- Email: [email protected]
The response of a NE102 scintillator to passing-through relativistic heavy ions
Nuclear Instruments and Methods in Physics Research A 437 (1999) 346}353
The response of a NE102 scintillator to passing-through relativistic heavy ions Naruhiro Matsufuji!,*, Tatsuaki Kanai!, Hideaki Komami", Toshiyuki Kohno" !Division of Accelerator Physics and Engineering, National Institute of Radiological Sciences, 263-8555 Chiba, Japan "Department of Energy Science, Tokyo Institute of Technology, 226-8502 Yokohama, Japan Received 17 June 1999; accepted 6 July 1999
Abstract The response of a NE102 scintillator to He, C, Ne, Si and Ar ions has been studied at incident energies from 120 MeV up to 18 GeV when used as a transmission (*E) counter. The energy of the incident beam was changed by inserting an absorber of various thicknesses made of PMMA plates. The primary particles were identi"ed and separated from projectile fragments by using the *E}E counter telescope method. A thin NE102 plastic scintillator and a BGO scintillator were used as a *E counter and a residual E counter, respectively. The energy of the primary particles was deduced by comparing the depth-dose distribution measured by an ionization chamber to those by a theoretical calculation. The light output of the NE102, d¸/dx, was found to be linear to dE/dx in the low-dE/dx region, while gradually being quenched in the high-dE/dx region. This quenching was regarded as being a phenomenon independent of the particle species. The residual particle-species dependency was well indexed as a function of log (AZ2). ( 1999 Elsevier Science B.V. All rights reserved. PACS: 29.40.M; 29.30; 07.77.K Keywords: NE102; response; high energy; heavy ion; *E
1. Introduction The fragmentation of projectiles has been regarded as being one of the most important reactions of relativistic heavy ions, i.e., several hundred MeV/n of heavy ions to be considered regarding its medical applications by the fact that the RBE (Relative Biological E!ectiveness) is strongly dependent on the particle species [1].
* Corresponding author. Tel.:#81-43-251-2111 6873; fax: #81-43-251-1840. E-mail address: [email protected] (N. Matsufuji)
The *E-E counter telescope method is one of the commonly used techniques used to identify the species of fragment particles. As suggested by the Bethe}Bloch's formula, *E(dE/dx) of a charged particle is proportional to the kind of particle (AZ2), and is inversely proportional to its energy (E) (A: mass number, Z: atomic number). When investigating the #uence of fragments, good separation between light fragment species (hydrogen and helium) is required, because the numbers of produced light fragments is much larger compared with that of other heavier fragment species [2]. In general, the response of a scintillator is governed not only by the energy deposited in the scintillator, but
0168-9002/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 7 5 5 - X
N. Matsufuji et al. / Nuclear Instruments and Methods in Physics Research A 437 (1999) 346}353
also by the species of the incident particles. Here, the particle-species dependence tends to enhance the response for lighter particle. Because the output of a semiconductor detector principally re#ects the energy loss only in the detector, using a scintillator as a *E counter can be superior to using a semiconductor detector for the sake of separating lighter particle species. NE102 (equivalent to BC400) is one of the most commonly used plastic scintillators aimed at particle counting. Beginning with the work by Birks [3,4], theoretical approaches to the scintillation mechanism have been carried out [5}9]. Many e!orts have also been made from experimental points of view [10}29]. Among them, Becchetti et al. comprehensively investigated the response of NE102 for various kinds of low-energy heavy ions stopped in a scintillator [10]. Salamon and Ahlen reported on the response at the relativistic energy region, but on di!erent types of scintillators [20]. In this paper, the response of the NE102 scintillator to passing-through He, C, Ne, Si and Ar ions at relativistic energy region is reported for the "rst time.
2. Experimental technique The experimental technique used in this study has been described in detail elsewhere [30]; therefore, only a brief description is given below. 2.1. Irradiation of heavy-ion beams An experiment was carried out at a beam port for biological experiments at HIMAC (Heavy Ion Medical Accelerator in Chiba) of NIRS. The kind of primary beams used for the experiment were as follows: 150 MeV/n of 4He, 290 MeV/n of 12C, 400 MeV/n of 20Ne, 490 MeV/n of 28Si and 550 MeV/n of 40Ar. Energy spreading is regarded to have been less than 0.1%. The counter complex used in this study was principally designed for the measurement of the #uence and energy spectra of a therapeutic beam for each fragment species from primaries down to hydrogen [2]. Therefore, the incident beam was broadened to 100 mm in diameter by a wobbler
347
method [31] with more than $2% of the lateral uniformity, in the same manner as in the general case of therapy at HIMAC. A stack of polymethyl methacrylrate (PMMA) plates, called binary "lter, was used to decrease the energies of the primary particles by inserting various thicknesses of the plates. The utilized scintillation counters were positioned at the irradiation point, 300 mm downstream from the binary "lter. Measurements were carried out while changing the thickness of the binary "lter variously. 2.2. Scintillation counters A disk-like NE102 plastic scintillator (5 mm in thickness and 23.9 mm in diameter) was used for investigating the response. The NE102 was polished well and mounted in a light guide made of PMMA. Entrance and exit windows were made of a "lm of aluminized polyethylene terephthalate (Mylar) 2.4 mg/cm2 in thickness. A HAMAMATSU H1161 PMT was attached to a NE102 scintillator via a PMMA light guide. The voltage applied to the PMT was !1400 V for helium, !1100 V for carbon, neon and silicon, and !1000 V for argon. The PMT signal was at "rst processed by an ORTEC 113 preampli"er. A resistor of 150 k) was added between the input signal line and the ground for the sake of reducing the input impedance. The output from the preampli"er was divided into two. One line was connected to an ampli"er with a higher gain than that of the other line so as to detect lighter fragments, such as hydrogen and helium. To eliminate fragment particles from primary particles by the *E}E counter telescope method, a BGO scintillator was attached next to the NE102 scintillator. The BGO crystal has a rectangular parallelepiped form of 40 mm in width and height and 300 mm in length. The length corresponds to the range of 1H-525 MeV. The energy response of the BGO scintillator is reported in Ref. [30]. As a calibration source, stabilized green light emitted from a 1N6094 LED, driven by a TENNELEC TC814 pulser, was used. An optical "ber cable was attached to the light guide of both the NE102 and the BGO scintillators to deliver the LED light directly. Calibration using the LED light
348
N. Matsufuji et al. / Nuclear Instruments and Methods in Physics Research A 437 (1999) 346}353
Fig. 1. Schematic layout of the detector system.
