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Ionization of tetramethylsilane by alpha particles
Volume 115, number 6
IO~ZATION
CHEMICAL
OF TE TRAMETI-IYLSILANE
Raul C. MUNOZ, James B. CUMMING C!zem&r~ Deptimenr.
19 April 1985
PHYSICS LETTERS
BY ALPHA
PARTICLFS
and Richard A. HOLROYD
Brookhutzn ~o~~onoI Labornlo~,
Upton, NY 11973, USA
Received 27 September 19&1; in final form 5 February 1985
The yield of electrons formed on exposure of liquid tetramethy%lane to 5.48 MeV alpha particles has been measured as a fun&on of eh~tsic field. The number of electrons released per alpha was determined by puise height analysis using a charge sensitive detector_ The yieIds are low and increase with field. At 40 kV/cm the yield is 4100 electrons/alpha (C= 0.075 electrons/100 eV). The results are shown to be consistent withthe major source of electrons being delta rays.
1, Introduction There are few data avaiIable at present on the yields of electrons produced by densely ionizing particles, like alpha particles, in non-polar liquids at room temperature. In contrast, electron yields are well known‘ [l] for fast electrons, which are minimum ionizing particles. A low yieId is expected for alpha particles because recombination of electrons with ions is more probable due to the very high density of ions along the track. This contrasts to the minimum ionizing case where individual ionizations occur widely separated. In the latter case yields of electrons are solvent dependent. Most of the data available for alpha particles pertains to ionization of liquid Ar and Xe at fow temperature. The yield in argon increases with field and at 10 kV/cm is O-45 per 100 eV [24], or about one tenth the fleid observed at this field for fast electron irradiation. The yield of ions in n-hexane exposed to 5.1 MeV alphas is 0.014 per 100 eV at 8 kV/cm [S] , again about one-tenth the yield found for fast electrons [l l] . We are measuring yields of electrons as a function of field in various liquids exposed to alpha particles. The results for tetramethylsilane (TMS) are reported here. These studies pJovide more information about track structure in liquids for densely ionizing particles, a topic which has been the subject of recent controversy [6-8]. In addition to the fundamental interest in track structure there is also a practical reason to measure 0 OOP-2614/85/$03.30 0 Elsevier Science Publishers B-V_ (North-Holland Physics Publishing Division)
electron yields in TMS for densely ionizing particles_ TMS and similar liquids for which the yield is high for minimum ionizing radiation are appropriate for room temperature liquid-filled ionization detectors [9,10] _Although these are designed to detect primarily very-high+rergy particles, which are minimum ionizing, some densely ionizing particles are produced in typical experiments for which it is necessary to know the electron yield_
2, Experimental The method used to measure electron yields is that of pulse height analysis with a charge sensitive preamplifier. This is a very sensitive technique abowing use of a weak radioactive source; knowing the strer&h of the source is not required. Current measurements were also made however for comparison purposes. The detector cell was constructed of Pyrex with two 1.27 cm diameter stainless steel electrodes separated by 0.079 cm. A rugged alpha particle source, suitable for immersion in liquids, was prepared by Isotope Products Laboratory by electrodeposition and diffusion bonding of O-23 &i of 241Am to an 0.18 cm2 area centered in one electrode. The alpha spectrum of this source, obtained with a silicon surface barrier detector, exhibited no shifts in position or line broadening compared to that of a thin 241Am standard. The cell was ftied with TMS (KR Research) 477
Volume 1 X5. numbs 6
CHEMIC&
19 April1985
PHYSICS LETTERS
which was further purified as described [1l].The TMS was shown to be of sufficient purity by measuring the electron lifetime following a short X-ray pulse from a van de Graaff accelerator. The electron Iifetime for the T&IS in the detector was >ZO ~.ts.In the pu?.se height analysis the drEt times were<06 ~.rsinsuring that the decrease in pulse height due to electron loss to impurities w2s <2%. The output of the charge sensitive preamplifier was fed to an amplifier (Ortec 472) with a shaping time constant of 6 p. The output of the amplifier was fed to an analogue to digital converter and the spectrum was collected on a multichannel analyzer and stored on magnetic tape. The calibration utilized a known voltage pulse across a 1.05 pF capacitor connected to the preamplifier input. The observed resolution, corresponding to the fwhm of the calibrating pulse, is 350 electrons. E (kV/cm)
3. Results A spectrum of counts versus channel number for TMS at an applied voltage of 2400 V is shown in fig, 1. The position of the peak corresponds to 3380 electrons and the fwhm to 1020 electrons. The absolute width increases with field from 460 electrons at the lowest field studied to 1030 electrons at 35 kV/cm. Fig. 2 shows the number of electrons per alpha, cal-
:
* . . . *
Fig. 1. Multichannel anaIyzer spectrum of alpllas from TMS at 2400 V appJ.ied_The small peak to the dght is due to the czdibration pulsar operating at 1-I mV, whkh cosrespoods to 7200 charges
478
Fig. 2. o Numb= of charges at peak per alpha observed for ThfS plotted versus elect& field Q; solid line - 3affe mode5 eq. Cl), &shed line - Krasnmer mcdeZ eq. (2). R&ht-)land vertical scale is elect;ions per 100 eV fr;).
