements of MeV F+ ions [5], 500–800 keV He+ and H+ ions through some botanic samples yielded a very small energy loss of the incident ions [6]. According to the general knowledge of biology and the investigations before, the long penetration depth for the low-energy heavy ions in the botanic samples was contributed to the non-uniform mass density and the mass compactness of the natural botanic samples [4,6], but the detailed physical process of the interaction between the energetic ions and the natural botanic materials is not cleared. The energy spectrum of the transmitted ions has been used widely to study the energy loss of the ions in various solid materials. Based on LSS theory and Chu’s improvement [7], the energy loss spectrum for energetic ions through a homogeneous random thin solid film could be described with Gaussian distribution approximately and can be simulated with T RIM code [8]. However, this theory cannot be used directly to picture the profile of the ion transmission energy spectrum through botanic samples which have a cellular structure and micromass distribution. In this Letter, transmission measurement of very low current ion beam was established to investigate the properties of the botanic samples, the applicability of LSS theory in these samples, and then a target model is present to describe the penetration process of the energetic ions in the thin botanic samples.
2. Experimental Two kinds of typical botanic samples were used in our experiment. One was kidney bean slices with average measured thickness of 30 and 50 µm (prepared with traditional biological technique), corresponding to average mass thickness of 3.5 and 6.9 mg/cm2 , respectively. The preparing method of this kind of sample has been described in Ref. [4]. The other kind of sample was onion (Allium cepa L.) scale
Fig. 1. The surface of an onion scale endocuticle observed with SEM.
endocuticle, a kind of botanic membrane with thin monolayer cellular structure with reference to botany, which is shown in Fig. 1. As shown in this figure, the onion endocuticle consists of two dominating parts with different thickness: the cell walls and the district within the walls. The onion endocuticle were carefully exfoliated and flattened, and the two-layer and three-layer onion endocuticle samples were prepared by stacking the endocuticle samples. Then the samples were put into a dryer (relative humidity is 5%) to be dehydrated. The purpose of drying procedure is to make the endocuticle samples closer to the situation when they are put into vacuum chamber for irradiation. After dehydration for two weeks, the onion endocuticle samples were split into quadrate sheets about 1 cm2 to be used. The mainly element atomic composition of the onion endocuticle measured with an element analysis equipment (vario EL) is: C : H : O : N = 24% : 54% : 21% : 1%, which is close to the element composition of carbohydrate. Two other solid materials, PET (ethylene terephthalate, Mylar) film (with a thickness of 38.3 µm) and Nylon filter films (76 µm) were used in the experiments for the purpose of comparison with the botanic samples. The protons with energies of 1.5, 2.5 and 3.0 MeV were selected as the incident ions. The transmission experiment was carried out with the 2 × 1.7 MV tandem accelerator of Peking University. The experimental set-up was shown in Fig. 2. A layer of Au film with a thickness about 0.1 µm was used to scatter the in-
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Fig. 2. Setup of low current H+ ion transmission measurement.
cident ions. The sample target was placed at the direction of 20◦ off the original beam direction. The transmitted protons were collected by a silicon barrier detector (SBD) placed just behind the sample. With this arrangement the fluence rate and the fluence that each sample received was about 2000 ions/cm2 s and about 106 ions/cm2 , respectively, so that the irradiation damage on the sample and the detector could be ignored. Finally, the energy straggling of the scattered proton ions was about 13 keV in this experiment, which is similar to the energy resolution of our SBD at the energies used. By using MeV H+ ions with small mass and energy loss, a complete energy spectrum could be obtained for all the samples in our experiments. Based on the energy spectrum, it is possible for us to character the stopping of the energetic ions and the mass distribution of the samples. 3. Results Fig. 3 showed the 2.5 MeV H+ ions transmission spectra of single-layer, two-layer and three-layer onion endocuticle samples, together with the spectra through Nylon filter film and Mylar film. As can be seen in this figure, the energy loss for Mylar film is the largest one but with the smallest FWHM, and the spectrum can be fitted roughly with a symmetrical Gaussian distribution. The spectrum for Nylon filter film is also symmetrical but cannot be fitted with a Gaussian function and the FWHM was much larger than that for Mylar film. Fig. 3 also shows that, all the spectra through onion endocuticle samples are asymmetrical, and the less the layer number of the onion endocuticle sample, the more the asymmetry of the spectrum. In the spectra of the single-layer samples, two dominating parts of energy loss could be seen clearly, which indicates that there exist two relative larger por-
