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Ionization and scintillation produced by relativistic Au, He and H ions in liquid argon
Nuclear Instruments and Methods in Physics Research North-Holland, Amsterdam
A260 (1987) 437-442
437
IONIZATION AND SCINTILLATION PRODUCED BY RELATIVISTIC An, He AND H IONS IN LIQUID ARGON E. SHIBAMURA 1), H.J . CRAWFORD 2), T. DOKE 3), J.M . ENGELAGE 2), I. FLORES 4), 4), A. HITACHI 3), J. KIKUCHI 3), P.J . LINDSTROM K. MASUDA 1> and K. OGURA 5) 1) Saitama College of Health, Kamiokubo, Urawa-shi, Sauama 338, Japan 21 University of California Space Science Laboratory, Berkeley, California 94720, USA s) Science and Engineering Research Laboratory Waseda University, Kikuicho, Shinluku-ku, Tokyo 162, Japan 4) Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720, USA sl College of Industrial Technology, Nihon University, Izumicho, Narashino-shi, Chiba 275, Japan
Received
5
May
1987
We have measured ionization and scintillation produced by relativistic ions of Au, He and H in liquid argon. The sum of ionization signal and scintillation signal per unit energy deposition is the same for He and H ions, which is also the same as that for relativistic Ne, Fe and La ions previously measured. We have found that quenching occurs when liquid argon is irradiated by relativistic Au ions and that the sum per unit energy deposition for the Au ions is 70-76% of that for the other ions mentioned above. 1. Introduction Ionization and scintillation have been investigated in liquid argon irradiated by various relativistic particles. One of the characteristic features is that the ionization and the scintillation are complementary. Our first experiment [1] with heavy ions showed that the scintillation yield per unit energy deposited in liquid argon is the same for relativistic Ne and Fe ions . On the other hand, the yield for slow alpha particles from 21° Po is 0.71 of that for relativistic Ne or Fe ions. The sum of ionization and scintillation per unit energy is independent of applied electric field and is the same for high energy electrons, Ne and Fe particles. It is concluded that all electron-ion pairs and excitons produced by a relativistic particle are converted to either ionization charge or scintillation light in liquid argon. So, by use of the proper conversion factor a, the summation I + aS of ionization I and scintillation S is proportional to the deposited energy 4 E. The fraction of the ionization or the scintillation depends on the applied electric field. Our second experiment [2] showed that the ratio of the sum to deposited energy for relativistic La ions is almost the same as that for Ne and Fe ions . It is therefore interesting to see the ratio as a function of atomic number of incident ions . Recently we made a measurement of ionization and scintillation in liquid argon for relativistic Au, He and H ions . Although the ratio was constant for He and H ions, a significant reduction in the ratio was found for Au ions . In this paper, we will present the experimental results 0168-9002/87/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
and give a perspective for fast incident ions from Z = 1 through Z = 79 on ionization and scintillation in liquid argon. 2. Experimental The experimental apparatus is the same as that used in a previous experiment [2] and will be briefly described here . The detector is a gridded ionization chamber filled with liquid argon, and consists of circular electrodes set in parallel as shown in fig. 1 . The distance is 4 mm between the cathode (K) and the grid (G) and 3 mm between G and the collector (C). The chamber was placed so that the incident beam passed through the chamber between K and G parallel to the electrodes.
Pyrex window
Fig. 1. Arrangement of electrodes . K; cathode, G; grid, C; collector, S; 21°Po source, W.L .S. ; wavelength shifter. All dimensions are in mm .
438
E Shibamura et al. / Ionization and scintillation to liquid argon
The effective diameter of the chamber is 2.0 cm, which gives the path length of the ion beam to be measured . The actual position of each ion track was determined by two-dimensional position-sensitive Si detectors located at the front and at the back of the chamber. A 210po alpha source was deposited in the central region of the cathode surface and was used for calibration of scintillation signals.
