SC -0.0047 2.5 should influence Ise as well as (asc)25. In order to prove this point, measurements of the exposure rate dependence of Ise, were carried out for the different detectors as shown in figures 14 and 15. For small short-circuit currents produced by 137Cs gamma rays, I is a linear function of exposure rate for all detectors. However, for large short-circuit currents produced at high exposure rates of lightly filtered 30 kV x rays obtained from a beryllium window-type tube, the exposure rate dependence of I is non-linear for detectors which have a high resistance ratio and show a relatively large current dependence of (asc) 25 (fig. 14). This nonlinearity can again be explained by the increase of R/R with increasing exposure rate, i.e., increasing short-circuit current, due to the voltage dependence of R. According to eq (12), Ise remains proportional to Ig, i.e., to exposure rate, only as long as the resistance ratio R/R remains constant. Shortcircuit currents observed with the p-i-n type detector (No. 2) (fig. 14) show only a slight deviation from a linear exposure rate dependence, but for the n-p type detector (2.1 cm2) which had a resistance ratio of about 0.015, I is proportional to exposure rate over the whole range investigated (fig. 15). 8. Summary and Conclusions The following conclusions can be drawn from this investigation of the temperature dependence of photocurrents produced by x and gamma rays in silicon radiation detectors of the diffused n-p, lithium drifted p-i-n, and surface barrier type in a temperature range between 20 and 50 °C approximately. (1) Generated photocurrents derived from photodiode measurements showed an increase with increasing temperature in all detectors investigated. Although there is no complete explanation for this positive temperature dependence it is, at least in part, due to an increase in the effective diffusion length of minority carriers with increasing temperature, as has previously been shown [3]. (2) The temperature dependence of the shortcircuit current was similar to that of the generated current in the diffused n-p type detectors only, but was in a varying degree non-linear and negative for the other detectors. It is shown that this different behavior can be explained by the influence of the strong positive temperature-dependence of the junction current which in the short-circuit mode is deducted from the generated photocurrent. This junction current is a function of the internal series resistence R, and the junction resistance R, of the respective detector. With increasing resistance ratio R/Rj, the junction current increases and the temperature coefficient of the short-circuit current decreases. Assuming a temperature coefficient at 25 °C of about 10 percent per °C for the junction current and about 0.4 percent per °C for the generated photocurrent, then the temperature coefficient (asc) 25 of the shortcircuit current should become negative when the junction current is larger than about 4 percent of the generated photocurrent (eq (16)) or R, is larger than approximately the same fraction of R. These values were confirmed by determining the resistance ratios from the current-voltage characteristics of the individual detectors. (3) Values of the temperature coefficients B25 of the generated photocurrent, determined by measurements of photodiode photocurrents, varied approximately between +0.004 per °C and +0.002 per °C for the different detectors (fig. 6). Values of (asc)5 for short-circuit currents changed with increasing resistance ratio of individual detectors of different types from about +0.004 per °C to -0.005 per °C. (4) Detectors with larger series resistances showed a current dependence of (asc)25 decreasing with increasing short-circuit currents. This can be explained by a decrease of R; at higher forward junction voltages resulting in an increase of the resistance ratio and a corresponding decrease of (asc)25. This explanation was confirmed by the measurement of the exposure rate dependence of the short-circuit current which is also a function of the resistance ratio. For resistance of small temperature dependence, the short-circuit currents showing a positive (asc)25 can AE be made nearly independent of temperature within a certain temperature range. In the case of a neg ative (asc)25, the additional resistance must be of the thermistor type having a strong negative temperature dependence. However, by using an additional resistance, care must be taken that a linear exposure rate