Fast Radiochromatographic Detection of Tritium with “Liquid” Scintillators at Low Temperatures Svein Prydz, T. B. Melo, J. F. Koren, and E. L. Eriksen Institute of Physics, University of Oslo, Blindern, Norway Relative scintillation yields have been measured under electron excitation (aH-glucose) from 0 O to -190 O C . Scintillators tested included ethanol, acetone, benzene, anthracene, PPO, and Nuclear Enterprise types NE-214, NE-231, and NE-240. The use of benzene at -190 O C is suggested for the detection of tritium (and other radionuclides) in thin-layer radiochromatography by the action of p-radioluminescence. The method should be applicable to film as well as photomultiplier detection. The sensitivity seems to be very high and the method is “noncontaminating” because the benzene can be completely evaporated at the end of the measurement. The freezing technique is still somewhat destructive to a few thin-layer preparations because of cracking during the cooling. However, this can perhaps be prevented. The temperature dependency observed in the p-radioluminescence of anthracene and PPO, when using photomultiplier detection, is much smaller than reported by others using film detection.
THEDETECTION OF TRITIUM (3H)in thin-layer radiochromatography (TLRC) is a difficult task because of the energy-weak /3-radiation from this radionuclide (electron energies: Emsx= 18.5 kev, E,, = 5.7 kev). The quantitative measurement as well as the localization of 3H spots is of great importance to TLRC work and a variety of methods have been proposed, tested, or discussed (1-12). Most of the methods are covered by review articles (1-4). Of the methods usable for 3H detection in TLRC, those not employing direct electron detection have been (very briefly) mentioned (13). As a consequence of the very short range in the chromatographic medium (about 1 mg per cm*),only a small proportion of the emitted 3H electrons emerge from the chromatogram surface, the larger part being absorbed in the sample itself. This inherent limit to the detection of 3H may be overcome, however, by application of @-radioluminescence (P-RL). This technique employs a scintillator (embedded in the chromatographic preparation) giving off light upon absorption of the @-particles. As the samples are translucent, the 3H activities may be registered by photographic film or by a (1) Nucleonics, 16, (3) 62 (1958). (2) E. Broda, “Radioactive Isotopes in Biochemistry,” Elsevier, Amsterdam, 1960. (3) A. T. Wilson and D. J. Speddiag, J. Chromatog., 18,76 (1965). (4) E. Snyder, Atomlight, 58, 1 (1967). (5) A. T. Wilson, Nature, 182,524 (1958). (6) G. Jolchine, Physiol. Vegetde, 2, 341 (1964). (7) H. H. Seliger and B. W. Agranoff, ANAL. CHEM.,31, 1607 (1959). (8) E. V. Parpus, I. Hoffman, and H. R. Jackson, Tulanta, 5, 75 (1960). (9) U. Liithi and P. G. Waser, Nature, 205, 1190 (1965). (10) J. C. Roucayroll, J. A. Bergner, G. Meyniel, and J. Perrin, Intern. J. Appl. Radiation Isotopes, 15,671 (1964). (11) S. Prydz, Chr. Petersen, and J. F. Koren, Phys. Noruegica, 2, No. 4, 343 (1967). (12) S.Prydz and K. S . Skammelsrud, J. Chromatog., 32,732 (1968). (13) S. Prydz, E. L. Eriksen, T. B. Melo, and J. F. Koren, Zbid., in press. 156
photomultiplier (PM). The sensitivity of the P-RL method is much greater than that of direct P-particle detection on film (autoradiography). Stimulated by the somewhat inconsistent nomenclature found in the literature describing such methods, some suggestions which might seem useful are added in an appendix. The first application of P-RL detection was reported by Wilson (5) in 1958. Whereas he used a liquid scintillator solution of p-diphenylbenzene for film detection of 3H on paper chromatograms (3, Jolchine was the first to apply a solid scintillator for the detection of 3H in TLRC (6). Solid scintillator detection by PM on paper-chromatogram strips had, however, been reported in 1959 by Seliger and Agranoff (7). Anthracene has been used as a scintillator, either dissolved in benzene for paper impregnation (7, 8) or as a powdered admixture (as much as 5Oz)to a commercial silica gel (9). Strip scanning for P-RL detection on paper or TL radiochromatograms impregnated with a scintillating gel has also been developed (IO). A careful examination of available cathodoluminophors (electron scintillators) would be useful for optimization of the P-RL detection method. Earlier we investigated the use in 3H detection of some solid scintillators as admixtures to the commercial media for TLC (11, 12). The results show the /3-RL to be a very promising method, lending itself especially to (quantitative) measurements of low-energy radionuclidesfor example, 3H and 14C. These investigations have recently been continued and extended and will soon be reported (13). The present communication reports results obtained by use of liquid scintillators in a frozen state. Their relative P-RL efficiencies were determined as functions of temperature, and were found generally to increase upon lowering of the temperature. This is in harmony with the idea that the probabilities of competing nonradiative transitions (thermal quenching) normally decrease with temperature. The efficiencies of high-sensitivity “liquid” p-scintillators have been compared with those of anthracene and PPO at temperatures down to - 190 “C. At the lowest temperature pure benzene had the highest P-RL efficiency. This suggests the use of this scintillator in routine measurements, rendering the chromatogram undamaged and free from scintillator upon subsequent slow heating to room temperature and complete evaporation of the benzene. EXPERIMENTAL