In the present experiment, the incident ions were Au with an incident energy of 1.08 GeV/amu, He of 1 .05 GeV/amu and H of 1 .05 GeV/amu. Hydrogen ions were prepared by spallation of He ions in a thin foil of Be . The average energy loss was estimated by integrating the Bethe-Mott-Bloch energy loss formula (eq. (4 .22) of ref. [3]) along the ion track using a mean excitation energy of 210 eV for argon [3]. The estimated
512
a 384
256 U
128
64
128
S(0)
S(0)
192
(channels)
(channels)
256
Fig. 2. Pulse height distribution of scintillation signal S(0) obtained under zero electric field for (a) Au ions, (b) He ions and H ions
E. Shibamura et al / Ionization and scintillation m liquid argon energies of the incident ions at the entrance of the 2 cm path in liquid argon are 870 MeV/amu for Au ions, 1 .04 GeV/amu for He ions and 1 .04 GeV/amu for H ions . The average energy losses in the liquid argon were 32 .4 GeV for Au, 17 .9 MeV for He and 4.47 MeV for H. Charge collected by the electric field was measured by a charge-sensitive amplifier. Scintillation light was detected by a photomultiplier through a transparent grid and collector and a Pyrex window with wavelength shifter. The photomultiplier was operated at constant high voltage throughout the experiment, so that the intensities of the scintillation light were directly comparable .
439
Fig . 3 . The scintillation intensity S(E) (0) and the collected charge I (O) for an electric field E between the cathode and the grid . The scintillation intensity is normalized to S(0) . The charge is normalized to I_ =,A El W.
3 . Results and discussion Pulse height spectra of scintillation at zero electric field are shown in fig . 2a for Au ions, and in fig. 2b for He and H ions . For H ions, a considerable asymmetry is seen . The shape of the energy loss distribution is determined by the parameter, K (
= 0wmax , =2anZ2e4x/mv2),
where win _ is the maximum energy transfer in collisions with atomic electrons of argon, n is the number density of electrons in liquid argon, Zt is the atomic number of the incident ion, v is the velocity of the incident ion, x is the thickness of liquid argon and e and m are the charge and mass of the electron . The value of K is 0 .070 for H ions and 0 .28 for He ions . The shape of the energy loss distribution for these values of K is given by Vavilov's distribution function . The most probable energy loss d Emp was calculated to be 4.04 MeV for H ions and 17 .0 MeV for He ions by a computer program given by Badhwar et al . [4] . It is difficult to obtain a mean pulse height from the experimental distribution, so we restrict our consideration to the most probable energy loss values . On the other hand, the parameter K is 700 for the Au ions which implies that the mean energy loss equals the most probable energy loss . Fig . 3 shows the relative signal intensity as a function of applied field . The symbols S(0) and S(E) denote the magnitude of the signals from scintillation for zero field and for an applied field E, respectively . Similarly, I and I,, denote the collected charge at field E and E = oc, respectively . The value of I. is given by I_ =dE/W, where W=23 .6 eV [5] . For Au ions, the collected charge and the reduction in scintillation seem too small at low electric field . This effect is believed to be due to a reduction in the electric field caused by a space charge effect similar to that
observed previously for La ions [2]. In the case of H and He ions, more than 80% of the produced charge was collected over 4 .5 kV/cm. In contrast, less than 30% of the produced charge was collected for Au ions at the highest electric field of E = 7 .5 kV/cm . This sharply reduced value is due to the recombination between electrons and ions along the highly ionized track of the Au ion in liquid argon. Fig. 4 is a scatter plot of scintillation (x-axis) and ionization (y-axis) signals obtained for Au ions under an electric field E of 1 .5 kV/cm (a) and 6 .0 kV/cm (b) . The groups of points (c) and (d) correspond to ions passing through the chamber between the grid and the collector at E = 1 .5 and 6.0 kV/cm, as was seen previously for La ions [2] . The figure demonstrates the complementary feature of scintillation and ionization signals which was already shown in fig . 3 . Here, let us make a summation of scintillation and ionization . The ionization signal can be measured in numbers of electrons by the use of a charge-sensitive preamplifier with a well calibrated capacitor for test pulse . On the other hand, the scintillation intensity can be obtained in numbers of emitted photons by comparing the measured pulse height with that from a source of known scintillation intensity under the same conditions . In the previous paper [1] we showed that the number of photons Nph _, produced by alpha particles from 21° Po under zero electric field is given by Nph-~ = gaE~1 W',
( 2)
if we assume that the quantum efficiency of photon production is unity [6], where q . (= 0.71) is the quenching factor * for the scintillation from alpha particles
* In ref . [1], the symbol rl a was called the quantum efficiency for the scintillation from alpha particles in liquid argon, instead of q. .