was periodically carried out between experimental runs together with additional calibration of the electronics by an ORTEC 448 research pulser. Another large-area NE102 scintillator of 1 mm thickness and 150 mm width and height was used as a beam monitor to count the total number of primary particles. The beam-monitor scintillator was "xed just behind a beam exit window, 4300 mm upstream from the irradiation site. A schematic layout of the detector system is summarized in Fig. 1. 2.3. Particle identixcation Fig. 2 shows a two-dimensional scatter plot of the light output of the NE102 scintillator (normal gain), which corresponds to the energy loss (*E), and that of the BGO scintillator, which corresponds to the residual energy (E). The plot was taken from the incidence of a 400 MeV/n of Ne beam to the binary "lter (90.0 mm in thickness). As shown in the "gure, fragment particles were clearly
Fig. 2. Two-dimensional scatter plot of the light output of the NE102 scintillator (low gain), which corresponds to the energy loss (*E) and that of the BGO scintillator which corresponds to the residual energy (E). The plot was taken from the incidence of a 400 MeV/n of Ne beam to the binary "lter (90.0 mm in thickness).
N. Matsufuji et al. / Nuclear Instruments and Methods in Physics Research A 437 (1999) 346}353
separated by the di!erence in the element as some di!erent bands. Primary particles were then well distinguished from fragments by marking ROI to the highest band, which corresponds to the primaries in the plot. 2.4. Energy scaling The depth}dose pro"le was measured using a parallel-plate ionization chamber on each incident beam for the sake of calibrating the response in terms of energy. The depth}dose pro"le was also calculated with a simulation code of the fragmentation reaction developed by Sihver et al. [32]. The incident energy as well as the residual energy of primary particles after passing through any thickness of binary "lter were derived by "tting the measured pro"le with the calculated one. Most of the error expected in the calculation was due to the error in the stopping power; however, its extent was estimated to be $2% at maximum.
3. Results and discussion 3.1. Global response 3.1.1. d¸/dx as a function of dE/dx Fig. 3 shows an example of the raw energy spectra from the NE102 scintillator for the inci-
349
dence of a 400 MeV/n-Ne beam to PMMA "90.0 mm. The solid and dashed lines correspond to normal- and higher gain, respectively. The pulse height of the NE102 for various kinds and energy of incident particles was calibrated using the pulse height by the LED light and that by the research pulser. The light output from the NE102 scintillator which we used was then directly regarded as being the response of the NE102 (d¸/dx). Fig. 4 indicates d¸/dx as a function of dE/dx at the entrance surface of the NE102 scintillator. The symbols correspond to our experimental result. The error bars from the FWHM of the measured spectra are plotted together in the "gure; however, most of them are smaller than the symbol size. The result by Becchetti et al. [10] is plotted together with one for the sake of a comparison, as drawn with narrow solid lines. Our d¸/dx by He at the lowest dE/dx data point (indicated with an arrow in the "gure) was normalized to their data in accordance with their d¸/dx given by H at the same dE/dx. Although the response curve indicates a slight particle-species dependency, its global behavior tends to indicate a gradual quenching at the relativistic energy region, while it shows a strong particle-species dependency for decelerated particles shown by Becchetti et al. At "rst, we discuss this universal quenching. 3.1.2. Theoretical scintillation model Some models have been proposed based on a scintillation mechanism. A brief description is given of general models. The response of the scintillator (d¸/dx) was at "rst notated by Birks [3,4], as d¸ S(dE/dx) " , dx 1#kB(dE/dx)
(1)
where S and k represent the scintillation e$ciency and the rate of quenching, respectively, and B is a proportional constant. The conjugation, kB, is often called Birks' parameter. Chou [5] expanded Birks' model by adding a second-order dE/dx term, as Fig. 3. Pulse-height spectra of the NE102 scintillator for the incidence of a Ne-400 MeV/n beam to PMMA"90.0 mm.