The P-RL intensity was measured in all cases with a sensitive photometer positioned close to the samples (ca. 5 cm) with the light-sensitive detector area parallel to the sample surface, both areas being 5 cm in diameter. The luminescence emission occurred in a spot of radioactivity in the center of the samples. (The spot areas were approximately 1 cm2.) The various liquid scintillators were added by pipetting constant volumes (0.8 ml) on to the samples which were quickly cooled down so as to avoid any substantial evaporation. Materials. The chromatographic media applied were the following paper or TL types: Whatman No. I. paper and Chromedia SG 81 (W. & R. Balston, Ltd., England) and
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Eastman Chromagram K301 R2 (Eastman Kodak Co., Rochester, N. Y . ) . The radioactive test materials used in the experiments were aH- or 1%-glucose solutions (obtained from the Radiochemical Center, Amersham, Buckinghamshire, England): ~ - g l u c o s e - 6 - ~(36.5 c mCi/mM; 0.05 mCi/l .O ml) product CFA 351, ~-glucose-6-T(500mCi/mM; 1 mCi/l ml) product TRA 85. and ~-~li1cose-6-T(2000 mCi/mM: 1 mCi/l ml) product TRK 85.The liauids tested as 8-scintillators were either Dure solvents (purity i r o analysis),' ethanol, acetone, and benzene, or commercially available liquid scintillator (purchased from Nuclear Enterprises, Ltd., Scotland) : NE-214 (based on xylene, quoted efficiency 82 % relative to anthracene), NE-231 (based on benzene, quoted light output 60 anthracene), and NE-240 (based on purified dioxane, quoted light output 6 7 x anthracene). As references were used the solid scintillators anthracene and PPO (2,5-diphenyloxazole) from E. Merck, Germany. Sample Preparation. The samples were cut as disks 5 cm in diameter to fit into the measuring equipment. The radioactivity was applied in various amounts (1 nCi to 5 pCi) by pipetting of constant volumes ( 5 or 10 pl) of glucose stock solutions made in wanted strengths. These again were produced from the commercial standard solutions. The diameter of the radioactive spots was measured in several samples of the applied types: SG 81, diameter of spot = 8 ( f1) mm, and K 301 R2, diameter of spot = 12 (i 1)mm. For the study of the temperature dependency in the P-RL efficiencv of the various scintillators standard activities of 5 pCi were used. The liauid scintillators were amlied in constant volume (0.8 ml) by pipetting onto the simples when positioned in the sample holder of the measuring unit. The samples were wetted to the extent that a continuous surface of liquid was formed over the sample. By use of liquid Nz as coolant the evaporation was quickly reduced to a minimum and the sample temperature then further reduced for the measurements to be performed. The solid scintillators, anthracene and PPO, were applied to the samples in solution in benzene (saturated) and diethyl ether. The amount of PPO per square centimeter of chromatogram surface was 2.8 mg. The corresponding amount of anthracene is not known, but it is an amount sufficient to give maximum output. Some details concerning the preparation and its importance are given below and in Figure 3. Methods of Measurement. All scintillators were measured during a linear increase of the temperature (9' per minute). Very often this technique revealed a thermoluminescence component, the intensity of which was proportional to the previous period of low-temperature storage. This phenomenon has been described for solid scintillators. However, the possible use of this kind of P-induced luminescence for 3H-detection seems very limited ( I I , 1 2 ) . The equipment used in these measurements has been described (11, 13, 14). To make sure that thermoluminescence does not add to the light intensity curves recorded as 0-RL, all measurements were repeated applying a slowly decreasing temperature. However, the sample temperature was allowed to stabilize for about 10 minutes before each reading. That a sufficiently good temperature equilibrium was obtained could be verified by simultaneous recording of P-RL intensity and temperature of the copper sample holder, the sample being kept in good thermal contact with this. The minimum obtainable temperature (ca. - 190 "C), which was applied for a series of measurements of relative P-RL efficiencies, seemed to reprcduce to within about 5 '. The P-RL intensities were measured by a sensitive PM (EM1 9558 AQ) applied for (anode) current measurement. (14) S. Prydz and K. S. Skammelsrud, Phys. Norvegica, 2,No. 4, 343 (1967).