E. Shibamura et al. / Ionization andscintillation in liquid argon 384
_N C C O -C U v C O
192
O N C O
0
0
Scintillation (channels) Fig. 4. Correlation between scintillation signals and charge signals. The data obtained at E =1 .5 kV/cm (a and c) and E = 6.0 kV/cm (b and d) are superposed.
and E. (= 5.3 MeV) the energy of the alpha particle . The symbol W' denotes the effective W value for photon production in liquid argon under zero electric field and W' = W/(1 + Nex /N,) = 19 .5 eV, where N., and N, are the numbers of produced excitons and electron-ion pairs, respectively . So the number of photons Nph produced by the incident ion is obtained from the scintillation signal S due to the ion by the relation
equals unity if there is no quenching, otherwise q gives the quenching factor [71. Fig. 5 shows q as a function of electric field E for Au, He and H ions . In the case of He or H ions, q is smaller than unity at zero electric field. This result is not due to quenching but to the presence of electrons
Npli - ( SIS. ) Nph-aIf ,
where Sq is the scintillation signal due to the alpha particle from the 21° po source at E = 0 and f (=1.15) is the ratio of the solid angle for detecting scintillation produced by incident ions to the solid angle for detecting scintillation produced by the alpha particles. The solid angle for incident ions was calculated by averaging along the track of the ion. The conversion factor a from scintillation signals S to emitted photon numbers Nph is given by a = Nph/S = (ga Ea)l( W'Saf ).
Thus the sum of ionization and scintillation signals I + aS gives the total number of emitted photons and collected electrons. Namely, the ratio q-(I+aS)/(aE/W')=(I+aS)/(N,+N,)
(5)
Electric Field (kV/cm)
Fig. 5. Sum signals of ionization and scintillation normalized on the basis of calibration with scintillation of alpha particles for Au ions, He ions and H ions (open circles and solid line). Closed circles and dashed line express the sum signals for Au ions obtained on the assumption that the sum should be equal at E = 0 and at E = 7.5 kV/cm.
E. Shibamura et al. / Ionization and scintillation m liquid argon that escape recombination . A similar effect was observed previously for incident electrons [8] . The value of q increases with E and becomes compatible with unity at high fields, which might be explained by the effect of electronegative impurities in the low field region [8] . In the case of Au ions the quenching factor q is about 0 .7, and varies only slightly with E. This means that about 30% of the energy deposited in liquid argon by relativistic Au ions was lost without making any ionization or scintillation signal . For La ions [2], q was almost equal to unity over the applied electric field except for the region of 0 .9 < E < 2 .7 kV/cm, where the effect of space charge was significant as was mentioned before . If we assume that q is independent of field E even for the case of q < l, we can obtain another conversion factor a for Au ions by equating q at zero field and high field . Closed circles in fig . 5 show the quenching factor q for various electric fields E obtained on the assumption that q is equal at E = 0 and at E = 7 .5 kV/cm . In this case, q equals 0 .76 . In this figure, there seems to be a small peak at E = 3 .0 kV/cm for Au ions . This might be due to the effect of positive ions produced by incident Au ions as explained below . The produced electron-ion pairs recombine to make scintillation in the region near the grid G, more than in the region near the cathode K, because the externally applied electric field is significantly weakened by the space charge field near the grid [2] . The scintillation emitted near the grid is detected by the photomultiplier with a solid angle larger than that for the beam passing through the middle of the K-G gap . Therefore, the nonuniform effect of the space charge increases the sum signal a little. The recent experiment and the theory of energy loss by relativistic heavy ions show a discrepancy of about 5% around Au ions [9] . The resulting uncertainty in the magnitude of the energy loss directly affects the value of q, as seen before. Moreover, the experimental uncertainties in the determination of beam position with respect to the electrodes directly affect the comparison of the scintillation yields from Au and alpha particles . Therefore, the values of q = 0 .7 and q = 0 .76 should be considered to be within the experimental uncertainties . As seen in fig. 5, q equals unity for He and H ions at high fields E . For No and Fe ions, q was also unity [1]. We can conclude here that the quenching factor q is unity for relativistic incident ions with atomic numbers Z from 1 to 57 and that q decreases for larger Z. The sum signal multiplied by W' is plotted in fig. 6 versus deposited energy AE for 1 MeV electrons [1] and relativistic H, He, Ne [1], Fe [1], La [2] and Au ions . This figure demonstrates a linear relation between the magnitude of the measured signal, (I + aS)W', and the energies absorbed in liquid argon . The maximum deviation is 2% except for Au ions .