tions with similar mass thickness in the single-layer onion endocuticle sample. It can be seen additionally from Fig. 3 that the average energy losses for all the onion endocuticle samples are smaller comparing with that for Mylar and Nylon filter film, while the FWHMs of the spectra for the onion endocuticle samples are much larger than that for the Mylar film, which means there exist some incident protons with very small energy loss. For example, the high-energy edge of some transmitted protons from the single-layer onion endocuticle sample is close to the incident energy. Since the value of FWHM in the spectrum is the result of energy straggling of the transmitted ions in the sample, it reflects the inhomogeneousness of the sample’s mass distribution besides the element proportion and the thickness of the sample. Nylon filter film is also a kind of polymer film as Mylar but with micropores. Therefore the incident ions experience different mass densities according to their penetration locus. It appears that initially monoenergetic ions penetrate layers of various densities resulting in a larger energy straggling than caused by the statistics of the energy loss process. In Fig. 3, the broader FWHM in Nylon filter film’s spectrum than that in Mylar’s verified that the dispersion in mass density is a critical factor of energy loss straggling. Consequently it is reasonable to contribute the broader FWHM in botanic sample’s spectrum to its inhomogeneous mass density, which means at different region in the sample the mass density or the mass thickness is different. Recently, SPM (scanning proton microscope) was carried out to characterize the botanic samples and confirmed the inhomogeneousness of the sample at different microregion [9]. For better understanding of the relationship of energy loss of the transmitted ions with the cellular structure and micromass distribution of the botanic sam-
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Fig. 3. Typical energy spectra of 2.5 MeV H+ through the experimental samples, including the single-layer, two-layer, three-layer onion endocuticle samples and Mylar film, Nylon porous filter film; each kind of endocuticle sample has three samples with 1# , 2# , 3# comment.
Fig. 4. The surface of etched CR-39 from microscope observation after the 1.0 MeV He+ ion lithography. The CR-39 substrate was placed behind the single-layer endocuticle with an irradiation fluence of 1014 He+ ions/cm2 . The lithography method is referenced in paper [10].
ple, 1.0 MeV He+ lithograph experiments of an endocuticle sample were performed with a piece of CR39 (allyl diglycol polycarbonate) used as a particlerecord substrate. After irradiation, the CR-39 substrate was etched in 100 ml of 6.25 M NaOH solution at (70±1 ◦ C) for 6 h. Fig. 4 showed the surface feature of
etched CR-39 observed with microscope. In the experiment the endocuticle sample was used as a mask and the contrast of the image results from the difference in energy loss of the transmitted ions when they pass through different micropart of the sample with different mass thickness. Fig. 4 gives the profile of the relation between the energy loss of transmitted ions and the endocuticle micromass distribution, as a result of natural cellar structure of this biological endocuticle. Similar to the SEM (scanning electronic microscope) image shown in Fig. 1, the onion endocuticle has near rectangle cell structure and each cell is separated by cell wall. Concerning about the mass distribution, it is also evident based on the contrast of the lithograph image that this monolayer onion endocuticle consists of two main mass density parts: one is from the part of cell wall and another is the inner part within the cell. It is interesting to find from this image that even within a cell the energy loss and also the mass density are different at different microregions.
4. Discussion As known, the LSS theory and T RIM code are based on the hypotheses that the target material has
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mogeneous mass density consists of a series of small parts, called “local density unit”. In each unit, the mass density is assumed being uniform and the LSS theory or T RIM code is suitable to describe the transport of the energetic ions. Assumption the frequency coefficient of a “local density unit” with a mass density ρi in such a sample is ai . Then we have ρ¯ = ai ρi , ai = 1, i
Fig. 5. 2.5 and 3.0 MeV H+ transmission spectra of 50 µm kidney bean slice and the fitting result of the average energy loss.