PI
-20 -40 -60 -80 -100 -120 -140 -160 -180 Tern pe rat u re, cent i g rad e
0
Figure 1. Temperature variation in P-RL efficiency of some liquid scintillators 1. Ethanol 2. Acetone 3. NE-214(heating curve included) 4. NE-231 5. NE-240 6. Benzene For Figures 1, 3, and 4, 5 pCi of a-H-glucose applied to Eastman Chromagram K 301 R2
For the P-RL in very slow scintillators (having very long decay times) PM pulse counting might equally well have been applied. However, for fast scintillators current measurement is the most appropriate technique. Further discussion is given in the Appendix and by Prydz et a!. (13). An output voltage from the PM current detector unit was recorded as a (relative) measure of the P-RL intensity. The photometer is a very sensitive one with good stability in zero-signal output and amplification. If the output voltage was not adjusted to zero (see Figure 4), the value was subtracted in all low-intensity recordings, as was a luminescence background signal due to the contamination and background radiation traversing the scintillator (Figure 4). RESULTS AND DISCUSSION
Temperature-Dependent P-RL Efficiencies. Applying the method described above, various scintillators were tested under as identical conditions as possible. In Figure 1 are shown their 0-RL intensities as functions of temperature. The photometric measurements are highly reproducible, while some uncertainties were involved in the temperature assignments (ca. & 5 " ) due to variations in the effective thermal contact to the sample holder. Alcohol and acetone were included among the liquids investigated, as these would, like benzene, constitute noncon-
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Table I. Low-Temperature p R L Efficiency of Benzene in Various TL Media TL-media wetted with benzene. RadioacCurrent tivity, 5 pCi 3H-glucose reading 720 Whatman paper No. 1 SiCkloaded paper 2400 Identical sample 2300 Eastman K 301 R2 Chromagram 5300
VI
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t
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J 10 Radioactivity in nCi
1
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104
Figure 2. Benzene luminescent yield us. applied amount of radioactivity
Whatman paper No. 1 used. Measurements at - 190°C. Upper. 14C-glucose Lower. 8H-glucose taminating systems. Their efficiencies, however, were rather small. On the other hand, acetone showed a very intense @-thermoluminescencein the temperature range from - 170" to -120 "C (not shown in the figure); after storage at - 190 "C for 15 minutes a peak amplitude (in units as used in Figure 1) of lo4was obtained (at 9" per minute heating rate). Intense thermoluminescence after low temperature x-ray irradiation of acetone had been reported ( 1 3 , so that our findings were to be expected. The relative efficiencies at a particular temperature may easily be obtained from the curves. The measurements were performed during slow cooling with temperature stabilization prior to each reading. The possibility of getting misleading results when readings are taken during heating is shown for NE-214 (curve 3), where a small thermoluminescence "glow peak" occurs at about - 110 "C. At the higher temperatures most of the N E scintillators are more efficient than benzene. They are actually selected as having the highest available sensitivity to energy-weak electrons in this temperature range. At lower temperatures, however, benzene increases its P-RL efficiency, to become superior at -190 "C. The low-temperature increase in efficiency was much larger in the liquid than in the solid scintillators. This is what might be expected if the viscosity of the system is a determining parameter for the nonradiative processes competing with those responsible for the luminescence. One process linked to viscosity would be a transfer of ex( 1 5 ) T. Rogeberg, Oslo, unpublished results, 1966.