44 1
10 5
AE
(MeV)
Fig . 6. Relation between the energy d E absorbed in liquid argon and the amplitude of the sum signal in units of energy .
4. Conclusion
Ionization and scintillation for relativistic Au, He and H ions have been observed in liquid argon . It was shown that the sum of the ionization and the scintillation is proportional to the absorbed energy A E for incident H ions (Z= 1) to La ions (Z = 57), and is given by AE/ W', where W' = 19 .5 eV. The sum per absorbed energy for Au ions is 70-76% of that for the ions mentioned above .
Acknowledgements
The authors would like to express their thanks to Prof. K . Nagata for his support in the calculation of energy loss and to Prof. P .N . Kirk for his reading of the manuscript and for his helpful comments . This work was supported by the Director, Office of Energy Research, Division of Nuclear Physics of the Office of High Energy and Nuclear Physics of the US Department of Energy under Contract DE-AC0376SF00098 .
References
[1] T . Doke, H .J . Crawford, C .R . Gruhn, A. Hitachi, J . Kikuchi, K . Masida, S . Naganuya, E. Shibamira and S . Tamada, Nucl . Instr. and Meth. A235 (1985) 136. [2] H .J . Crawford, T . Doke, A . Hitachi, J . Kikuchi, P.J . Lind-
442
[31 [41
[51
[61
E. Shubamura et al. / Ionization and scintillation in liquid argon strom, K. Masuda, S. Nagamiya and E. Shibamura, Nucl . Instr. and Meth . A256 (1987) 47 . S.P. Ahlen, Rev. Mod. Phys . 52 (1980) 121 . G.D. Badhwar, Nucl. Instr. and Meth. 109 (1973) 119; J.H . Adams, Jr ., R. Silberberg and G.D . Badhwar, Nucl. Instr and Meth . 124 (1975) 551 . M. Miyajima, T. Takahashi, S. Konno, T. Hamada, S. Kubota, E. Slnbamura and T. Doke, Phys. Rev. A9 (1974) 1438 . S. Suzuki, T. Doke, A. Hitachi, J. Kikuchi, A. Yunoki and K. Masuda, Nucl . Instr. and Meth . A245 (1986) 366.
[71 A. Hitachi, A. Yunoki, T. Doke and T. Takahashi, Phys . Rev. A35 (1987) 3956 . [81 T. Doke, A. Hitachi, J. Kikuchi, K. Masuda, S. Tamada, A Mozumder, E. Siibamura and T. Takahashi, Chem . Phys. Lett . 115 (1985) 164. [91 C.J. Waddington, D.J . Fixen, H.J . Crawford, P.J. Lmdstrom and H.H . Heckman, Phys . Rev. A34 (1986) 3700 .