certain element composition, a uniform mass density and a determined target surface. However, for natural botanic sample, all these features are not given. Therefore a special model has to be established to describe the penetration process of the energetic ions in the botanic samples. The proton transmission experiments using twoenergies were carried out to obtain the energy spectra for the botanic samples. A typical one is shown in Fig. 5. For a homogeneous sample of certain thickness, the simulation program of S IMNRA [11], which is also based on LSS theory, was used to fit the peak position in one experimental spectrum. Using the same set of the optimized parameters the peak position of the other spectrum was calculated as shown in Fig. 5. From the excellent agreement of the results it is reasonable to assume that for a botanic sample of homogeneous mass density the average energy loss and average penetration depth of the energetic ions in this samples could be predicted by LSS theory. The same result that the LSS theory is still suitable to describe the penetration of the energetic heavy ions in a very thin region has been confirmed in Ref. [5] from the fitting result of high-energy edge of transmission spectrum of MeV F+ at different energy in single-layer onion endocuticle. Based on these experimental facts and fitting results, a target model with localized density approximation is suggested to describe the penetration behavior of the energetic ions in the botanic samples. It is supposed in this model that a botanic sample with inho-
i
here ρ¯ is the average mass density of the sample. If the transmission spectrum in “local density unit” i is described by a function of fi (ρi ) which could be simulated by T RIM or S IMNRA, the total spectrum F (ρ) of the sample will be ai fi (ρi ). F (ρ) = i
For the purpose of fitting convenience, the mass density ρ was replaced by an areal density δ to fit the spectrum. Then, we have F (δ) = ai fi (δi ). i
According to these target model assumptions, the measured 1.5 MeV H+ and 2.5 MeV H+ transmission spectrum through onion endocuticle samples were analyzed as follows: a series of areal density δi with area weight coefficient ai is used to describe the onion endocuticle sample, while ai is not fixed at this moment. Then the 1.5 MeV H+ transmission spectrum through unit i with areal density δi is calculated via T RIM code (each spectrum is obtained by calculating 20 000 incident 2.5/1.5 MeV protons), which presents the transmission spectrum function f (δi )2.5 MeV /f (δi )1.5 MeV . Using the calculated f (δi )1.5 MeV to fit the experimental 1.5 MeV H+ transmission spectrum will get a series of frequency coefficient ηi , then the normalized value of ηi will be the value of ai . After fitting the 1.5 MeV H+ transmission spectrum, the same series of the parameters of δi and ai were applied to calculate the 2.5 MeV H+ spectrum. If the 2.5 MeV H+ transmission spectrum could be reproduced well, the target model with local density approximation and this series of parameters can be used to describe the botanic sample in penetration study.
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(a)
(b) Fig. 6. The transmission spectra and the fitting result of 1.5 and 2.5 MeV H+ ion through a single-layer onion endocuticle sample.
Fig. 6 is the fitting result of ion transmission spectra through a single-layer onion endocuticle sample with H+ ion energy at 1.5 and 2.5 MeV. The low-energy spectrum (1.5 MeV) was fitted first with 33 groups parameter of the local areal density δi , the calculated transmission spectrum f (δi ), and the frequency coefficient ai, as shown in Fig. 6(a). It could be seen in Fig. 6(b) that the second spectrum with energy of 2.5 MeV could be fitted well also with these para-
meters. From the excellent results of the proton transmission spectra of two different energies obtained after penetration of a typical biological sample like a single-layer onion endocuticle, it could be concluded that a botanic sample with inhomogeneous mass density could be considered to consist of a series of small parts, called “local density unit”. The set of parameters of ρi (or δi ) and ai are material parameters, and they will not change when the energy of the incident
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(a)
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shown in Fig. 7(a). Based on the SEM image in Fig. 1 and lithography image in Fig. 4, the two peaks at the smaller and larger energy are due to its cellularity as mentioned before. With the same procedure, the areal density proportion of a three-layer endocuticle sample was also simulated based on the transmission spectrum and is shown in Fig. 7(b). Since it is difficult to obtain the information about a “local density unit” with very small frequency coefficient ai from this complete transmission spectrum, an additional lower energy proton transmission experiment is prepared to collect enough counts of the transmitted ions at high-energy edge. If it could be performed and the surface roughness could be considered in this target model and the influence of the irradiation damage could be evaluated, the overall range distribution of the energetic ions in the botanic sample could also be predicted based on this target model of localized density approximation.