d¸ S(dE/dx) " , dx 1#kB(dE/dx)#C(dE/dx)2
(2)
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N. Matsufuji et al. / Nuclear Instruments and Methods in Physics Research A 437 (1999) 346}353
Here, C is a proportional constant for the secondorder term. Wright also handled the second-order dE/dx effect in a di!erent way [6],
A
B
d¸ dE "A log 1#a , dx dx
(3)
where A and a are both proportional constants. Although these three models are identical at the low-dE/dx region, they show a di!erent behavior at the high-dE/dx region. Voltz et al. showed another notation on the response by taking into account the response of an inorganic scintillator proposed by Meyer and Murray [33,34], as [7]
C
G A
d¸ dE "S (1!F )exp !B 1!F s s s dx dx
D
dE #F . s dx
BH (4)
Here, B is a quenching parameter and F is s s a function of Z2/(dE/dx). Ahlen et al. [8] gave a notation of F applicable in the relativistic s region. The F also showed a particle-species des pendency. Tarle expanded the exponential term of Voltz's model as follows [9]:
G
d¸ "S dx
1!F s
dE 1#B (1!F ) s s dx
H
dE #F . s dx
dE/dx is so small that the e!ect of quenching can be negligible. d¸ dE "S . dx dx
(6)
Starting from this relationship, the following simple equation was used to denote the global response by taking into account the universal quenching e!ect independent of particle species:
A B A B A B
d¸ dE dE q "S !Q #const. dx dx dx
(7)
Here, the second term on the right-hand side represents the universal-quenching e!ect with two "tting parameters, Q and q. The bold line in Fig. 4 denotes the result of this "tting. As clearly shown, this function gave a good "t to the data. The values of the "tting parameters were S"1.478, Q"0.299, q"1.56 and const"!1.66. The good agreement between our data and Eq. (7) in Fig. 4 brings about the existence of another, particle-species-independent quenching mechanism on NE102, apart from the previously known particle-species-dependent quenching that is found in the data of Becchetti et al. [10], and is well described by the former theoretical models. The
(5)
This model is often called the BTV model. The latter two models (Voltz and BTV) are both particle-species dependent due to the F term. s Although the former 3 models (Birks, Chou and Wright) are all particle-species independent in the equation itself, a later investigation revealed that the constants used in every equation cannot be global, but particle-species dependent [16}29]. Hence, all of the models are unacceptable to describe the observed particle-species independent universal relationship between d¸/dx and dE/dx. As already mentioned, the models of Birks, Chou and Wright are all simpli"ed as follows when
Fig. 4. d¸/dx as a function of dE/dx at the entrance surface of the NE102 scintillator.
N. Matsufuji et al. / Nuclear Instruments and Methods in Physics Research A 437 (1999) 346}353
particle-species-independent (universal) quenching only depends on dE/dx, as shown in the form of Eq. (7). The channel of the universal quenching or particle-species-dependent quenching becomes dominant for the relativistic and non-relativistic energy region, respectively. Another feature that should be pointed out concerning the universal quenching is that the quenching curve principally bears o! from a linear relation between dE/dx and d¸/dx, even in the low-dE/dx region. This suggests that universal quenching can be caused by any relativistic (low dE/dx) particles from a gross point of view. According to Voltz and Laustrait, the scintillation mechanism can be divided into two components [7,35]. One is a scintillation that occurs in a densely ionized region that is formed along a particle's track (&core'). The other is a scintillation that occurs in a &blob' or &spur', which is caused by high-energy secondary electrons (d-rays) in a sparsely ionized region surrounding the core, called a &halo'. The quenching phenomenon is considered to take place only in the core region. Here, the track structure depends not only on the velocity, but also on the charge of the incident particle. This particle-species dependency in the track structure causes the particle-species-dependent quenching observed in the non-relativistic region. This means that a decrease in dE/dx of the incident particle causes a decrease in the particle-speciesdependent quenching, because the ionization density in the core region also decreases. To summarize the feature of universal quenching, at "rst a decrease in the incident particle's dE/dx leads to a decrease in the ionization density in the core region. This results in a decrease in the particle-species-dependent quenching. Then, the less steep universal quenching is consequently uncovered. By the fact that the extent of universal quenching increases when dE/dx increases, it is suggested that another quenching center might exist for the occurrence of universal quenching. The density of the quenching center relates to dE/dx, but not to the ionization density in the core region. It is expected that future theoretical research about the scintillation mechanism at the relativistic energy region will expand our current understanding of the scintillation model and give a reasonable explanation for universal quenching.
351
Fig. 5. Scintillation e$ciency, d¸/dE, as a function of dE/dx at the entrance surface of the NE102 scintillator.