A260 (1987) 437-442
437
IONIZATION AND SCINTILLATION PRODUCED BY RELATIVISTIC An, He AND H IONS IN LIQUID ARGON E. SHIBAMURA 1), H.J . CRAWFORD 2), T. DOKE 3), J.M . ENGELAGE 2), I. FLORES 4), 4), A. HITACHI 3), J. KIKUCHI 3), P.J . LINDSTROM K. MASUDA 1> and K. OGURA 5) 1) Saitama College of Health, Kamiokubo, Urawa-shi, Sauama 338, Japan 21 University of California Space Science Laboratory, Berkeley, California 94720, USA s) Science and Engineering Research Laboratory Waseda University, Kikuicho, Shinluku-ku, Tokyo 162, Japan 4) Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720, USA sl College of Industrial Technology, Nihon University, Izumicho, Narashino-shi, Chiba 275, Japan
Received
5
May
1987
We have measured ionization and scintillation produced by relativistic ions of Au, He and H in liquid argon. The sum of ionization signal and scintillation signal per unit energy deposition is the same for He and H ions, which is also the same as that for relativistic Ne, Fe and La ions previously measured. We have found that quenching occurs when liquid argon is irradiated by relativistic Au ions and that the sum per unit energy deposition for the Au ions is 70-76% of that for the other ions mentioned above. 1. Introduction Ionization and scintillation have been investigated in liquid argon irradiated by various relativistic particles. One of the characteristic features is that the ionization and the scintillation are complementary. Our first experiment [1] with heavy ions showed that the scintillation yield per unit energy deposited in liquid argon is the same for relativistic Ne and Fe ions . On the other hand, the yield for slow alpha particles from 21° Po is 0.71 of that for relativistic Ne or Fe ions. The sum of ionization and scintillation per unit energy is independent of applied electric field and is the same for high energy electrons, Ne and Fe particles. It is concluded that all electron-ion pairs and excitons produced by a relativistic particle are converted to either ionization charge or scintillation light in liquid argon. So, by use of the proper conversion factor a, the summation I + aS of ionization I and scintillation S is proportional to the deposited energy 4 E. The fraction of the ionization or the scintillation depends on the applied electric field. Our second experiment [2] showed that the ratio of the sum to deposited energy for relativistic La ions is almost the same as that for Ne and Fe ions . It is therefore interesting to see the ratio as a function of atomic number of incident ions . Recently we made a measurement of ionization and scintillation in liquid argon for relativistic Au, He and H ions . Although the ratio was constant for He and H ions, a significant reduction in the ratio was found for Au ions . In this paper, we will present the experimental results 0168-9002/87/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
and give a perspective for fast incident ions from Z = 1 through Z = 79 on ionization and scintillation in liquid argon. 2. Experimental The experimental apparatus is the same as that used in a previous experiment [2] and will be briefly described here . The detector is a gridded ionization chamber filled with liquid argon, and consists of circular electrodes set in parallel as shown in fig. 1 . The distance is 4 mm between the cathode (K) and the grid (G) and 3 mm between G and the collector (C). The chamber was placed so that the incident beam passed through the chamber between K and G parallel to the electrodes.
Pyrex window
Fig. 1. Arrangement of electrodes . K; cathode, G; grid, C; collector, S; 21°Po source, W.L .S. ; wavelength shifter. All dimensions are in mm .
438
E Shibamura et al. / Ionization and scintillation to liquid argon
The effective diameter of the chamber is 2.0 cm, which gives the path length of the ion beam to be measured . The actual position of each ion track was determined by two-dimensional position-sensitive Si detectors located at the front and at the back of the chamber. A 210po alpha source was deposited in the central region of the cathode surface and was used for calibration of scintillation signals.
In the present experiment, the incident ions were Au with an incident energy of 1.08 GeV/amu, He of 1 .05 GeV/amu and H of 1 .05 GeV/amu. Hydrogen ions were prepared by spallation of He ions in a thin foil of Be . The average energy loss was estimated by integrating the Bethe-Mott-Bloch energy loss formula (eq. (4 .22) of ref. [3]) along the ion track using a mean excitation energy of 210 eV for argon [3]. The estimated
512
a 384
256 U
128
64
128
S(0)
S(0)
192
(channels)
(channels)
256
Fig. 2. Pulse height distribution of scintillation signal S(0) obtained under zero electric field for (a) Au ions, (b) He ions and H ions
E. Shibamura et al / Ionization and scintillation m liquid argon energies of the incident ions at the entrance of the 2 cm path in liquid argon are 870 MeV/amu for Au ions, 1 .04 GeV/amu for He ions and 1 .04 GeV/amu for H ions . The average energy losses in the liquid argon were 32 .4 GeV for Au, 17 .9 MeV for He and 4.47 MeV for H. Charge collected by the electric field was measured by a charge-sensitive amplifier. Scintillation light was detected by a photomultiplier through a transparent grid and collector and a Pyrex window with wavelength shifter. The photomultiplier was operated at constant high voltage throughout the experiment, so that the intensities of the scintillation light were directly comparable .
439
Fig . 3 . The scintillation intensity S(E) (0) and the collected charge I (O) for an electric field E between the cathode and the grid . The scintillation intensity is normalized to S(0) . The charge is normalized to I_ =,A El W.
