LUMINESCENCE AND COLOR EXCITED BY RADIUM I N ZINC BORATE GLASSES WHICH CONTAIN MANGANESE BYRON E. COHK Department of Physics, University of Denver, Denver, Colorado AND
S. C. LIND School of Chemistry, University of Minnesota, Minneapolis, Minnesota
Received September 19, 1997 INTRODUCTION
It is well known that radiations from radium color certain soda-silicate glasses a deep violet color. Rutherford (9) attributes to M.and Mme. Curie the first observations of this effect. Clarke (1) found that glasses colored purple by exposure to radium contained manganese, whereas the purple color was not produced in the absence of manganese. As early as 1901 Wiedemann (10) showed that certain solids when exposed to radium exhibited the property of thermoluminescence. Because exposure to radium produces both coloration and thermoluminescence and because heat discharges both the luminescence and the color, these effects have been associated by many of the workers in this field. However, on the basis that the thermoluminescence could be liberated without markedly changing the visible color, Lind (6) indicated that the connection between color and luminescence was not always so close as had commonly been supcosed. An investigation of the effects of exposure to radium of synthetic zinc borate glasses, each of which contained a known concentration of manganese, offered promise of more exact information than has hitherto been available. This paper is the report of such an investigation. THERMOLUMINESCENCE
A series of zinc borate glasses in the form of 6-mm. squares, each 2.7 mm. thick, which contained known amounts of manganese ranging between 0 and 5 per cent were prepared as indicated by Cohn and Harkins (3). These samples were exposed to 140 mg. of radium at a distance of 1.7 cm. for the equivalent of approximately seven gram-days.' Under the conditions of exposure the radiation responsible for the energizing of the Exposure to 1g, of radium for one day is referred to as a gram-day; if a glass were exposed t o 0.1 g. for ten days, this also would be indicated as one gram-day.
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BYRON E. COHN AND S. C . LIND
glass specimens 15 as mainly gamma rays, although beta radiation may have contributed a small fraction of the energy. The thermoluminescence was determined by a modification of the total light area method of Nysvvander and Lind (8). Intensity-time areas of glass saniples heated at a constant temperature of 100°C. were taken as a measure of the energy emitted in the process of luminescence. The values of the areas nere then each multiplied by the same factor, so that the maximuni luminescenee would have the value 100. The results are presented in figure 1, curve -4. The ralues of manganese concentration are plotted 011 a non-uniform scale in order t o spread the values for the lower concentrations, as the greatest changes were in this region. Semilogarithmic cross-section paper was uqed for this purpose. From this graph it can be seen that a change in the amount of manganese in a glass qpeciinen has an effect yhich is quite
P e e d Manyome FIG 1 Thermoluminescence in zinc borate glasses n hich contain manganese C uive A , thermoluminescence excited by exposure t o radium; cuive B, thermoluminescence excited by exposuie t o quartz nieicury arc.
h i i l a r t o the effect of activators when other sources of energy arc used to excite the luminescence. It nil1 be observed that the optimum concrntration of manganeqe i. approximately 0.05 per cent. T o determine the effect of the time of exposure to radium upon the optiniuni concentration the luminescence nac inpawred in a set of samples whose exposure to ratliuni n a s equivaleiit to approxiinately 120 gram-days a t a distancp of 1.7 cm. This time the niaxiiiiuni intensity method of Nyswander and Cohn ( 7 ) 1va5 employed. I t na5 found that the optimum concentration reniaincd a t 0.05 per cent nianganev. The optimum concentration is therefore independent of time of e x p o w e This is of considerable interest, a, i, a i d e n t from the comparison of curves A and R in figure 1. Curve B presents the value.. for the thernioluminewence excited by ultraviolet light (mercury arc 111 quartz) and nai: obtained by Cohn and Harkins (3). It can be wen that the optimum concrntration for the therinoluiiiii~escencc
COLOII EXCITED I N GLASSES BY I i A D I C h l
is :~l)l)roxiiii:ttc,ly0.2 I W I . (wit i i i tliv 1:rt tc.1. ( w v . sliift of thv optiiiiiiin will
1 x 3
iiitlir:itc\tl 1:itc.r
iii
443
.\ possil)lo I lliis p y o r .