dependence is retained in the range of interest. COMPENSATION VOLTAGE VCS GAMMA RAYS), mV IGURE 14. Exposure rate dependence of compensation voltage, determining the short-circuit currents, as measured with silicon p-i-n and surface-barrier type detectors at high exposure rates of lightly filtered 30 kV x rays and at low exposure rates of 137Cs gamma rays. etectors with high resistance ratios, the exposure ate dependence is linear at low current levels and ecomes non-linear at large currents. (5) According to the above findings it is possible > change the value of (asc)25 by changing the resistnce ratio. By adding to a detector an additional (6) By measuring the resistance ratio of an individual detector, its behavior with regard to temperature and exposure rate dependence can be predicted qualitatively. (7) The diffused n-p type detectors showed the best performance characteristic with regard to temperature and exposure rate dependence of short-circuit currents. Obviously no generalization can be made for the behavior of individual detectors of different types. But the larger resistance ratios observed with the other detectors seem to be typical, being due either to large series resistances as in the case of surface-barrier type detectors, or to relatively small junction resistances as in the case of p-i-n type detectors. 9. References [1] Parker, R. P. and Morley, B. J., Silicon p-n junction surface [2] Fowler, J. F., Solid state electrical conductivity dosimeters, [3] Scharf, K., and Sparrow, J. H., Steady-State response of silicon [4] Scharf, K. and Sparrow, J. H., Steady-State response of silicon [6] Bailey, N. A., and Kramer, G., The lithium-drifted silicon p-i-n [7] Whelpton, D., and Watson, B. W., A p-n junction photovoltaic detector for use in radiotherapy, Phys. Med. Biol. 8, No. 1, 33-42 (April 1963). [8] Scharf, K., Exposure rate measurements of x- and gamma-rays with silicon radiation detectors, Health Phys. 13, No. 6, 575-586 (June 1967). [9] White, G. N., Measurement of exposure dose, Chapter 5 in Principles of Radiation Dosimetry, pp. 62-64 (John Wiley & Sons, Inc., New York 1959). (Paper 75A6-690) JOURNAL OF RESEARCH of the National Bureau of Standards - A. Physics and Chemistry somerization Processes in lons of the Empirical Formula CH + * 8 S. G. Lias Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234 and The American University, Washington, D.C. 20016 and P. Ausloos Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234 (June 9, 1971) Ions of the formula C4H have been generated with different initial energies by ionizing ethylene (C2H++ C2HA CH, where the CH ion is formed with an initial energy of > 11.51 eV), cyclobutane (initial energy of C4H, > 10.84 eV), methylcyclopropane (> 10.15 eV), 1-C4Hs (>9.58 eV), and i-CH (>9.06 eV) with 11.6–11.8 eV photons, and in some cases also with 10 eV photons and with gamma radiation. The structures of the ions have been determined from the structures of the C4H8 products formed in charge transfer reaction between the ions and charge acceptors such as dimethylamine and nitric oxide, as well as from the structures of the butanes formed in D2 transfer reactions with methylcyclopentane-d12 (CH₫ +C6D12→ C4H8D2+C6D †0). At low pressures the C.Hg ions initially formed in ethylene, cyclobutane, and methylcyclopropane isomerize to the thermodynamically most stable configurations, i-CH₫ and 2-C1H. The 2-CH structure predominates in all the experiments. As the pressure is raised, the i-CH ion yield diminishes as that of 2-CH increases, indicating that when the precursor of the i-C4Hg ion is collisionally deactivated, it ends up as 2-C,Hg. At high pressures, 1-C4H₫ ions are intercepted; their yield increases with increasing pressure, indicating that 1-C4H₫ is an intermediate which isomerizes further unless it is collisionally deactivated. The 1-C.H ion formed in methylcyclopropane (initial energy > 10.15 eV) is more easily deactivated than that formed in cyclobutane (initial energy > 10.84 eV). That the isomerization of the 1-C4H ion to lower energy structures such as i-C4H and 2-C.H requires excess internal energy is demonstrated by the fact that in the photolysis with 10 eV photons, a negligible amount of isomerization is observed, but with 11.6-11.8 eV photons, more than half of the 1CH ions isomerize to the 2-C4H structure at a pressure of 2 torr. Isomerization of the low energy i-C.H ions formed in the photolysis of i-C4H8 to other structures is relatively unimportant at 11.6-11.8 eV. Taking the