3.1.3. Scintillation ezciency (d¸/dE) The scintillation e$ciency (d¸/dE) can be derived by dividing the response d¸/dx by dE/dx, as
A B
d¸ d¸ dE ~1 " . dE dx dx
(8)
Fig. 5 shows the scintillation e$ciency as a function of dE/dx together with the data obtained by Becchetti et al. [10]. A global relationship between d¸/dE and dE/dx also appears. The solid line in the "gure corresponds to the universal line between d¸/dx and dE/dx as described in Eq. (7). 3.2. Particle-species dependency As already mentioned in Section 3.1.2, it is suggested that the quenching mechanism comprises two components, i.e., particle-species-dependent quenching is observed in the low-energy region related to track structure, and independent quenching is observed in the relativistic energy region. The obscure particle-species dependency found in our data may suggest a transition of the two mechanisms in this energy region. Therefore, a precise description of the response in this energy region, the establishment of an alternative response model
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N. Matsufuji et al. / Nuclear Instruments and Methods in Physics Research A 437 (1999) 346}353
Table 1 Parameter a and b for each incident beam Beam
a
b
150 MeV/n-He 290 MeV/n-C 400 MeV/n-Ne 490 MeV/n-Si 550 MeV/n-Ar
0.038 0.135 0.246 0.384 0.430
0.929 0.646 0.557 0.507 0.500
Fig. 7. d¸/dx as a function of dE/dx for each beam species. The solid lines in the "gure correspond to the reproduced relationship deduced by this parameterization technique.
Fig. 6. Dependency of the two parameters to log (AZ2).
is required by taking into account any moderate particle-species dependency. Parameterization of the scintillation response with an index log(AZ2) is a useful method to represent for the particle-species dependency of the scintillator [36]. The derived d¸/dx was examined by following this technique. The following simple power equation given below gave good "t to the data on each beam species.
A B A B
d¸ dE bi "a . i dx dx
(9)
Therefore, the equation was adopted at this transitional region instead of theoretical equations described in Section 3.1.2. The right-hand side of the equation involves both scintillation and quenching e!ects in Eq. (7). a and b are the amplii i tude and exponent parameter of element i, respectively. The parameters are summarized in Table 1.
The "tting parameters a and b were then parameterized as a function of log (AZ2). Fig. 6 indicates the dependency of the two parameters to log (AZ2). It is obvious that the dependency of d¸/dx to a particle-species is well presented as a function of log (AZ2). An exponential function was used for the sake of "tting to both parameters as a"0.013721]expM0.37529log(AZ2)N,
(10)
b"1.1878]expM!0.096382log(AZ2)N.
(11)
The increase in the amplitude a as log (AZ2)'s increase is consistent with Becchetti et al.'s data [10]. The decrease in exponent b can be caused by two factors: one is the in#uence of the universal quenching in the high-dE/dx region; the other is that quenching in the core in the case of heavier particles is more signi"cant than that of lighter particles. It is necessary to expand the range of dE/dx to the lower side and to obtain experimental data for the sake of a more precise discussion. The parameterization technique is useful when estimating the response for any unmeasured particle species in between. Fig. 7 demonstrates d¸/dx as a function of dE/dx for each beam species. The solid lines in the "gure correspond to the
N. Matsufuji et al. / Nuclear Instruments and Methods in Physics Research A 437 (1999) 346}353
reproduced relationship deduced by this parameterization technique. As shown in the "gure, it agrees well to the data.
4. Conclusion The response of the NE102 scintillator to passing-through He, C, Ne, Si and Ar ions was investigated. The results reveal: f A relationship between dE/dx and d¸/dx was derived at incident energies from 120 MeV up to 18 GeV for the "rst time. This result suggests the existence of a universal quenching mechanism in the high-dE/dx region that is independent of the particle species. f The parameter log (AZ2) well represents the particle-species dependency of the NE102's response. The amplitude and exponent parameters when power-"tting d¸/dx with dE/dx show an exponential relationship with log (AZ2).
Acknowledgements The authors wish to express their gratitude to all members in the division of accelerator physics and engineering of NIRS for their kind support to this work. The skillful work of the HIMAC operation sta! is also acknowledged. This work was carried out as a part of the Research Project with Heavy Ions at HIMAC.
References [1] [2] [3] [4]
M. Belli et al., Int. J. Rad. Biol. 55 (1989) 93. N. Matsufuji, T. Kanai, T. Kohno, Rad. Res, submitted. J.B. Birks, Proc. Phys. Soc. A 64 (1951) 874. J.B. Birks, The Theory and Practice of Scintillation Counting, Pergamon, London, 1964.