158
citation energy (instead of photon emission) from one molecule to another type of molecule with a smaller probability of photoemissive de-excitation. Lowering of the temperature will normally decrease the probability of the energy transfer by reducing the wave function overlap. In ultrapure (one-component) systems another mechanism must be responsible for observed thermal quenching (or its reduction at low temperature). Neither our anthracene nor our benzene was an ultrapure substance. A possible explanation for the reduced thermal quenching seen in benzene at low temperature might be the quenching of light-induced benzene fluorescence. This mechanism, however, is not thought to be linked to viscosity. From about - 110 "C most of the scintillators tend to show an increased efficiency enhancement upon further cooling. The exact reason is not well known and the phenomenon is not discussed further at the present stage, except for a short comment. Importance of TL Medium. In the measurements presented in Figure 1 Eastman K 301 R2 plates were used [silica gel bound to poly(ethy1ene terephthalate) plastic sheets by means of poly(viny1 alcohol) and containing no ultravioletindicator additives]. It was felt that a simple comparison of its usefulness with those of other TL-media was necessary. As is shown in Table I, its efficiency (in supporting the 0-RL) is paramount. The silica-loaded Whatman paper, as well, gave a high light output. The differences in the effectiveness of the @-RLin benzene shown on the types of media investigated are supposed to be due to differences in their benzene absorption and scattering (and self-absorption) of the emitted P-RL. The K 301 R2 as well as the SG 81 types of sample could be measured several times, even if the handling tended to reduce the registered P-RL intensity, probably because of loss of glucose microcrystals from the sample. (A further comment on this is given below in relation to Figure 4 in a later section.) A tendency of the TL material to slip from the base in K 301 R2 due to some crack formation during too rapid temperature variations was observed, while the silica load of the SG 81 looked somewhat more resistant to the repeated freezing and thawing of the scintillator liquid. For the rest of the experiments the K 301 R2 medium was chosen because of its strong and reproducible support of the P-RL process in the applied scintillators. (No sample had to be read more than once, so the tendency to lose active material upon repeating was not serious.) Linear @-RL Response to Radionuclide Activity. In Figure 2 are shown two curves, for 3H and I4C, respectively, resulting from two series of measurements where the applied activities were varied from 1 nCi to 5 pCi. Equal amounts (0.8 ml) of benzene were added to each sample and the intensities at - 190 "C were measured. A fairly good linearity governs the P-RL response to the activity, for 14Cas well as for 3H, in the range studied. Such a linear relationship (which was to be expected) is very convenient for possible practical applications of the method. From Figure 2 it may be judged that the sensitivity is rather good. Furthermore, the photometer signal could have been increased ca. five times by placing the PM photocathode closer to the sample. The time constant of the detector unit was 15s and with the uncertainties shown one may easily estimate activities down to about 1 nCi of 3H or 0.2 nCi of 14C,say for a few (2 to 5 ) minutes of registration. Detection sensitivity and noise, and the lower limit of detection. will be discussed in more detail elsewhere.