culated from the position of the maximum, versus the applied electric field. The most salient feature of these resuhs is the low yield of electrons. A a field of 10 kV/cm the yield is only 0.029 per 100 eV. Measurements of the current resulting from the alpha bombardment confirmed this value: A current of 1.05 pA, corresponding to a yield of 0.028 per 100 eV, was observed at 10 kV/cm. There is no truly linear portion to the curve; the curve is concave downward throughout. The intercept at zero field is &400 c~rges/a~p~, which corresponds to 7.0 X 10-T electrons/l00 eV.
4. i%cussZon The striking result observed here is the low yield of electrons from TMS exposed to 5.48 MeV alphas. The yields are considerably less t&n _jhqvalue of 0.74 per 100 eV found for fast electron bombardment at zero field [l 1, and are only a factor of two larger than the yieid of ions for aIpha-bombardment of nhexane [5] - fonization yields from alphas are notas solvent dependent as they are in the case of minimum ionizing radiation.
Volume 115,number
CHEMICAL
6
PHYSICS
The yield of electrons increases with field as shown in fig. 2. Because of the recent controversy [6--83 regarding the field dependence of electron yields in liquid argon, it is of interest to compare our data with the prediction of the Onsager model [12]. This model predicts a slope to intercept ratio at low fields of 6.0 X 10m5 cm/V for TMS. The data show no linear region but a straight line through the lowest points yields a slope-to-intercept ratio that is three times larger than predicted by the Onsager model. Another model for alpha particle tracks is that proposed by Jaffe 1131. His model includes the effect of field on a column of ion pairs undergoing recombination and diffusion, where the latter is dominant. According to Jaffe the maximum yield of electrons is observed for the electric field perpendicular to the a-track, the number of electrons (Q) which escape the track is given by: l/Q(E) = l/P,
+ =(z)lE
Cl)
3
where &, is the total number of electrons along the track initially, E is the electric field, S(z) = n-If2
s
ems [s(l + s/z)] -Ii2
ds ,
0 = bpE/21/2D. b is 1.13 times the average track radius of the charges, P is the mobility of electrons, D is diffusion constant of electrons, X is constant involving the parameter b, dE’/dx = 9.8 eV/A [14]*. The best fit of the high-field data to eq. (1) was obtained by assuming a value for b (to calculate z) that was consistent with the b value derived from the value of h obtained when a plot of l/Q(E) versus S(z)/E was made. The resulting fit (shown by the solid line in fig. 2) yielded b = 300 + 30 A and a Q_ value which corresponded to 0.28 + 0.03 electrons/100 eV. The theoretical line deviates at low field from the data but does predict a sharp break at low field. Although the fit is somewhat reasonable, the parameters derived are unexpected. The value of 300 A is much larger than the slowing down distance (b = 160 A) of and electron as derived from the free ion yield for TMS under X-irradiation [l]_ The yield of 0.28 per
&I2
* The value of d,F/dx for TMS was calculated from that for n-hexane by correcting for the electron densities.