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[5] C.N. Chou, Phys. Rev. 87 (1952) 904. [6] G.T. Wright, Phys. Rev. 91 (1953) 1282. [7] R. Voltz, J. Ropes da Silva, G. Laustrait, A. Coche, J. Chem. Phys. 45 (1966) 3306. [8] S.P. Ahlen, B.G. Cartwright, G. Tarle, Nucl. Instr. and Meth. 147 (1977) 321. [9] G. Tarle, S.P. Ahlen, B.G. Cartwright, Astrophys. J. 230 (1979) 607. [10] F.D. Becchetti, C.E. Thorn, M.J. Levine, Nucl. Instr. and Meth. 138 (1976) 93. [11] B.J. McParland, Nucl. Instr. and Meth. A 251 (1986) 412. [12] L. Muga, G. Gri$th, Phys. Rev. B 9 (1974) 3639. [13] F.D. Brooks, W.A. Cilliers, M.S. Allie, Nucl. Instr. and Meth. A 240 (1985) 338. [14] Z. Milosevich et al., Nucl. Instr. and Meth. Phys. Res. A A 320 (1992) 471. [15] Z. Papandreou et al., Nucl. Instr. and Meth. Phys. Res. B 34 (1988) 454. [16] T.J. Gooding, H.G. Pugh, Nucl. Instr. and Meth. 7 (1960) 189. [17] M. Buenerd et al., Nucl. Instr. and Meth. 136 (1976) 173. [18] W.R. Webber, J.F. Ormes, T. Von Rosenvinge, Proceedings of the 9th International Conference on Cosmic Rays, London, 1965, Vol. 1, The Inst. of Phys. and the Phys. Soc., London, p. 407. [19] R. Dwyer, D. Zhou, Nucl. Instr. and Meth. A 242 (1985) 171. [20] M.H. Salamon, S.P. Ahlen, Nucl. Instr. and Meth. 195 (1982) 557. [21] J.R. Prescott, A.S. Rupaal, Can. J. Phys. 39 (1961) 221. [22] H.C. Evans, E.H. Bellamy, Proc. Phys. Soc. 74 (1959) 483. [23] M. Gettner, W. Selove, Rev. Sci. Inst. 31 (1960) 450. [24] W.W. Daehnick, J.M. Fowler, Phys. Rev. 111 (1958) 1309. [25] D.E. Groom, M.G. Hauser, Nucl. Instr. and Meth. 46 (1967) 301. [26] E. Newmann, A.M. Smith, F.E. Steigert, Phys. Rev. 122 (1961) 1520. [27] G.D. Badhwar, C.L. Deney, B.R. Dennis, M.F. Kaplon, Nucl. Instr. and Meth. 57 (1967) 116. [28] M. Hirschberg et al., IEEE Trans. Nucl. Sci. 39 (1992) 511. [29] M.A. McMahan, IEEE Trans. Nucl. Sci. 35 (1988) 42. [30] N. Matsufuji et al., Nucl. Instr. and Meth. A 430 (1999) 60. [31] T. Kanai et al., Int. J. Rad. Oncol. Biol. Phys. 44 (1999) 201. [32] L. Sihver, D. Schardt, T. Kanai, Jpn. J. Med. Phys. 18 (1998) 1. [33] R.B. Murray, A. Meyer, Phys. Rev. 122 (1961) 815. [34] A. Meyer, R.B. Murray, Phys. Rev. 128 (1962) 98. [35] G. Laustrait, Mol. Cryst. 4 (1968) 127. [36] Z. Dlouhy et al., Nucl. Instr. and Meth. A 317 (1992) 604.
The response of a NE102 scintillator to passing-through relativistic heavy ions Naruhiro Matsufuji!,*, Tatsuaki Kanai!, Hideaki Komami", Toshiyuki Kohno" !Division of Accelerator Physics and Engineering, National Institute of Radiological Sciences, 263-8555 Chiba, Japan "Department of Energy Science, Tokyo Institute of Technology, 226-8502 Yokohama, Japan Received 17 June 1999; accepted 6 July 1999
Abstract The response of a NE102 scintillator to He, C, Ne, Si and Ar ions has been studied at incident energies from 120 MeV up to 18 GeV when used as a transmission (*E) counter. The energy of the incident beam was changed by inserting an absorber of various thicknesses made of PMMA plates. The primary particles were identi"ed and separated from projectile fragments by using the *E}E counter telescope method. A thin NE102 plastic scintillator and a BGO scintillator were used as a *E counter and a residual E counter, respectively. The energy of the primary particles was deduced by comparing the depth-dose distribution measured by an ionization chamber to those by a theoretical calculation. The light output of the NE102, d¸/dx, was found to be linear to dE/dx in the low-dE/dx region, while gradually being quenched in the high-dE/dx region. This quenching was regarded as being a phenomenon independent of the particle species. The residual particle-species dependency was well indexed as a function of log (AZ2). ( 1999 Elsevier Science B.V. All rights reserved. PACS: 29.40.M; 29.30; 07.77.K Keywords: NE102; response; high energy; heavy ion; *E
1. Introduction The fragmentation of projectiles has been regarded as being one of the most important reactions of relativistic heavy ions, i.e., several hundred MeV/n of heavy ions to be considered regarding its medical applications by the fact that the RBE (Relative Biological E!ectiveness) is strongly dependent on the particle species [1].
* Corresponding author. Tel.:#81-43-251-2111 6873; fax: #81-43-251-1840. E-mail address: [email protected] (N. Matsufuji)
The *E-E counter telescope method is one of the commonly used techniques used to identify the species of fragment particles. As suggested by the Bethe}Bloch's formula, *E(dE/dx) of a charged particle is proportional to the kind of particle (AZ2), and is inversely proportional to its energy (E) (A: mass number, Z: atomic number). When investigating the #uence of fragments, good separation between light fragment species (hydrogen and helium) is required, because the numbers of produced light fragments is much larger compared with that of other heavier fragment species [2]. In general, the response of a scintillator is governed not only by the energy deposited in the scintillator, but
0168-9002/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 7 5 5 - X
N. Matsufuji et al. / Nuclear Instruments and Methods in Physics Research A 437 (1999) 346}353
also by the species of the incident particles. Here, the particle-species dependence tends to enhance the response for lighter particle. Because the output of a semiconductor detector principally re#ects the energy loss only in the detector, using a scintillator as a *E counter can be superior to using a semiconductor detector for the sake of separating lighter particle species. NE102 (equivalent to BC400) is one of the most commonly used plastic scintillators aimed at particle counting. Beginning with the work by Birks [3,4], theoretical approaches to the scintillation mechanism have been carried out [5}9]. Many e!orts have also been made from experimental points of view [10}29]. Among them, Becchetti et al. comprehensively investigated the response of NE102 for various kinds of low-energy heavy ions stopped in a scintillator [10]. Salamon and Ahlen reported on the response at the relativistic energy region, but on di!erent types of scintillators [20]. In this paper, the response of the NE102 scintillator to passing-through He, C, Ne, Si and Ar ions at relativistic energy region is reported for the "rst time.