ratio i-C4H/2-CH as an indicator of the amount of energy removed by collisions from the intermediate CH species under conditions where only i- and 2-CH ions are intercepted, it is shown that the efficiency of energy transfer from the ions to helium, hydrogen, neon, krypton, xenon, nitrogen, and carbon dioxide is related to the polarizability of the added deactivator. Key words: Butene; cyclobutane; ion structure; isomerization; methylcyclopropane; photoioniza- 1. Introduction Determinations of the structures of organic ions have been the subject of a great deal of interest in the literature recently [1]. In particular, n.m.r. specEra of organic ions formed in very strong acids are now *This research, which was supported by the U.S. Atomic Energy Commission, was preSented by S. G. Lias in partial fulfillment of the requirements for the Ph.D. degree at the American University, Washington, D.C. 20006. Figures in brackets indicate the literature references at the end of this paper. being recorded [2]. More classical studies, using analysis of the products formed from ionic reactions in thermal organic systems in the liquid phase [3], or from ions generated by high energy radiation in the gas phase [4], have recently elucidated isomerization mechanisms of the CH and CH carbonium ions. Indirect evidence bearing on ionic structures has been inferred from mass spectrometric results; in particular, many studies recently have attempted to derive information about ionic structures from the modes and rates of reaction of ions as observed in ion cyclotron resonance mass spectrometry [5]. This investigation is devoted to a detailed examination of the structures and isomerization reactions of the ions having the empirical formula, C.H. These ions are not known to be formed in thermal chemical reactions, and to our knowledge have not been studied except in systems where they are generated by high-energy radiation. Several years ago, it was reported [6] that when ethylene was irradiated in the presence of a compound having an ionization energy lower than or equal to 9.54 eV, the C4Hion formed in the reaction: acquired the same butene structures as those seen for the CH ions in the ethylene system. In those early studies it was reported that the CH ions formed in ethylene and cyclobutane acquired the 2-CH; and 1-CH structures. A mass spectrometric study compared the rate of charge transfer to NO from the CH ions in cyclobutane with the rates of the same reaction for the 1-CH, 2-CH, and i-CH ions, and seemingly corroborated these results [9]. However, more recently it was noted [10] that when the CH ions formed in the photoionization of cyclobutane at a pressure of 20 torr were allowed to undergo D, transfer reactions with methylcyclopentane-d, the butane products consisted of CHCHDCHDCH, from the 2-CH reaction: rather than the 1-CH, ion is also formed in irradiated ethylene [11]. (The original misassignment came about because 1-C4H8 and i-C4H8 have nearly identical retention times on the gas chromatographic columns used for analysis in these studies.) The structures of the CH ions formed in the dissociation of cyclohexane and methylcyclopentane parent ions were recently determined by an examination of the structures of the butanes formed in D1⁄2 transfer reactions, (such as 4 and 5), as well as by the structures of the butenes formed in charge transfer reactions [12]. This study was carried out at a single pressure, so pressure effects on the distribution of the isomeric CH ions could not be ob served; because in the 10 eV photolysis of 1-C4H and 2-C4Hs in the presence of methylcyclopentaned12, no evidence was seen [12] for isomerization of 1-CH to the 2-CH structure, or 2-CH to the 1-CH structure, isomerizations between the different C4H8 structures were not considered. It was suggested that the equilibrium position is affected by the internal energy of the ion, which is gradually diminished as the entity undergoes successive collisions. Because such a rapid equilibrium between two skeletally different isomers has not been seen for carbonium ions [1], and because detailed information about isomerization process in CH ions is generally lacking, it was considered worthwhile to undertake a study of isomerization processes in these ions. In particular, it was felt that since CH; ions formed in ethylene can undergo further reaction with the parent molecule: Further, an examination of pressure effects on the structures of CH ions over a wide pressure range seemed warranted. |