19 April 1985
LETTERS
100 eV is considerably less than the yield of 4.0 electrons/lOO eV expected **. A modification of Jaffe’s model was proposed by Krammers [16] _ Although he also assumed that the distribution of positive and negative species were the same, he took track recombination as the dominant term. In this theory the number of electrons escaping the track is: QQ
= Qno(EI~‘)~(~)
+ e,
3
(2)
where fix)
= S
SIP (esex + 1)-l
d.r ,
0
both x and h’ are functions of b and the electric field and Q. is the yield at zero field. The best fit of eq. (2) to the data is shown by the dashed line in fig. 2. The parameters used in this fit were Qu = 490 charges, an initital yield of 1.25 + 0.13 electrons/100 eV and b = 125 5 13 A. The fit to the data is best at low field but deviations are observed at higher fields. Although an apparent fit to the data is obtained, the two models lead to quite different vahies of the width of the track and the yield of electrons at in% nite field. This disagreement suggests either or both may not be representative. A major criticism of both Jaffe’s and l&unrner’s models is the assumption of the same initial distrr%utions of positively and negatively charged species. Whereas we expect, based on current knowledge that initially the positive ions will be close to the alpha track and the e!e&rons will be more widely distributed. Because of tiic strong Coulombic forces, electrons close to the track recombine rapidly and those at larger distances are more likely to be drawn out by the electric field. The above theories attribute the maximum yield to tracks oriented perpendicular to the electric field and, as suggested by Swan for argon, the shape of the number versus charge plot (as in fig. 1) is attributed to the effect of orientation, where the least number of charges is observed for tracks parallel to the field. This may not be the case for hydrocarbons. Richards’ [17] results indicate that the yield of ions from alpha ** The total ion yield in neopentane for minimum ionizing particles is 4.03 X lO+/eV
[15].
479
Volume 115, number 6
CHEMICAL
PHYSICS
particles is independent of angle for n-hexane. He attributed the observed yield to those ions formed in delta ray tracks. Several facts are consistent with the idea that delta rays are responsible for a large share of the eLectrona observed here. First, the yields are only a factor of two larger than the yields in hexane. Second, the dc current measurement gave the same yield as the pulse-height measurement_ This would not be the case if most electrons came from the alpha tracks since then the effect of the field would depend on the angle between the track and the field. This would lead to an asymmetric spectrum [2] and a smaller yield in a dc experiment. Finally, the observed spectrum at low fields is nearly symmetric and the fwhm of the peak is only slightly greater than the detector resolution. Is the delta ray hypothesis plausrbie? For a 5-5 MeV alpha the maximum energy of delta rays is (4&J jI$.&& or 3 keV. The number of delta rays decreases with energy but the ranges of electrons with energy !ess than 1 keV are comparable to the width of the c!ectron distribution. Thus only delta rays with energy between 1 and 3 keV would contribute. Burch [18] estimates there are 670 delta rays in this energy range for a 5.5 MeV alpha. The yield of ion pans for such low-energy electrons in TMS is unknown. However, Hummell and Allen [19] reported the zero-field yield of ion pairs in hexane for 2.4 keV electrons to be 0.05 per 100 eV which corresponds to j-ust over 1 ion pair/ track. The yield rises rapidly with field and is four times this value at lo4 V/cm. As a conservative estimate one would expect at least one ion pair per delta ray for TMS or 670 ion pairs at zero field. Considering that delta rays are forward scattered and that not all would leave the alpha track, this value is in reasonable agreement with our “intercept” of 490 electrons. The yield from alphas is 1600 electrons at 10 kV/cm. Although the yield from l-3 keV delta ray electrons is not known for TMS, the yield is estimated to be 0.15 electrons/100 eV at this field for 2 keV electrons for a very similar molecule: 2,2,4&tetramethylpentane [20 ] . Approxhnately 1 MeV of the alpha particle energy goes into delta rays of 1-3 keV energy [18] _ Therefore (I.5 X 10e3) X lo6 = 1500 electrons should be formed by delta rays at 10 kV/cm which is in agree-
480
LETTERS
19 April 1985
ment with experiment Thus the delta ray hypothesis could account for the observed results. Measurements of other liquids are currently underway along with model calculations to test the validity of this hypothesis.