2. Experimental technique The experimental technique used in this study has been described in detail elsewhere [30]; therefore, only a brief description is given below. 2.1. Irradiation of heavy-ion beams An experiment was carried out at a beam port for biological experiments at HIMAC (Heavy Ion Medical Accelerator in Chiba) of NIRS. The kind of primary beams used for the experiment were as follows: 150 MeV/n of 4He, 290 MeV/n of 12C, 400 MeV/n of 20Ne, 490 MeV/n of 28Si and 550 MeV/n of 40Ar. Energy spreading is regarded to have been less than 0.1%. The counter complex used in this study was principally designed for the measurement of the #uence and energy spectra of a therapeutic beam for each fragment species from primaries down to hydrogen [2]. Therefore, the incident beam was broadened to 100 mm in diameter by a wobbler
347
method [31] with more than $2% of the lateral uniformity, in the same manner as in the general case of therapy at HIMAC. A stack of polymethyl methacrylrate (PMMA) plates, called binary "lter, was used to decrease the energies of the primary particles by inserting various thicknesses of the plates. The utilized scintillation counters were positioned at the irradiation point, 300 mm downstream from the binary "lter. Measurements were carried out while changing the thickness of the binary "lter variously. 2.2. Scintillation counters A disk-like NE102 plastic scintillator (5 mm in thickness and 23.9 mm in diameter) was used for investigating the response. The NE102 was polished well and mounted in a light guide made of PMMA. Entrance and exit windows were made of a "lm of aluminized polyethylene terephthalate (Mylar) 2.4 mg/cm2 in thickness. A HAMAMATSU H1161 PMT was attached to a NE102 scintillator via a PMMA light guide. The voltage applied to the PMT was !1400 V for helium, !1100 V for carbon, neon and silicon, and !1000 V for argon. The PMT signal was at "rst processed by an ORTEC 113 preampli"er. A resistor of 150 k) was added between the input signal line and the ground for the sake of reducing the input impedance. The output from the preampli"er was divided into two. One line was connected to an ampli"er with a higher gain than that of the other line so as to detect lighter fragments, such as hydrogen and helium. To eliminate fragment particles from primary particles by the *E}E counter telescope method, a BGO scintillator was attached next to the NE102 scintillator. The BGO crystal has a rectangular parallelepiped form of 40 mm in width and height and 300 mm in length. The length corresponds to the range of 1H-525 MeV. The energy response of the BGO scintillator is reported in Ref. [30]. As a calibration source, stabilized green light emitted from a 1N6094 LED, driven by a TENNELEC TC814 pulser, was used. An optical "ber cable was attached to the light guide of both the NE102 and the BGO scintillators to deliver the LED light directly. Calibration using the LED light
348
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Fig. 1. Schematic layout of the detector system.
was periodically carried out between experimental runs together with additional calibration of the electronics by an ORTEC 448 research pulser. Another large-area NE102 scintillator of 1 mm thickness and 150 mm width and height was used as a beam monitor to count the total number of primary particles. The beam-monitor scintillator was "xed just behind a beam exit window, 4300 mm upstream from the irradiation site. A schematic layout of the detector system is summarized in Fig. 1. 2.3. Particle identixcation Fig. 2 shows a two-dimensional scatter plot of the light output of the NE102 scintillator (normal gain), which corresponds to the energy loss (*E), and that of the BGO scintillator, which corresponds to the residual energy (E). The plot was taken from the incidence of a 400 MeV/n of Ne beam to the binary "lter (90.0 mm in thickness). As shown in the "gure, fragment particles were clearly
Fig. 2. Two-dimensional scatter plot of the light output of the NE102 scintillator (low gain), which corresponds to the energy loss (*E) and that of the BGO scintillator which corresponds to the residual energy (E). The plot was taken from the incidence of a 400 MeV/n of Ne beam to the binary "lter (90.0 mm in thickness).
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separated by the di!erence in the element as some di!erent bands. Primary particles were then well distinguished from fragments by marking ROI to the highest band, which corresponds to the primaries in the plot. 2.4. Energy scaling The depth}dose pro"le was measured using a parallel-plate ionization chamber on each incident beam for the sake of calibrating the response in terms of energy. The depth}dose pro"le was also calculated with a simulation code of the fragmentation reaction developed by Sihver et al. [32]. The incident energy as well as the residual energy of primary particles after passing through any thickness of binary "lter were derived by "tting the measured pro"le with the calculated one. Most of the error expected in the calculation was due to the error in the stopping power; however, its extent was estimated to be $2% at maximum.