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I
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6
8
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Droplets of anthracene -benzene solution added Figure 3. 0-RL efficiency dependency on degree of anthracene impregnation
VI C
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Temp., "C Relative 8-RL efficiency
G
0
Loading with anthracene increased stepwise by repeated application of concentratedbenzene solution (pipettingof constant volumes) Benzene, Anthracene, and PPO Systems. It is of general interest to compare a new possible P-RL system for 3H detection with other well known systems. For this reason we also wanted to measure with our equipment, and under as identical conditions as possible, samples containing a solid scintillator such as anthracene or PPO. From identical registrations of the P-RL intensities of benzene and, say, anthracene, the relative efficiency of the former would then be easily obtainable at any wanted temperature within the studied range. As a prerequisite it would be necessary to know that both scintillators were applied under optimal conditions. Our procedure in the case of the frozen-benzene samples resulted from several trial-and-error experiments. Various methods for the anthracene application to TLC samples have been tried and discussed (7, 9 , I I , 13). (However, for samples prepared by grinding together SiOzand anthracene a general difference is found for different degrees of grinding the anthracene. Large anthracene grains give a poorer degree of reproducibility, probably due to uneven wetting by and drying of the 3H-glucose solution.) For the purpose of this investigation a rather simple procedure was applied: Onto the K 301 R2 sample, already tritiated, were applied repeatedly small volumes of a saturated anthracene-benzene solution by pipetting. As shown in Figure 3, one may obtain a degree of anthracene impregna-
o c- - - - : - - - - : - _ _ _ :_ - - _ I - _ _ -_I _ _ __ _: _ _ !?_-'-!-__.-I I -20 - 4 0
-60 - 8 0 -100 -120 -140 -160 -18O'C
Temperature, centigrade Figure 4. Temperature dependency in P-RL efficiency of benzene, compared to those of anthracene and PPO Measurements during slow cooling with temperature stabilization prior to each PM reading 1. Background noise from PM detector 2. Total noise with anthracene (addition of noise scintillationsdue to background radiation) 3. Benzene (cooling) 4. Benzene (heating) 5. Benzene (repeated cooling of sample 3) 6. Anthracene 7. Anthracene and benzene 8. PPO 5 and 8 reproduced from simultaneousrecordings of light output and temperature during very slow (equilibrium)cooling
tion where additional loading gives negligible enhancement of the P-RL intensity. 111the experiments the anthracene loading was always kept well within this range. In Figure 4 are shown the curves obtained for the temperature variation in the P-RL intensity of the systems benzene, anthracene, PPO, and anthracene-benzene (combined). The relative efficiencies at particular temperatures may easily be found from the curves. (Table I1 gives the relative P-RL efficiency of benzene compared to that of anthracene for several temperatures.) Again, as for the curves in Figure 1, slow cooling with temperature stabilization is applied. At the bottom of the diagram is seen the level of PM detector noise, l , and further the background signal, 2, con-
Table 11. Relative p-Radioluminescence Efficiency of Benzene at Low Temperatures Compared to anthracene under specified conditions 0 -20 -40 -60 -80 -100 -120 -140 - 160 0.16
0.20
0.22
0.27
0.29
I
0.36
1 .o
1.8
2.7
- 180
- 190
3.5
4 ~~
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taining as well a component of very faint luminescence due to background or contamination radiation traversing the samples. Curves 3 to 5 are those of pure benzene; the little maximum in 4 is obtained as a thermoluminescence component during slow heating, while branch 3 is of interest for our comparison. Upon a rough repeating of the heating-cooling process curve 5 is obtained, showing an extreme example of poor reproducibility after severe cracking in the sample. (Some of the microcrystalline glucose had probably been washed away during the second application of benzene. Such sources of error would normally not exist in a practical application of the method.) Curves 6 and 8 are for the pure anthracene and PPO systems, respectively, while curve 7 is for the binary system anthracene-benzene. The sensitivity to 3Helectrons of benzene changes from the lowest at high temperatures to the highest at temperatures below - 150 “C. The P-RL efficiency of the mixed system anthracene-benzene is seen as an intermediate between those of the components. A remarkably small temperature dependency is found for the pure anthracene efficiency, in accordance with results given earlier (11, 12), in contrast to findings reported by others (9,17). These contradictions are not thought to be due to preparative differences. As discussed elsewhere (16), all the methods applied (9, 11, 12) for anthracene loading give results consistent with those reported here. However, it is not astonishing that the greater temperature dependence in 0-RL efficiency is found in “liquid” systems undergoing a greater change in viscosity upon the temperature variation. The reason why the efficiency ratio 4 to 3 for anthracene-PPO (crystals at 30 “C) (18) is not obtained might be our special method of preparation. Anthracene impurities (in our case possibly from the benzene) might give a reduction to as little as 1/3 