Acknowledgement We wish to thank V. Radeka for his help and advice and for providing the charge sensitive preamplifier. We also wish to thank H. Schwarz for his suggestions and encouragement and K. Walther for fabricating the Pyrex detector cell. This research was carried out at Brookhaven National Laboratory under contract DEACO2-76CHOOO16 with the U.S. Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences.
References [ 1 J W.F. Schmidt and A.O. Allen, J. Chem. Phys. 52 (1970) 2345. [2J D-W_ D-W. Swan, Proc. Phys. Sot 85 (1965) 1297. [3] W.J. Willis and V. Radeka. NucL Instr. Methods 120 (1974) 221. [a] S. Kormo and S. Kobayashi, ScL Papers Inst. Phys. Chem. Res. 67 (1973) 57. [S ] M. Chybicki, Acta Phys Polon. 30 (1966) 927. [6] CR. Gruhn and M.D. Edmiston, Phys. Rev. Letters 40 (1978) 407. [7] G.R Freeman, Phys. Rev. 320 (1979) 3518. [8] C.R. Gruhn and A. Mozumder, Phys. Rev. B20 (1979) 3520. [9] J. Engler and H. Keim, NucL Instr. Methods 223 (1984) 47. [IO] RA. Hoboyd and D.F. Anderson, to be published. [ll] R.C. Munoz and G. Ascarelli, Chem. Phys. Letters 94 (1983) 235; J. Phys. Chem. 88 (1984) 3712. [ 123 L. Onsager, Phys. Rev. 54 (1938) 554. [ 13] G. Jaffe, Ann_ Physik 42 (1912) 303. 1143 RB.J. Palmer, J. Phys. B6 (1973) 384. [15] W-F. Schmidt, Rad. Res. 42 (1970) 73. [16] H.A. Krammers. Physica 18 (1952) 665. [ 171 E.W.T. Richards,Proc. Phys. Sot. 66A (1953) 631. El83 P.RJ. Burch. Rad. Res_ 6 (1957) 289. [ 193 A. Humm~+ and k0. Allen, J. Chem. Phys. 46 (1967) 1602. [20] R.A. Holroyd and T.K. Sham, to be published.
IO~ZATION
CHEMICAL
OF TE TRAMETI-IYLSILANE
Raul C. MUNOZ, James B. CUMMING C!zem&r~ Deptimenr.
19 April 1985
PHYSICS LETTERS
BY ALPHA
PARTICLFS
and Richard A. HOLROYD
Brookhutzn ~o~~onoI Labornlo~,
Upton, NY 11973, USA
Received 27 September 19&1; in final form 5 February 1985
The yield of electrons formed on exposure of liquid tetramethy%lane to 5.48 MeV alpha particles has been measured as a fun&on of eh~tsic field. The number of electrons released per alpha was determined by puise height analysis using a charge sensitive detector_ The yieIds are low and increase with field. At 40 kV/cm the yield is 4100 electrons/alpha (C= 0.075 electrons/100 eV). The results are shown to be consistent withthe major source of electrons being delta rays.