3. Results and discussion 3.1. Global response 3.1.1. d¸/dx as a function of dE/dx Fig. 3 shows an example of the raw energy spectra from the NE102 scintillator for the inci-
349
dence of a 400 MeV/n-Ne beam to PMMA "90.0 mm. The solid and dashed lines correspond to normal- and higher gain, respectively. The pulse height of the NE102 for various kinds and energy of incident particles was calibrated using the pulse height by the LED light and that by the research pulser. The light output from the NE102 scintillator which we used was then directly regarded as being the response of the NE102 (d¸/dx). Fig. 4 indicates d¸/dx as a function of dE/dx at the entrance surface of the NE102 scintillator. The symbols correspond to our experimental result. The error bars from the FWHM of the measured spectra are plotted together in the "gure; however, most of them are smaller than the symbol size. The result by Becchetti et al. [10] is plotted together with one for the sake of a comparison, as drawn with narrow solid lines. Our d¸/dx by He at the lowest dE/dx data point (indicated with an arrow in the "gure) was normalized to their data in accordance with their d¸/dx given by H at the same dE/dx. Although the response curve indicates a slight particle-species dependency, its global behavior tends to indicate a gradual quenching at the relativistic energy region, while it shows a strong particle-species dependency for decelerated particles shown by Becchetti et al. At "rst, we discuss this universal quenching. 3.1.2. Theoretical scintillation model Some models have been proposed based on a scintillation mechanism. A brief description is given of general models. The response of the scintillator (d¸/dx) was at "rst notated by Birks [3,4], as d¸ S(dE/dx) " , dx 1#kB(dE/dx)
(1)
where S and k represent the scintillation e$ciency and the rate of quenching, respectively, and B is a proportional constant. The conjugation, kB, is often called Birks' parameter. Chou [5] expanded Birks' model by adding a second-order dE/dx term, as Fig. 3. Pulse-height spectra of the NE102 scintillator for the incidence of a Ne-400 MeV/n beam to PMMA"90.0 mm.
d¸ S(dE/dx) " , dx 1#kB(dE/dx)#C(dE/dx)2
(2)
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Here, C is a proportional constant for the secondorder term. Wright also handled the second-order dE/dx effect in a di!erent way [6],
A
B
d¸ dE "A log 1#a , dx dx
(3)
where A and a are both proportional constants. Although these three models are identical at the low-dE/dx region, they show a di!erent behavior at the high-dE/dx region. Voltz et al. showed another notation on the response by taking into account the response of an inorganic scintillator proposed by Meyer and Murray [33,34], as [7]
C
G A
d¸ dE "S (1!F )exp !B 1!F s s s dx dx
D
dE #F . s dx
BH (4)
Here, B is a quenching parameter and F is s s a function of Z2/(dE/dx). Ahlen et al. [8] gave a notation of F applicable in the relativistic s region. The F also showed a particle-species des pendency. Tarle expanded the exponential term of Voltz's model as follows [9]:
G
d¸ "S dx
1!F s
dE 1#B (1!F ) s s dx
H
dE #F . s dx
dE/dx is so small that the e!ect of quenching can be negligible. d¸ dE "S . dx dx
(6)
Starting from this relationship, the following simple equation was used to denote the global response by taking into account the universal quenching e!ect independent of particle species:
A B A B A B
d¸ dE dE q "S !Q #const. dx dx dx
(7)
Here, the second term on the right-hand side represents the universal-quenching e!ect with two "tting parameters, Q and q. The bold line in Fig. 4 denotes the result of this "tting. As clearly shown, this function gave a good "t to the data. The values of the "tting parameters were S"1.478, Q"0.299, q"1.56 and const"!1.66. The good agreement between our data and Eq. (7) in Fig. 4 brings about the existence of another, particle-species-independent quenching mechanism on NE102, apart from the previously known particle-species-dependent quenching that is found in the data of Becchetti et al. [10], and is well described by the former theoretical models. The
(5)
This model is often called the BTV model. The latter two models (Voltz and BTV) are both particle-species dependent due to the F term. s Although the former 3 models (Birks, Chou and Wright) are all particle-species independent in the equation itself, a later investigation revealed that the constants used in every equation cannot be global, but particle-species dependent [16}29]. Hence, all of the models are unacceptable to describe the observed particle-species independent universal relationship between d¸/dx and dE/dx. As already mentioned, the models of Birks, Chou and Wright are all simpli"ed as follows when
Fig. 4. d¸/dx as a function of dE/dx at the entrance surface of the NE102 scintillator.