3 . Results and discussion Pulse height spectra of scintillation at zero electric field are shown in fig . 2a for Au ions, and in fig. 2b for He and H ions . For H ions, a considerable asymmetry is seen . The shape of the energy loss distribution is determined by the parameter, K (
= 0wmax , =2anZ2e4x/mv2),
where win _ is the maximum energy transfer in collisions with atomic electrons of argon, n is the number density of electrons in liquid argon, Zt is the atomic number of the incident ion, v is the velocity of the incident ion, x is the thickness of liquid argon and e and m are the charge and mass of the electron . The value of K is 0 .070 for H ions and 0 .28 for He ions . The shape of the energy loss distribution for these values of K is given by Vavilov's distribution function . The most probable energy loss d Emp was calculated to be 4.04 MeV for H ions and 17 .0 MeV for He ions by a computer program given by Badhwar et al . [4] . It is difficult to obtain a mean pulse height from the experimental distribution, so we restrict our consideration to the most probable energy loss values . On the other hand, the parameter K is 700 for the Au ions which implies that the mean energy loss equals the most probable energy loss . Fig . 3 shows the relative signal intensity as a function of applied field . The symbols S(0) and S(E) denote the magnitude of the signals from scintillation for zero field and for an applied field E, respectively . Similarly, I and I,, denote the collected charge at field E and E = oc, respectively . The value of I. is given by I_ =dE/W, where W=23 .6 eV [5] . For Au ions, the collected charge and the reduction in scintillation seem too small at low electric field . This effect is believed to be due to a reduction in the electric field caused by a space charge effect similar to that
observed previously for La ions [2]. In the case of H and He ions, more than 80% of the produced charge was collected over 4 .5 kV/cm. In contrast, less than 30% of the produced charge was collected for Au ions at the highest electric field of E = 7 .5 kV/cm . This sharply reduced value is due to the recombination between electrons and ions along the highly ionized track of the Au ion in liquid argon. Fig. 4 is a scatter plot of scintillation (x-axis) and ionization (y-axis) signals obtained for Au ions under an electric field E of 1 .5 kV/cm (a) and 6 .0 kV/cm (b) . The groups of points (c) and (d) correspond to ions passing through the chamber between the grid and the collector at E = 1 .5 and 6.0 kV/cm, as was seen previously for La ions [2] . The figure demonstrates the complementary feature of scintillation and ionization signals which was already shown in fig . 3 . Here, let us make a summation of scintillation and ionization . The ionization signal can be measured in numbers of electrons by the use of a charge-sensitive preamplifier with a well calibrated capacitor for test pulse . On the other hand, the scintillation intensity can be obtained in numbers of emitted photons by comparing the measured pulse height with that from a source of known scintillation intensity under the same conditions . In the previous paper [1] we showed that the number of photons Nph _, produced by alpha particles from 21° Po under zero electric field is given by Nph-~ = gaE~1 W',
( 2)
if we assume that the quantum efficiency of photon production is unity [6], where q . (= 0.71) is the quenching factor * for the scintillation from alpha particles
* In ref . [1], the symbol rl a was called the quantum efficiency for the scintillation from alpha particles in liquid argon, instead of q. .
E. Shibamura et al. / Ionization andscintillation in liquid argon 384
_N C C O -C U v C O
192
O N C O
0
0
Scintillation (channels) Fig. 4. Correlation between scintillation signals and charge signals. The data obtained at E =1 .5 kV/cm (a and c) and E = 6.0 kV/cm (b and d) are superposed.
and E. (= 5.3 MeV) the energy of the alpha particle . The symbol W' denotes the effective W value for photon production in liquid argon under zero electric field and W' = W/(1 + Nex /N,) = 19 .5 eV, where N., and N, are the numbers of produced excitons and electron-ion pairs, respectively . So the number of photons Nph produced by the incident ion is obtained from the scintillation signal S due to the ion by the relation
equals unity if there is no quenching, otherwise q gives the quenching factor [71. Fig. 5 shows q as a function of electric field E for Au, He and H ions . In the case of He or H ions, q is smaller than unity at zero electric field. This result is not due to quenching but to the presence of electrons
Npli - ( SIS. ) Nph-aIf ,
where Sq is the scintillation signal due to the alpha particle from the 21° po source at E = 0 and f (=1.15) is the ratio of the solid angle for detecting scintillation produced by incident ions to the solid angle for detecting scintillation produced by the alpha particles. The solid angle for incident ions was calculated by averaging along the track of the ion. The conversion factor a from scintillation signals S to emitted photon numbers Nph is given by a = Nph/S = (ga Ea)l( W'Saf ).