1, Introduction There are few data avaiIable at present on the yields of electrons produced by densely ionizing particles, like alpha particles, in non-polar liquids at room temperature. In contrast, electron yields are well known‘ [l] for fast electrons, which are minimum ionizing particles. A low yieId is expected for alpha particles because recombination of electrons with ions is more probable due to the very high density of ions along the track. This contrasts to the minimum ionizing case where individual ionizations occur widely separated. In the latter case yields of electrons are solvent dependent. Most of the data available for alpha particles pertains to ionization of liquid Ar and Xe at fow temperature. The yield in argon increases with field and at 10 kV/cm is O-45 per 100 eV [24], or about one tenth the fleid observed at this field for fast electron irradiation. The yield of ions in n-hexane exposed to 5.1 MeV alphas is 0.014 per 100 eV at 8 kV/cm [S] , again about one-tenth the yield found for fast electrons [l l] . We are measuring yields of electrons as a function of field in various liquids exposed to alpha particles. The results for tetramethylsilane (TMS) are reported here. These studies pJovide more information about track structure in liquids for densely ionizing particles, a topic which has been the subject of recent controversy [6-8]. In addition to the fundamental interest in track structure there is also a practical reason to measure 0 OOP-2614/85/$03.30 0 Elsevier Science Publishers B-V_ (North-Holland Physics Publishing Division)
electron yields in TMS for densely ionizing particles_ TMS and similar liquids for which the yield is high for minimum ionizing radiation are appropriate for room temperature liquid-filled ionization detectors [9,10] _Although these are designed to detect primarily very-high+rergy particles, which are minimum ionizing, some densely ionizing particles are produced in typical experiments for which it is necessary to know the electron yield_
2, Experimental The method used to measure electron yields is that of pulse height analysis with a charge sensitive preamplifier. This is a very sensitive technique abowing use of a weak radioactive source; knowing the strer&h of the source is not required. Current measurements were also made however for comparison purposes. The detector cell was constructed of Pyrex with two 1.27 cm diameter stainless steel electrodes separated by 0.079 cm. A rugged alpha particle source, suitable for immersion in liquids, was prepared by Isotope Products Laboratory by electrodeposition and diffusion bonding of O-23 &i of 241Am to an 0.18 cm2 area centered in one electrode. The alpha spectrum of this source, obtained with a silicon surface barrier detector, exhibited no shifts in position or line broadening compared to that of a thin 241Am standard. The cell was ftied with TMS (KR Research) 477
Volume 1 X5. numbs 6
CHEMIC&
19 April1985
PHYSICS LETTERS
which was further purified as described [1l].The TMS was shown to be of sufficient purity by measuring the electron lifetime following a short X-ray pulse from a van de Graaff accelerator. The electron Iifetime for the T&IS in the detector was >ZO ~.ts.In the pu?.se height analysis the drEt times were<06 ~.rsinsuring that the decrease in pulse height due to electron loss to impurities w2s <2%. The output of the charge sensitive preamplifier was fed to an amplifier (Ortec 472) with a shaping time constant of 6 p. The output of the amplifier was fed to an analogue to digital converter and the spectrum was collected on a multichannel analyzer and stored on magnetic tape. The calibration utilized a known voltage pulse across a 1.05 pF capacitor connected to the preamplifier input. The observed resolution, corresponding to the fwhm of the calibrating pulse, is 350 electrons. E (kV/cm)
3. Results A spectrum of counts versus channel number for TMS at an applied voltage of 2400 V is shown in fig, 1. The position of the peak corresponds to 3380 electrons and the fwhm to 1020 electrons. The absolute width increases with field from 460 electrons at the lowest field studied to 1030 electrons at 35 kV/cm. Fig. 2 shows the number of electrons per alpha, cal-
:
* . . . *
Fig. 1. Multichannel anaIyzer spectrum of alpllas from TMS at 2400 V appJ.ied_The small peak to the dght is due to the czdibration pulsar operating at 1-I mV, whkh cosrespoods to 7200 charges
478
Fig. 2. o Numb= of charges at peak per alpha observed for ThfS plotted versus elect& field Q; solid line - 3affe mode5 eq. Cl), &shed line - Krasnmer mcdeZ eq. (2). R&ht-)land vertical scale is elect;ions per 100 eV fr;).
culated from the position of the maximum, versus the applied electric field. The most salient feature of these resuhs is the low yield of electrons. A a field of 10 kV/cm the yield is only 0.029 per 100 eV. Measurements of the current resulting from the alpha bombardment confirmed this value: A current of 1.05 pA, corresponding to a yield of 0.028 per 100 eV, was observed at 10 kV/cm. There is no truly linear portion to the curve; the curve is concave downward throughout. The intercept at zero field is &400 c~rges/a~p~, which corresponds to 7.0 X 10-T electrons/l00 eV.
4. i%cussZon The striking result observed here is the low yield of electrons from TMS exposed to 5.48 MeV alphas. The yields are considerably less t&n _jhqvalue of 0.74 per 100 eV found for fast electron bombardment at zero field [l 1, and are only a factor of two larger than the yieid of ions for aIpha-bombardment of nhexane [5] - fonization yields from alphas are notas solvent dependent as they are in the case of minimum ionizing radiation.