5. Conclusion
(b) Fig. 7. The obtained areal density of experimental samples in detail based on the target model and T RIM code. (a) Simulated areal density composition for a single-layer endocuticle sample; (b) simulated areal density composition for a three-layer endocuticle sample.
ion changes. In each “local density unit” the universal usage of the LSS theory or T RIM still could be applied to predict the penetration process and the target model with localized density approximation is suitable to describe the transmission of the energetic ions through the botanic samples. From the results in Fig. 6, it could be seen that in this typical botanic membrane there are some units with small areal density δi and small frequency coefficient ai at the high-energy edge of the spectrum. There, the energetic ions lose only a little of their incident energy and will penetrate much deeper in these units. The detailed areal density proportion of a single-layer endocuticle sample, which showed double peak as well as its transmission spectrum, was
In summary, MeV H+ ion transmission spectra confirmed the botanic sample is inhomogeneous in mass density. A target model with local density approximation was suggested to describe the penetration of the energetic ions in such kind of materials. From the fitting of the proton transmission spectra of two-energies, this target model was verified primarily. With further study of the influence of surface roughness and irradiation damage, this target model could be improved to predict the penetration depth and range distribution of energetic ions in botanic samples.
Acknowledgements Authors are grateful to Prof. Lu Xiting and Shen Dingyu for their helpful assistance and advice. This work was supported by the National Science Foundation of China (Grant No. 19890300).
References [1] Z. Yu, J. Deng, J. He, Y. Hou, Y. Wu, X. Wang, G. Liu, Nucl. Instrum. Methods B 59-60 (1991) 705.
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[2] J. Lindhard, M. Scharff, H.E. Schiott, Mat. Fys. Medd. Dan. Vid. Selsk. 14 (1963) 33. [3] T. Lu, W. Yu, H. Zhou, G. Zhu, X. Wang, C. Wang, Chin. Phys. 10 (2) (2001) 145. [4] F. Liu, Y. Wang, J. Xue, S. Wang, S. Yan, W. Zhao, Phys. Lett. A 283 (2001) 360. [5] J. Xue, Y. Wang, F. Liu, S. Wang, S. Yan, W. Zhao, Surf. Coat. Technol. 128-129 (2000) 139. [6] Y. Xia, C. Tan, Y. Yu, R. Wang, J. Zhang, X. Liu, J. Liu, Z. Yu, Phys. Lett. A 256 (1999) 205.
[7] W.-K. Chu, J.W. Mayer, M.-A. Nicolet, Backscattering Sperctrometry, Academic Press, 1978. [8] J.F. Ziegler, J.P. Biersack, U. Littmark, Stopping and Ranges of Ions in Matter, Pergamon Press, New York, 1985. [9] Y. Xia, private communication. [10] B.E. Fischer, R. Spohr, Rev. Mod. Phys. 55 (4) (1983). [11] M. Mayer, SIMNRA User’s Guide, Max-Planck-Institut für Plasmaphysik, Garching, Germany, 1997, Technical Report IPP 9/113.
Physics Letters A 300 (2002) 611–618 www.elsevier.com/locate/pla
The primary target model of energetic ions penetration in thin botanic samples Yugang Wang ∗ , Guanghua Du, Jianming Xue, Feng Liu, Sixue Wang, Sha Yan, Weijiang Zhao Ion Beam Group, Institute of Heavy Ion Physics, Peking University, Beijing 100871, PR China Received 2 January 2002; received in revised form 3 July 2002; accepted 4 July 2002 Communicated by B. Fricke
Abstract The ion transmission spectra of very low current MeV H+ ions through two kinds of botanic samples, kidney bean slices and onion endocuticle, were carried out. The experimental spectra confirmed the botanic sample is inhomogeneous in mass density. A target model with local density approximation was suggested to describe the penetration of the energetic ions in such kind of materials. From the fitting of proton transmission spectra of two-energies, this target model was verified primarily. Including the influence of surface roughness and irradiation damage, this target model could be improved to predict the profile of penetration depth and range distribution of the energetic ions in the botanic samples. 2002 Elsevier Science B.V. All rights reserved. PACS: 61.82.-d; 34.50.Bw; 87.50.-a Keywords: Energetic ion penetration; Transmission spectrum; Biological membrane; Target model
1. Introduction The mutation effect of low energy heavy ions on crop seeds has gained more and more attentions since it was found in 1980’s due to its prospective application in biological study and agriculture [1]. However, the mechanism of this new application has not been well studied. Based on LSS theory [2], the range of such low-energy ions in condensed organic materials is not more than 1 µm, which is far less than the actual distance between the embryo and the surface of the seeds (usually 100 µm). One of the * Corresponding author.