Thus the sum of ionization and scintillation signals I + aS gives the total number of emitted photons and collected electrons. Namely, the ratio q-(I+aS)/(aE/W')=(I+aS)/(N,+N,)
(5)
Electric Field (kV/cm)
Fig. 5. Sum signals of ionization and scintillation normalized on the basis of calibration with scintillation of alpha particles for Au ions, He ions and H ions (open circles and solid line). Closed circles and dashed line express the sum signals for Au ions obtained on the assumption that the sum should be equal at E = 0 and at E = 7.5 kV/cm.
E. Shibamura et al. / Ionization and scintillation m liquid argon that escape recombination . A similar effect was observed previously for incident electrons [8] . The value of q increases with E and becomes compatible with unity at high fields, which might be explained by the effect of electronegative impurities in the low field region [8] . In the case of Au ions the quenching factor q is about 0 .7, and varies only slightly with E. This means that about 30% of the energy deposited in liquid argon by relativistic Au ions was lost without making any ionization or scintillation signal . For La ions [2], q was almost equal to unity over the applied electric field except for the region of 0 .9 < E < 2 .7 kV/cm, where the effect of space charge was significant as was mentioned before . If we assume that q is independent of field E even for the case of q < l, we can obtain another conversion factor a for Au ions by equating q at zero field and high field . Closed circles in fig . 5 show the quenching factor q for various electric fields E obtained on the assumption that q is equal at E = 0 and at E = 7 .5 kV/cm . In this case, q equals 0 .76 . In this figure, there seems to be a small peak at E = 3 .0 kV/cm for Au ions . This might be due to the effect of positive ions produced by incident Au ions as explained below . The produced electron-ion pairs recombine to make scintillation in the region near the grid G, more than in the region near the cathode K, because the externally applied electric field is significantly weakened by the space charge field near the grid [2] . The scintillation emitted near the grid is detected by the photomultiplier with a solid angle larger than that for the beam passing through the middle of the K-G gap . Therefore, the nonuniform effect of the space charge increases the sum signal a little. The recent experiment and the theory of energy loss by relativistic heavy ions show a discrepancy of about 5% around Au ions [9] . The resulting uncertainty in the magnitude of the energy loss directly affects the value of q, as seen before. Moreover, the experimental uncertainties in the determination of beam position with respect to the electrodes directly affect the comparison of the scintillation yields from Au and alpha particles . Therefore, the values of q = 0 .7 and q = 0 .76 should be considered to be within the experimental uncertainties . As seen in fig. 5, q equals unity for He and H ions at high fields E . For No and Fe ions, q was also unity [1]. We can conclude here that the quenching factor q is unity for relativistic incident ions with atomic numbers Z from 1 to 57 and that q decreases for larger Z. The sum signal multiplied by W' is plotted in fig. 6 versus deposited energy AE for 1 MeV electrons [1] and relativistic H, He, Ne [1], Fe [1], La [2] and Au ions . This figure demonstrates a linear relation between the magnitude of the measured signal, (I + aS)W', and the energies absorbed in liquid argon . The maximum deviation is 2% except for Au ions .
44 1
10 5
AE
(MeV)
Fig . 6. Relation between the energy d E absorbed in liquid argon and the amplitude of the sum signal in units of energy .
4. Conclusion
Ionization and scintillation for relativistic Au, He and H ions have been observed in liquid argon . It was shown that the sum of the ionization and the scintillation is proportional to the absorbed energy A E for incident H ions (Z= 1) to La ions (Z = 57), and is given by AE/ W', where W' = 19 .5 eV. The sum per absorbed energy for Au ions is 70-76% of that for the ions mentioned above .
Acknowledgements
The authors would like to express their thanks to Prof. K . Nagata for his support in the calculation of energy loss and to Prof. P .N . Kirk for his reading of the manuscript and for his helpful comments . This work was supported by the Director, Office of Energy Research, Division of Nuclear Physics of the Office of High Energy and Nuclear Physics of the US Department of Energy under Contract DE-AC0376SF00098 .
References
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