Volume 115,number
CHEMICAL
6
PHYSICS
The yield of electrons increases with field as shown in fig. 2. Because of the recent controversy [6--83 regarding the field dependence of electron yields in liquid argon, it is of interest to compare our data with the prediction of the Onsager model [12]. This model predicts a slope to intercept ratio at low fields of 6.0 X 10m5 cm/V for TMS. The data show no linear region but a straight line through the lowest points yields a slope-to-intercept ratio that is three times larger than predicted by the Onsager model. Another model for alpha particle tracks is that proposed by Jaffe 1131. His model includes the effect of field on a column of ion pairs undergoing recombination and diffusion, where the latter is dominant. According to Jaffe the maximum yield of electrons is observed for the electric field perpendicular to the a-track, the number of electrons (Q) which escape the track is given by: l/Q(E) = l/P,
+ =(z)lE
Cl)
3
where &, is the total number of electrons along the track initially, E is the electric field, S(z) = n-If2
s
ems [s(l + s/z)] -Ii2
ds ,
0 = bpE/21/2D. b is 1.13 times the average track radius of the charges, P is the mobility of electrons, D is diffusion constant of electrons, X is constant involving the parameter b, dE’/dx = 9.8 eV/A [14]*. The best fit of the high-field data to eq. (1) was obtained by assuming a value for b (to calculate z) that was consistent with the b value derived from the value of h obtained when a plot of l/Q(E) versus S(z)/E was made. The resulting fit (shown by the solid line in fig. 2) yielded b = 300 + 30 A and a Q_ value which corresponded to 0.28 + 0.03 electrons/100 eV. The theoretical line deviates at low field from the data but does predict a sharp break at low field. Although the fit is somewhat reasonable, the parameters derived are unexpected. The value of 300 A is much larger than the slowing down distance (b = 160 A) of and electron as derived from the free ion yield for TMS under X-irradiation [l]_ The yield of 0.28 per
&I2
* The value of d,F/dx for TMS was calculated from that for n-hexane by correcting for the electron densities.
19 April 1985
LETTERS
100 eV is considerably less than the yield of 4.0 electrons/lOO eV expected **. A modification of Jaffe’s model was proposed by Krammers [16] _ Although he also assumed that the distribution of positive and negative species were the same, he took track recombination as the dominant term. In this theory the number of electrons escaping the track is: QQ
= Qno(EI~‘)~(~)
+ e,
3
(2)
where fix)
= S
SIP (esex + 1)-l
d.r ,
0
both x and h’ are functions of b and the electric field and Q. is the yield at zero field. The best fit of eq. (2) to the data is shown by the dashed line in fig. 2. The parameters used in this fit were Qu = 490 charges, an initital yield of 1.25 + 0.13 electrons/100 eV and b = 125 5 13 A. The fit to the data is best at low field but deviations are observed at higher fields. Although an apparent fit to the data is obtained, the two models lead to quite different vahies of the width of the track and the yield of electrons at in% nite field. This disagreement suggests either or both may not be representative. A major criticism of both Jaffe’s and l&unrner’s models is the assumption of the same initial distrr%utions of positively and negatively charged species. Whereas we expect, based on current knowledge that initially the positive ions will be close to the alpha track and the e!e&rons will be more widely distributed. Because of tiic strong Coulombic forces, electrons close to the track recombine rapidly and those at larger distances are more likely to be drawn out by the electric field. The above theories attribute the maximum yield to tracks oriented perpendicular to the electric field and, as suggested by Swan for argon, the shape of the number versus charge plot (as in fig. 1) is attributed to the effect of orientation, where the least number of charges is observed for tracks parallel to the field. This may not be the case for hydrocarbons. Richards’ [17] results indicate that the yield of ions from alpha ** The total ion yield in neopentane for minimum ionizing particles is 4.03 X lO+/eV
[15].