E-mail address: [email protected] (Y. Wang).
possible explanations of this new phenomenon is that the penetration depth of a small part of those lowenergy ions in the plant seed is much longer than that predicted by T RIM code assuming a homogeneous target material. In this case the DNA molecules in the embryo of the seed can be broken directly by the energetic incident ions. In order to understand the whole process happened during and after the lowenergy ion irradiation of plant seeds, the first step—the physical process of the penetration of energetic ions in these samples should be studied carefully. To investigate the penetration process and the depth of the energetic ions in the botanic samples, in the past many analytic methods have been applied like particle induced X-ray emission (PIXE), positron annihilation and secondary ion mass spectrometry
0375-9601/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 9 6 0 1 ( 0 2 ) 0 0 9 3 8 - 6
612
Y. Wang et al. / Physics Letters A 300 (2002) 611–618
(SIMS) experiments have been used to detect the depth profile of implanted low-energy ions in some kinds of dry seeds. It was reported that a part of these ions was detected at a depth of more than 100 µm [3]. With STM (scanning tunneling microscope) observation, the penetration depth of some of 40 keV N+ ions in kidney bean slices was measured beyond 50 µm [4]. And in our previous work, the transmission measurements of MeV F+ ions [5], 500–800 keV He+ and H+ ions through some botanic samples yielded a very small energy loss of the incident ions [6]. According to the general knowledge of biology and the investigations before, the long penetration depth for the low-energy heavy ions in the botanic samples was contributed to the non-uniform mass density and the mass compactness of the natural botanic samples [4,6], but the detailed physical process of the interaction between the energetic ions and the natural botanic materials is not cleared. The energy spectrum of the transmitted ions has been used widely to study the energy loss of the ions in various solid materials. Based on LSS theory and Chu’s improvement [7], the energy loss spectrum for energetic ions through a homogeneous random thin solid film could be described with Gaussian distribution approximately and can be simulated with T RIM code [8]. However, this theory cannot be used directly to picture the profile of the ion transmission energy spectrum through botanic samples which have a cellular structure and micromass distribution. In this Letter, transmission measurement of very low current ion beam was established to investigate the properties of the botanic samples, the applicability of LSS theory in these samples, and then a target model is present to describe the penetration process of the energetic ions in the thin botanic samples.
2. Experimental Two kinds of typical botanic samples were used in our experiment. One was kidney bean slices with average measured thickness of 30 and 50 µm (prepared with traditional biological technique), corresponding to average mass thickness of 3.5 and 6.9 mg/cm2 , respectively. The preparing method of this kind of sample has been described in Ref. [4]. The other kind of sample was onion (Allium cepa L.) scale
Fig. 1. The surface of an onion scale endocuticle observed with SEM.
endocuticle, a kind of botanic membrane with thin monolayer cellular structure with reference to botany, which is shown in Fig. 1. As shown in this figure, the onion endocuticle consists of two dominating parts with different thickness: the cell walls and the district within the walls. The onion endocuticle were carefully exfoliated and flattened, and the two-layer and three-layer onion endocuticle samples were prepared by stacking the endocuticle samples. Then the samples were put into a dryer (relative humidity is 5%) to be dehydrated. The purpose of drying procedure is to make the endocuticle samples closer to the situation when they are put into vacuum chamber for irradiation. After dehydration for two weeks, the onion endocuticle samples were split into quadrate sheets about 1 cm2 to be used. The mainly element atomic composition of the onion endocuticle measured with an element analysis equipment (vario EL) is: C : H : O : N = 24% : 54% : 21% : 1%, which is close to the element composition of carbohydrate. Two other solid materials, PET (ethylene terephthalate, Mylar) film (with a thickness of 38.3 µm) and Nylon filter films (76 µm) were used in the experiments for the purpose of comparison with the botanic samples. The protons with energies of 1.5, 2.5 and 3.0 MeV were selected as the incident ions. The transmission experiment was carried out with the 2 × 1.7 MV tandem accelerator of Peking University. The experimental set-up was shown in Fig. 2. A layer of Au film with a thickness about 0.1 µm was used to scatter the in-
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Fig. 2. Setup of low current H+ ion transmission measurement.