that of the fluorescent grade material (18). Eventually it was investigated whether any substantial degree of oxygen quenching of the benzene luminescence could be expected as a source of error in a practical application. The P-RL intensity curves were recorded for three identical samples to which benzene had been added after three different pretreatments: (1) taken directly from a new bottle (“stored under nitrogen”) with no further treatment; (2) benzene bubbled for 10 minutes with Nzbefore use; and (3) benzene bubbled for 10 minutes with O2before use. No significant deviations in 0-RL intensity could be observed within the temperature range 0” to -190 “C. Even for a one-component scintillator liquid quenching could have been expected (18). However, oxygen exchange in liquids is known to take place very rapidly (18) and may to some extent have happened with our benzene samples after purification. The Nz (18) applied was not prepurified. CONCLUSIONS
The data reported are thought IO indicate the general usefulness of the suggested method of applying frozen liquid scintillators to enhance the radionuclide detection sensitivity in TLRC by the action of P-RL. The method should also be useful both for exposure of photographic emulsions (oneor two-dimensional RC) and for PM measurement of separate (16) J. F.Koren, T. B. Melo, and S.Prydz, J. Chromafog., in press. (17) K.Randerath, ANAL.CHEM., 41,991 (1969). (18) E. Schram, “Organic Scintillation Detectors,” Elsevier, Amsterdam, 1963. 160
*
spot activities. Further, a technique for strip scanning of frozen preparations might be developed. These methods, based on benzene scintillator, would have the advantage of high sensitivity with no contamination of the fractionated components in the chromatogram. The response of the system is linear with respect to radionuclide activity and the reproducibility is fairly good. The benzene system is virtually free from oxygen-quenching effects. Drawbacks include the tendency of cracking of the K 301 media during cooling with added scintillator liquid. Further, in practical application, there might be some difficulties concerned with the establishment of a useful cooling technique. Intense spikes of luminescence are seen in benzene (and acetone) during fast cooling of the samples. This luminescence is independent of radionuclide application and seems t o be a triboluminescence phenomenon possibly correlated to crack formations. However, the integrated spike luminescence may be ignored as a background component. The temperature data given for the scintillator efficiencies are of more general interest. The relative P-RL efficiency of benzene (compared to that of anthracene, both determined under optimal conditions) is given in Table I1 for the temperature range studied. It is greater than 1 below about - 120 “C. Bearing in mind results given by others (9, 17), the small temperature dependency found for anthracene- and PPO p-RL is to be noted. The discrepancy does not seem to be due to differences in sample preparation (16). The main difference between the experiments quoted (9, 17) and ours is that we applied PM detection instead of film exposure at low temperature. The PM detection is not influenced by any electron emission, as the film technique might be. An electron component, being temperature-independent, would partly mask the temperature effect under investigation. In addition, the film measurements would be thought more sensitive to differences in the tritium distribution within the sample. This is only partly due to the electron exposure component. While PM measurements are rather insensitive to small variations in spot area (for a constant radionuclide activity), fainter film exposures obviously result from expanded spots. This, by visual inspection, will give erroneous results, and even densitometric integration might be inaccurate, especially for very low or high densities. If long-lasting film exposures are involved, two more complications arise in comparative work. First, fading of the latent image might give errors in judgments based on the final exposures observed after development. The fading is absent from some emulsions, but might be rather large in others. Secondly, a deterioration of the scintillator sensitivity @-RL efficiency) can be an important source of error, at least for exposures lasting more than 1 to 2 weeks. Very recent results (16) from a study of the temperature dependence of the film emulsion sensitivity to ordinary and scintillating light indicate a possible explanation of the great discrepancy between the PM and the film measurements. Thus we believe (16) the low temperature increase in efficiency found for scintillation detection on film material reported earlier to be a pure emulsion effect, when using anthracene (9) as well as PPO (17) as scintillator. Any differences in spectral distribution of emitted P-RL will easily result in “wrong” relative values when a wavelength-dependent technique of detection is used for comparison of scintillators. Similarly, when two detection systems with different wavelength dependencies in their sensitivities are applied for evaluation of scintillators, different re-