YIP clniissioii spc,ctriun of thcx Iiunincsccnre induced in thcsc ziiic borate band in the rcd, ycllo\v, and g r m i with its inaximiini intensity in thc rod at ahout 6000 b . ' I ' l i ~ sprrtriim was cxamiiird visually with the aid of a transmission grating, iising the edgr of a glowing spccimen as its own qlit, The rmission spectrum appear3 the same whcthcr radium or riltra.r.iolct light is i1sr.d to rxrite the thermoluminescence. Indeed, thc spectruni eniittrd apprars to be indrpendent of the means of excitation or of the typr of Iuniiiiescrnw (fluorescence or thermoluminescence). Similar rmission in the type of glasses iised was found by Cohn (2) for ultraviolet fluorescence, and by Kabakjian ( 5 ) in t'he case of cathode-ray fluorrswnrc>. Both worlirrs used zinr borate glasses which contained manganesr. g1:tsscs is a
COLOR PRODUCED BY EXPOSURE TO RADJiTM
series of zinc borate glasses as indicated in thc preceding scction ww cixposed t o radium. Before irradiation the glass squares appear colorlcss \dien they contain less than 1 per cent of manganese. They have a w r y slight yellowish tint beginning at 1 per cent of maqganese and increasing in intmsity for samples up to 3 per cent of manganese. Samples whicali cmtained 4 and 5 per rent of manganese are a light amber color. Upon rxposiirc to radium the samples which contain manganese acquire a yiol(,tpiirplc tint of much smaller intensity than is the case in soda lime glasses. Tlir sample which contained the highest concentration of mangancsc ncqiiirrd the violet-purple tint first, and so on in the order of manganesr cwntcnt. After exposure at a distance of 1.7 cm. for the equivalrnt of approximately seven gram-days the specimen which contained 0 per cent of manganese appeared slightly yellow; the 0.01 per cent manganese glass was of neutral gray tint; the 0.02 per cent sample exhibited a trace of I,lue-violet, which tint appearpd to increase in intensity with increase of manganese concentration t o samples which contained 0.2 per cent manganese. The samples which contained more than 0.2 per cent manganese rxhibited about the same intensity of coloration. However, the color t,irit in sprcimrns which contained more than 1 per cent of manganese was of pink-violet hue as opposed to the blue-violet color of the others. It W ~ found S that thc color changr appeared to approach a maximum value for a given concrntration of manganeiie. For example, t h r change in the yisihle absorption determined in samples exposed approximately 120 equivalent gram-days at x distanw of 1.7 cm. was almost the same as for glassrs exposed 7 gram-days at the same distanre. The small differcww in the absorption in these glasses after a strong exposure agrees with a similar finding by Kabakjim (4), who found that the coloring duc \I
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Bl7tON E. COHN AND S. C. LIND
to exposure to radium approarlwd a ronrtant valne providrd the intensity of thr source remained coimtant. LIQHT ABYOIIPTION I N THE ULTIWVIOLET IlEGION
Spectra were photographed of the transmission of the lines from a quartz mercury arc through the original samples as compared with samples which had been exposed to radium. This was done by placing the glass squares one above the other in front of the slit of a Gaertner quartz prism spectrograph, and photographing the resulting spectrum. Figure 2 shows a set of the compariqon spectra for a zinc borate glass which contained 0.1 per cent of
h a . 2. Comparison spectra through zinc borate glasa which contains 0.1 per cent of manganese. Upper speotrogram: top, through 2.7 mm. of gl- exposed to radium; bottom, mercury arc lines. Lower 8peCtrOgram: top, through 2.7 mm. of glsss exposed to radium; bottom, through 2.7 mm. of same gl-8 not exposed to radium.
manganese. The upper spectrogram presents (upper half) the spectrum of a quartz mercury arc when the light was caused to pass through 2.7 mm. of Bine borate glass which had been exposed to radium. The lower half of the upper spectrogram shows the mercury spectrum as received upon the slit of the spectrograph. The lower spectrogram presents a direct comparison between a zinc borate glass which had been exposed to radium (upper Half) as Comp8red with a zinc borate glass of the same thickness which had not previously been expcmed to radium (lower half). The Concentration of manganese in both glarrses was 0.1 per cent. From the lower spectrogram it is evident that exposure of the g l w to radium produced 8 striking change in its transmiasion for shorter wave lengths of light.
COLOR EXCITED IN GLASSES BY RADIUM
445
Quantitative measurements of the change in transmission in the ultraviolet region were made. The measurement of the density of the photographic deposit caused by ultraviolet light passing through (a) glasses which had not previously been exposed to radium and (b) glasses of the same manganese content which had previously been exposed to radium mere made with the aid of a densitometer arranged for the purpose. The photographic plates were Eastnian D.C. Orthochromatic and were developed by Rodiiial (1 part) and water (15 parts) for 4 minutes at 20°C. using rocked-tray technique. They were placed for 45 minutes in a fixing
FIG.
3
FIG.
4
FIG.3. Change in absorption caused by exposure to radium. The absorption is indicated as density changes a t given wave lengt$s as a function of the concentrati?n of manganese. Curve A, 2537 A,; curve B, 2654 A , ; curve C, 2804 i.; curve D, 2967 -4. FIG.4. Per cent absorption change caused by exposure to radium. The absorption is indicated as per cent density change graphed as a function of wave length. Curve A, 0.2 per cent manganese; curve B, 0.05 per cent manganese; curve C, 0.01 per cent manganese.