cident ions. The sample target was placed at the direction of 20◦ off the original beam direction. The transmitted protons were collected by a silicon barrier detector (SBD) placed just behind the sample. With this arrangement the fluence rate and the fluence that each sample received was about 2000 ions/cm2 s and about 106 ions/cm2 , respectively, so that the irradiation damage on the sample and the detector could be ignored. Finally, the energy straggling of the scattered proton ions was about 13 keV in this experiment, which is similar to the energy resolution of our SBD at the energies used. By using MeV H+ ions with small mass and energy loss, a complete energy spectrum could be obtained for all the samples in our experiments. Based on the energy spectrum, it is possible for us to character the stopping of the energetic ions and the mass distribution of the samples. 3. Results Fig. 3 showed the 2.5 MeV H+ ions transmission spectra of single-layer, two-layer and three-layer onion endocuticle samples, together with the spectra through Nylon filter film and Mylar film. As can be seen in this figure, the energy loss for Mylar film is the largest one but with the smallest FWHM, and the spectrum can be fitted roughly with a symmetrical Gaussian distribution. The spectrum for Nylon filter film is also symmetrical but cannot be fitted with a Gaussian function and the FWHM was much larger than that for Mylar film. Fig. 3 also shows that, all the spectra through onion endocuticle samples are asymmetrical, and the less the layer number of the onion endocuticle sample, the more the asymmetry of the spectrum. In the spectra of the single-layer samples, two dominating parts of energy loss could be seen clearly, which indicates that there exist two relative larger por-
tions with similar mass thickness in the single-layer onion endocuticle sample. It can be seen additionally from Fig. 3 that the average energy losses for all the onion endocuticle samples are smaller comparing with that for Mylar and Nylon filter film, while the FWHMs of the spectra for the onion endocuticle samples are much larger than that for the Mylar film, which means there exist some incident protons with very small energy loss. For example, the high-energy edge of some transmitted protons from the single-layer onion endocuticle sample is close to the incident energy. Since the value of FWHM in the spectrum is the result of energy straggling of the transmitted ions in the sample, it reflects the inhomogeneousness of the sample’s mass distribution besides the element proportion and the thickness of the sample. Nylon filter film is also a kind of polymer film as Mylar but with micropores. Therefore the incident ions experience different mass densities according to their penetration locus. It appears that initially monoenergetic ions penetrate layers of various densities resulting in a larger energy straggling than caused by the statistics of the energy loss process. In Fig. 3, the broader FWHM in Nylon filter film’s spectrum than that in Mylar’s verified that the dispersion in mass density is a critical factor of energy loss straggling. Consequently it is reasonable to contribute the broader FWHM in botanic sample’s spectrum to its inhomogeneous mass density, which means at different region in the sample the mass density or the mass thickness is different. Recently, SPM (scanning proton microscope) was carried out to characterize the botanic samples and confirmed the inhomogeneousness of the sample at different microregion [9]. For better understanding of the relationship of energy loss of the transmitted ions with the cellular structure and micromass distribution of the botanic sam-
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Fig. 3. Typical energy spectra of 2.5 MeV H+ through the experimental samples, including the single-layer, two-layer, three-layer onion endocuticle samples and Mylar film, Nylon porous filter film; each kind of endocuticle sample has three samples with 1# , 2# , 3# comment.
Fig. 4. The surface of etched CR-39 from microscope observation after the 1.0 MeV He+ ion lithography. The CR-39 substrate was placed behind the single-layer endocuticle with an irradiation fluence of 1014 He+ ions/cm2 . The lithography method is referenced in paper [10].
ple, 1.0 MeV He+ lithograph experiments of an endocuticle sample were performed with a piece of CR39 (allyl diglycol polycarbonate) used as a particlerecord substrate. After irradiation, the CR-39 substrate was etched in 100 ml of 6.25 M NaOH solution at (70±1 ◦ C) for 6 h. Fig. 4 showed the surface feature of
etched CR-39 observed with microscope. In the experiment the endocuticle sample was used as a mask and the contrast of the image results from the difference in energy loss of the transmitted ions when they pass through different micropart of the sample with different mass thickness. Fig. 4 gives the profile of the relation between the energy loss of transmitted ions and the endocuticle micromass distribution, as a result of natural cellar structure of this biological endocuticle. Similar to the SEM (scanning electronic microscope) image shown in Fig. 1, the onion endocuticle has near rectangle cell structure and each cell is separated by cell wall. Concerning about the mass distribution, it is also evident based on the contrast of the lithograph image that this monolayer onion endocuticle consists of two main mass density parts: one is from the part of cell wall and another is the inner part within the cell. It is interesting to find from this image that even within a cell the energy loss and also the mass density are different at different microregions.