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particle-species-independent (universal) quenching only depends on dE/dx, as shown in the form of Eq. (7). The channel of the universal quenching or particle-species-dependent quenching becomes dominant for the relativistic and non-relativistic energy region, respectively. Another feature that should be pointed out concerning the universal quenching is that the quenching curve principally bears o! from a linear relation between dE/dx and d¸/dx, even in the low-dE/dx region. This suggests that universal quenching can be caused by any relativistic (low dE/dx) particles from a gross point of view. According to Voltz and Laustrait, the scintillation mechanism can be divided into two components [7,35]. One is a scintillation that occurs in a densely ionized region that is formed along a particle's track (&core'). The other is a scintillation that occurs in a &blob' or &spur', which is caused by high-energy secondary electrons (d-rays) in a sparsely ionized region surrounding the core, called a &halo'. The quenching phenomenon is considered to take place only in the core region. Here, the track structure depends not only on the velocity, but also on the charge of the incident particle. This particle-species dependency in the track structure causes the particle-species-dependent quenching observed in the non-relativistic region. This means that a decrease in dE/dx of the incident particle causes a decrease in the particle-speciesdependent quenching, because the ionization density in the core region also decreases. To summarize the feature of universal quenching, at "rst a decrease in the incident particle's dE/dx leads to a decrease in the ionization density in the core region. This results in a decrease in the particle-species-dependent quenching. Then, the less steep universal quenching is consequently uncovered. By the fact that the extent of universal quenching increases when dE/dx increases, it is suggested that another quenching center might exist for the occurrence of universal quenching. The density of the quenching center relates to dE/dx, but not to the ionization density in the core region. It is expected that future theoretical research about the scintillation mechanism at the relativistic energy region will expand our current understanding of the scintillation model and give a reasonable explanation for universal quenching.
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Fig. 5. Scintillation e$ciency, d¸/dE, as a function of dE/dx at the entrance surface of the NE102 scintillator.
3.1.3. Scintillation ezciency (d¸/dE) The scintillation e$ciency (d¸/dE) can be derived by dividing the response d¸/dx by dE/dx, as
A B
d¸ d¸ dE ~1 " . dE dx dx
(8)
Fig. 5 shows the scintillation e$ciency as a function of dE/dx together with the data obtained by Becchetti et al. [10]. A global relationship between d¸/dE and dE/dx also appears. The solid line in the "gure corresponds to the universal line between d¸/dx and dE/dx as described in Eq. (7). 3.2. Particle-species dependency As already mentioned in Section 3.1.2, it is suggested that the quenching mechanism comprises two components, i.e., particle-species-dependent quenching is observed in the low-energy region related to track structure, and independent quenching is observed in the relativistic energy region. The obscure particle-species dependency found in our data may suggest a transition of the two mechanisms in this energy region. Therefore, a precise description of the response in this energy region, the establishment of an alternative response model
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Table 1 Parameter a and b for each incident beam Beam
a
b
150 MeV/n-He 290 MeV/n-C 400 MeV/n-Ne 490 MeV/n-Si 550 MeV/n-Ar
0.038 0.135 0.246 0.384 0.430
0.929 0.646 0.557 0.507 0.500
Fig. 7. d¸/dx as a function of dE/dx for each beam species. The solid lines in the "gure correspond to the reproduced relationship deduced by this parameterization technique.
Fig. 6. Dependency of the two parameters to log (AZ2).
is required by taking into account any moderate particle-species dependency. Parameterization of the scintillation response with an index log(AZ2) is a useful method to represent for the particle-species dependency of the scintillator [36]. The derived d¸/dx was examined by following this technique. The following simple power equation given below gave good "t to the data on each beam species.
A B A B
d¸ dE bi "a . i dx dx
(9)
Therefore, the equation was adopted at this transitional region instead of theoretical equations described in Section 3.1.2. The right-hand side of the equation involves both scintillation and quenching e!ects in Eq. (7). a and b are the amplii i tude and exponent parameter of element i, respectively. The parameters are summarized in Table 1.
The "tting parameters a and b were then parameterized as a function of log (AZ2). Fig. 6 indicates the dependency of the two parameters to log (AZ2). It is obvious that the dependency of d¸/dx to a particle-species is well presented as a function of log (AZ2). An exponential function was used for the sake of "tting to both parameters as a"0.013721]expM0.37529log(AZ2)N,
(10)
b"1.1878]expM!0.096382log(AZ2)N.
(11)
The increase in the amplitude a as log (AZ2)'s increase is consistent with Becchetti et al.'s data [10]. The decrease in exponent b can be caused by two factors: one is the in#uence of the universal quenching in the high-dE/dx region; the other is that quenching in the core in the case of heavier particles is more signi"cant than that of lighter particles. It is necessary to expand the range of dE/dx to the lower side and to obtain experimental data for the sake of a more precise discussion. The parameterization technique is useful when estimating the response for any unmeasured particle species in between. Fig. 7 demonstrates d¸/dx as a function of dE/dx for each beam species. The solid lines in the "gure correspond to the
N. Matsufuji et al. / Nuclear Instruments and Methods in Physics Research A 437 (1999) 346}353
reproduced relationship deduced by this parameterization technique. As shown in the "gure, it agrees well to the data.
4. Conclusion The response of the NE102 scintillator to passing-through He, C, Ne, Si and Ar ions was investigated. The results reveal: f A relationship between dE/dx and d¸/dx was derived at incident energies from 120 MeV up to 18 GeV for the "rst time. This result suggests the existence of a universal quenching mechanism in the high-dE/dx region that is independent of the particle species. f The parameter log (AZ2) well represents the particle-species dependency of the NE102's response. The amplitude and exponent parameters when power-"tting d¸/dx with dE/dx show an exponential relationship with log (AZ2).
Acknowledgements The authors wish to express their gratitude to all members in the division of accelerator physics and engineering of NIRS for their kind support to this work. The skillful work of the HIMAC operation sta! is also acknowledged. This work was carried out as a part of the Research Project with Heavy Ions at HIMAC.
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