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sults may be obtained. This, however, cannot explain any large fault in the curve for the anthracene temperature dependency if no great wavelength changes in the emission take place during the cooling, effectively shifting the luminescence out of the sensitive range of one (but not the other) of the detector systems. (This is very improbable in our case.) Finally, one way of giving a quantitative assignment to the limit of detection is to state (for film methods) the number of pCi necessary per square centimeter (of spot area) to give a detectable blackening during 24 hours. This figure has been estimated for the various film techniques available up to 1964 by Chamberlain et al. (19). They obtained the limit of 0.033 pCi x day per cm2by impregnation of the chromatogram by an x-ray-sensitive film emulsion. Luthi and Waser, using large quantities of anthracene as a scintillator admixture, find (at low temperature) a limit of 0.08 pCi for 50% anthracene in SiOz or even 0.005 pCi for 100% anthracene thin layers (for an exposure time of 24 hours). They do not give their spot areas, making a direct comparison difficult. These values cannot be directly compared with what we quote for PM detection. Still, we believe our observed temperature variations for the 0-RL efficienciesof the benzene, PPO, and anthracene systems to be correct. For pure benzene, 50% anthracene (in SiOz), and pure anthracene preparations (measured at -190 “C in units as used in Figure 4), we get the relative P-RL intensities (13) of 5000, 4000, and 13,000. Referring to very simplified models for the TL media, pure anthracene should be more effective by a factor of 2 than the ideally impregnated types where the active material (adsorbed as a “layer” on the SiOz grains) has an anthracene scintillator on one side only. (For a particular point on an SiOzgrain surface the scintillator covers 2 x only of the total 4x solid angle.) For accurate and fast measurements of single-spot activities the low-temperature benzene method combined with PM detection seems very useful. It might be convenient in combination with film exposure (for one- or two-dimensional chromatograms) when noncontaminating detection is essential. However, the feasibility for many purposes of using admixtures in the TL media of solid scintillators (9, 12, 13) (which might be equally p-sensitive at room temperature as at lower ones) should be kept in mind. ACKNOWLEDGMENT
The authors are grateful to other members of the Institute
of Physics, University of Oslo, for their helpfulness and interest in the investigation. APPENDIX-METHODS AND NOMENCLATURE
One might suggest the term “autoradiography” to be used exclusively for the exposure of film material (or of a positionsensitive kind of detector) by radiation from radionuclides. (19) J. Chamberlain,A. Hughes, A. W. Rogers, and G. H. Thomas, Nature, 201, No. 4921, 774 (1964).
The term ‘‘@-radioluminescence@-RL) detection” is used to cover all processes involving the emission and registration of photons released by electron absorption (in solid as well as liquid phosphors or scintillators). The detection may be obtained by use of a photodetector or a photomultiplier (PM) as well as by film exposure. The term “scintillation autoradiography” thus appears as a “mixed term.” Its use correspondingly should be restricted to mixed cases only-namely, those where both light and electron exposures are taking part in the process responsible for the final film image. Similar “scintillation fluorography” is a mixed term according to our suggestions, while @-RL is a useful term when neither fluorescence nor scintillation detection can be used exclusively. (By fluorescence is normally understood a singlet-singlet de-excitation with photon emission.) In photoelectric detection of the luminescence from @excited phosphors or scintillators two available methods may be applied: measurement of the anode current of the PM or counting of PM pulses. When (solid or liquid) scintillators with short enough decay times are used (in the last case), so that each scintillation gives rise to one count only, the correct term would be “scintillation counting.” [In addition to the mere counting, scintillation detectors often use a direct analysis (by electronic networks) of the detailed time distribution and the size (total number of photons) of the individual scintillations.] However, as has been argued by Randerath (20), the term “P-RL detection” has a much wider coverage than “scintillation fluorography” which he prefers for describing the specific method (or process) of solid scintillation detection of radionuclides on film materials. Certainly one should pay attention to his reasons-namely, that the term “fluorography” is already well established in x-ray technology, implying that the photo image obtained “is caused chiefly by photons” (as opposed to autoradiography). Further, he points out that “-graphy” designates the class of techniques to which photography and autoradiography belong-those employing visual imaging. On the main points we fully agree in his comments, even if “-graphy” strictly says nothing about the use of film materials. Following more or less the intentions of Randerath and Waser (21) one might perhaps suggest a term “scintillography” or, even shorter, “scintography” to cover the combined use of scintillator and film for radionuclide detection. The requirements would be that the scintillator decay time is short enough to produce scintillations really separated in time and that no direct electron exposure of the film takes place. (Benzene, anthracene, and PPO are fast enough to fulfill the first requirement. The second criterion normally is fulfilled for tritium, but seldom for other radionuclides.) RECEIVED for review June 9, 1969. Accepted November 3, 1969. (20) K. Randerath, private communication. (21) P. G. Waser, private communication.
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