479
Volume 115, number 6
CHEMICAL
PHYSICS
particles is independent of angle for n-hexane. He attributed the observed yield to those ions formed in delta ray tracks. Several facts are consistent with the idea that delta rays are responsible for a large share of the eLectrona observed here. First, the yields are only a factor of two larger than the yields in hexane. Second, the dc current measurement gave the same yield as the pulse-height measurement_ This would not be the case if most electrons came from the alpha tracks since then the effect of the field would depend on the angle between the track and the field. This would lead to an asymmetric spectrum [2] and a smaller yield in a dc experiment. Finally, the observed spectrum at low fields is nearly symmetric and the fwhm of the peak is only slightly greater than the detector resolution. Is the delta ray hypothesis plausrbie? For a 5-5 MeV alpha the maximum energy of delta rays is (4&J jI$.&& or 3 keV. The number of delta rays decreases with energy but the ranges of electrons with energy !ess than 1 keV are comparable to the width of the c!ectron distribution. Thus only delta rays with energy between 1 and 3 keV would contribute. Burch [18] estimates there are 670 delta rays in this energy range for a 5.5 MeV alpha. The yield of ion pans for such low-energy electrons in TMS is unknown. However, Hummell and Allen [19] reported the zero-field yield of ion pairs in hexane for 2.4 keV electrons to be 0.05 per 100 eV which corresponds to j-ust over 1 ion pair/ track. The yield rises rapidly with field and is four times this value at lo4 V/cm. As a conservative estimate one would expect at least one ion pair per delta ray for TMS or 670 ion pairs at zero field. Considering that delta rays are forward scattered and that not all would leave the alpha track, this value is in reasonable agreement with our “intercept” of 490 electrons. The yield from alphas is 1600 electrons at 10 kV/cm. Although the yield from l-3 keV delta ray electrons is not known for TMS, the yield is estimated to be 0.15 electrons/100 eV at this field for 2 keV electrons for a very similar molecule: 2,2,4&tetramethylpentane [20 ] . Approxhnately 1 MeV of the alpha particle energy goes into delta rays of 1-3 keV energy [18] _ Therefore (I.5 X 10e3) X lo6 = 1500 electrons should be formed by delta rays at 10 kV/cm which is in agree-
480
LETTERS
19 April 1985
ment with experiment Thus the delta ray hypothesis could account for the observed results. Measurements of other liquids are currently underway along with model calculations to test the validity of this hypothesis.
Acknowledgement We wish to thank V. Radeka for his help and advice and for providing the charge sensitive preamplifier. We also wish to thank H. Schwarz for his suggestions and encouragement and K. Walther for fabricating the Pyrex detector cell. This research was carried out at Brookhaven National Laboratory under contract DEACO2-76CHOOO16 with the U.S. Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences.
References [ 1 J W.F. Schmidt and A.O. Allen, J. Chem. Phys. 52 (1970) 2345. [2J D-W_ D-W. Swan, Proc. Phys. Sot 85 (1965) 1297. [3] W.J. Willis and V. Radeka. NucL Instr. Methods 120 (1974) 221. [a] S. Kormo and S. Kobayashi, ScL Papers Inst. Phys. Chem. Res. 67 (1973) 57. [S ] M. Chybicki, Acta Phys Polon. 30 (1966) 927. [6] CR. Gruhn and M.D. Edmiston, Phys. Rev. Letters 40 (1978) 407. [7] G.R Freeman, Phys. Rev. 320 (1979) 3518. [8] C.R. Gruhn and A. Mozumder, Phys. Rev. B20 (1979) 3520. [9] J. Engler and H. Keim, NucL Instr. Methods 223 (1984) 47. [IO] RA. Hoboyd and D.F. Anderson, to be published. [ll] R.C. Munoz and G. Ascarelli, Chem. Phys. Letters 94 (1983) 235; J. Phys. Chem. 88 (1984) 3712. [ 123 L. Onsager, Phys. Rev. 54 (1938) 554. [ 13] G. Jaffe, Ann_ Physik 42 (1912) 303. 1143 RB.J. Palmer, J. Phys. B6 (1973) 384. [15] W-F. Schmidt, Rad. Res. 42 (1970) 73. [16] H.A. Krammers. Physica 18 (1952) 665. [ 171 E.W.T. Richards,Proc. Phys. Sot. 66A (1953) 631. El83 P.RJ. Burch. Rad. Res_ 6 (1957) 289. [ 193 A. Humm~+ and k0. Allen, J. Chem. Phys. 46 (1967) 1602. [20] R.A. Holroyd and T.K. Sham, to be published.