4. Discussion As known, the LSS theory and T RIM code are based on the hypotheses that the target material has
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mogeneous mass density consists of a series of small parts, called “local density unit”. In each unit, the mass density is assumed being uniform and the LSS theory or T RIM code is suitable to describe the transport of the energetic ions. Assumption the frequency coefficient of a “local density unit” with a mass density ρi in such a sample is ai . Then we have ρ¯ = ai ρi , ai = 1, i
Fig. 5. 2.5 and 3.0 MeV H+ transmission spectra of 50 µm kidney bean slice and the fitting result of the average energy loss.
certain element composition, a uniform mass density and a determined target surface. However, for natural botanic sample, all these features are not given. Therefore a special model has to be established to describe the penetration process of the energetic ions in the botanic samples. The proton transmission experiments using twoenergies were carried out to obtain the energy spectra for the botanic samples. A typical one is shown in Fig. 5. For a homogeneous sample of certain thickness, the simulation program of S IMNRA [11], which is also based on LSS theory, was used to fit the peak position in one experimental spectrum. Using the same set of the optimized parameters the peak position of the other spectrum was calculated as shown in Fig. 5. From the excellent agreement of the results it is reasonable to assume that for a botanic sample of homogeneous mass density the average energy loss and average penetration depth of the energetic ions in this samples could be predicted by LSS theory. The same result that the LSS theory is still suitable to describe the penetration of the energetic heavy ions in a very thin region has been confirmed in Ref. [5] from the fitting result of high-energy edge of transmission spectrum of MeV F+ at different energy in single-layer onion endocuticle. Based on these experimental facts and fitting results, a target model with localized density approximation is suggested to describe the penetration behavior of the energetic ions in the botanic samples. It is supposed in this model that a botanic sample with inho-
i
here ρ¯ is the average mass density of the sample. If the transmission spectrum in “local density unit” i is described by a function of fi (ρi ) which could be simulated by T RIM or S IMNRA, the total spectrum F (ρ) of the sample will be ai fi (ρi ). F (ρ) = i
For the purpose of fitting convenience, the mass density ρ was replaced by an areal density δ to fit the spectrum. Then, we have F (δ) = ai fi (δi ). i
According to these target model assumptions, the measured 1.5 MeV H+ and 2.5 MeV H+ transmission spectrum through onion endocuticle samples were analyzed as follows: a series of areal density δi with area weight coefficient ai is used to describe the onion endocuticle sample, while ai is not fixed at this moment. Then the 1.5 MeV H+ transmission spectrum through unit i with areal density δi is calculated via T RIM code (each spectrum is obtained by calculating 20 000 incident 2.5/1.5 MeV protons), which presents the transmission spectrum function f (δi )2.5 MeV /f (δi )1.5 MeV . Using the calculated f (δi )1.5 MeV to fit the experimental 1.5 MeV H+ transmission spectrum will get a series of frequency coefficient ηi , then the normalized value of ηi will be the value of ai . After fitting the 1.5 MeV H+ transmission spectrum, the same series of the parameters of δi and ai were applied to calculate the 2.5 MeV H+ spectrum. If the 2.5 MeV H+ transmission spectrum could be reproduced well, the target model with local density approximation and this series of parameters can be used to describe the botanic sample in penetration study.
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(b) Fig. 6. The transmission spectra and the fitting result of 1.5 and 2.5 MeV H+ ion through a single-layer onion endocuticle sample.
Fig. 6 is the fitting result of ion transmission spectra through a single-layer onion endocuticle sample with H+ ion energy at 1.5 and 2.5 MeV. The low-energy spectrum (1.5 MeV) was fitted first with 33 groups parameter of the local areal density δi , the calculated transmission spectrum f (δi ), and the frequency coefficient ai, as shown in Fig. 6(a). It could be seen in Fig. 6(b) that the second spectrum with energy of 2.5 MeV could be fitted well also with these para-
meters. From the excellent results of the proton transmission spectra of two different energies obtained after penetration of a typical biological sample like a single-layer onion endocuticle, it could be concluded that a botanic sample with inhomogeneous mass density could be considered to consist of a series of small parts, called “local density unit”. The set of parameters of ρi (or δi ) and ai are material parameters, and they will not change when the energy of the incident
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shown in Fig. 7(a). Based on the SEM image in Fig. 1 and lithography image in Fig. 4, the two peaks at the smaller and larger energy are due to its cellularity as mentioned before. With the same procedure, the areal density proportion of a three-layer endocuticle sample was also simulated based on the transmission spectrum and is shown in Fig. 7(b). Since it is difficult to obtain the information about a “local density unit” with very small frequency coefficient ai from this complete transmission spectrum, an additional lower energy proton transmission experiment is prepared to collect enough counts of the transmitted ions at high-energy edge. If it could be performed and the surface roughness could be considered in this target model and the influence of the irradiation damage could be evaluated, the overall range distribution of the energetic ions in the botanic sample could also be predicted based on this target model of localized density approximation.
5. Conclusion
(b) Fig. 7. The obtained areal density of experimental samples in detail based on the target model and T RIM code. (a) Simulated areal density composition for a single-layer endocuticle sample; (b) simulated areal density composition for a three-layer endocuticle sample.
ion changes. In each “local density unit” the universal usage of the LSS theory or T RIM still could be applied to predict the penetration process and the target model with localized density approximation is suitable to describe the transmission of the energetic ions through the botanic samples. From the results in Fig. 6, it could be seen that in this typical botanic membrane there are some units with small areal density δi and small frequency coefficient ai at the high-energy edge of the spectrum. There, the energetic ions lose only a little of their incident energy and will penetrate much deeper in these units. The detailed areal density proportion of a single-layer endocuticle sample, which showed double peak as well as its transmission spectrum, was
In summary, MeV H+ ion transmission spectra confirmed the botanic sample is inhomogeneous in mass density. A target model with local density approximation was suggested to describe the penetration of the energetic ions in such kind of materials. From the fitting of the proton transmission spectra of two-energies, this target model was verified primarily. With further study of the influence of surface roughness and irradiation damage, this target model could be improved to predict the penetration depth and range distribution of energetic ions in botanic samples.
Acknowledgements Authors are grateful to Prof. Lu Xiting and Shen Dingyu for their helpful assistance and advice. This work was supported by the National Science Foundation of China (Grant No. 19890300).
References [1] Z. Yu, J. Deng, J. He, Y. Hou, Y. Wu, X. Wang, G. Liu, Nucl. Instrum. Methods B 59-60 (1991) 705.
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[2] J. Lindhard, M. Scharff, H.E. Schiott, Mat. Fys. Medd. Dan. Vid. Selsk. 14 (1963) 33. [3] T. Lu, W. Yu, H. Zhou, G. Zhu, X. Wang, C. Wang, Chin. Phys. 10 (2) (2001) 145. [4] F. Liu, Y. Wang, J. Xue, S. Wang, S. Yan, W. Zhao, Phys. Lett. A 283 (2001) 360. [5] J. Xue, Y. Wang, F. Liu, S. Wang, S. Yan, W. Zhao, Surf. Coat. Technol. 128-129 (2000) 139. [6] Y. Xia, C. Tan, Y. Yu, R. Wang, J. Zhang, X. Liu, J. Liu, Z. Yu, Phys. Lett. A 256 (1999) 205.
[7] W.-K. Chu, J.W. Mayer, M.-A. Nicolet, Backscattering Sperctrometry, Academic Press, 1978. [8] J.F. Ziegler, J.P. Biersack, U. Littmark, Stopping and Ranges of Ions in Matter, Pergamon Press, New York, 1985. [9] Y. Xia, private communication. [10] B.E. Fischer, R. Spohr, Rev. Mod. Phys. 55 (4) (1983). [11] M. Mayer, SIMNRA User’s Guide, Max-Planck-Institut für Plasmaphysik, Garching, Germany, 1997, Technical Report IPP 9/113.