bath at 20°C., and then were thoroughly washed and dried in a dust-free atmosphere. The densitometer consisted of a light source, a microscope, a photronic cell equipped with a slit, and a galvanometer. The image of the spectrum line on the photographic plate was thrown upon the slit of the photronic cell, and the light intensity was recorded by means of the galvanometer. The intensity o€ the light source mas held constant by means of a voltage regulator. The densities of the deposits on the photographic plate were computed, and the curves for the changes in density due to the exposure of the glasses to radium were determined. The photographic density (D) for each spectrum line was computed by the formula
D
=
loglo Go/G
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B Y R O S E. COHN AKD S. C. LIND
where GOwas the value of the galvanometer deflection given by the projection of light through the clear plate in the vicinity of the spectrum line, and G was the galvanometer deflection when the image of the spectrum line covered the slit aperture of the photroiiic cell. The change in density was expressed as the difference D a - DR, q-vhere D, was the density of the spectrum line obtained by passage of the light through the original glass and D E was its density after passage through a glass which had been exposed to radium .? In figuie 3 are presented curves which shorr- the change in density ( D , - DR)due to exposure to radiuniplotted as a function of the concentration of manganese. Curve A represents the change in absorption at The approximate relationship beta een the values of t h e change in density ( A D ) computed from the measurements on a photographic plate with the value of the change in extinction coefficient ( A E ) nhich could be obtained from direct measure-
ment is indicated in the follovring given by
By definition the extinction coefficient ( E ) is
E’
l / d log,, Io/I
where I o is the light transmitted initially, I is the intensity transmitted thiough the specimen, and d is the thickness of the specimen If measurements were made with two specimens, one exposed t o radium giving a transmitted intensity I R , the second of t h e same original material b u t not exposed t o radium, giving a transmitted intensity I,y, and with the same light intensity from a monochromator, I O ,one would obtain
OP
Assuming (I) the Abney-Schuarzschild law of the photographic plate,
D
=y
log,, I t ”
-i
(8) a fixed time of pxposure, and (3) a constant developmental procedure, one obtaina
for (AD) froin this equation
AD =
D,y
- D R = y login I . ~ / I R
From the definition for D above in terms of galvanometer deflections arid the a s s u m p tion of t h e direct proportionality of galvanometer deflection t o light a t the slit 01
the photronic cell, one obtains AD = 1og:o GRIGN =
Y
log
io
Is/IR
From these t w o values one obtains for AE the result 1 1 AE = - logio GR/G.v = - AD dr dr The change in extinction coefficient is therefore approximately equal to the changc in density divided )ST. the product of the thickness of the sample and the photographic constant gamma.
COLOR EXCITED IN GLASSES BY RADIUM
447
2537 A., curve B at 2654 A., curve C a t 2804 A., and curve D at 2967 A. A progressive shift of the maximum absorption toward higher manganese concentration with increasing wave length is quite apparent. It will be noted that the exposure to radium has caused the appearance of an absorption band in the ultraviolet region of the spectrum; that for the region in the vicinity of 2500 A. the absorption band has its maximum concentration at approximately 0.05 per cent manganese, but that at longer wave lengths this absorption band is displaced toward a higher concentration of manganese. At 2967 A. the maximum of the absorption band corresponds approximately to 0.3 per cent manganese. From this it would seem that the optimum concentration for the thermoluminescence is very closely related to this absorption in the ultraviolet region, as a similar shift in the optimum thermoluminescence was observed in figure 1. It is readily understood that the optimum condition for luminescence will be at that concentration where conditions are most favorable for the production of that luminescence. If' the luminescence due to radium is an optimum at 0.05 per cent manganese, it means that the conditions are most favorable M hen the ratio of zinc to manganese atoms in the zinc borate glass has a given value. Since the optimum for thermoluminescence induced by exposure to radi2m closely approximates a change of absorption in the region of 2500 A., it is probable that the particular reaction between material and light ahich produces luminescence is directly related to the absorption in the ultraviolet region. It is of some interest t o obtain the approximate form of the absorption curve as a fuiiction of the wave length. To do this the values of the changes in density similar to those presented in figure 3 were divided by the corresponding initial density for each concentration and spectrum line. These values were graphe4as a function of wave length. Curve A of figure 4 gives the values of the per cent density change due to exposure to radium for zinc borate glasses which contain 0.2 per cent of manganese. Curve B presents the same relation for samples which contain 0.05 per cent of manganese, whereas curve C presents the values for samples which contain 0.01 per cent of manganese. This curve can be taken only as an approximate indication of the absorption, owing t o the fact that alrariation of the value of the photographic constant gamma with change of wave length is to be expected. An even more striking fact is brought out when the difference in densities is again determined after the samples have been heated to discharge the luminescence. This was done by heating the samples for one hour at 135°C. Most of the luminescence is discharged by this process, although a small portion of energy available as luminescence may still remain. These values of change in density are subtracted from the original change in density to give a measure of the difference in the absorption which m'as
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BYRON E. COHN AND S. C. LIND
caused by the process discharging the luminescence. The process of obtaining these data is illustrated by figure 5. The change in density for wave length 2537 A. before heating is presented in curve A. The difference of density after heating is indicated in curve B. The difference between the densities in curve A and curve B represents the change in absorption which occurred during the interval the luminescence was emitted, and is represented by curve C. The similarity in the shapes of each of these curves is apparent and appears to indicate the diminution in intensity of an absorption band. I n figure 6 are graphed the changes in
FIG.5
FIG.6
FIG. 5. Absorption change during luminescence emission. The curves are all given for absorption changes a t 2537 A. Curvp A, absorption caused by exposure t o radium; curve B, absorption which remains after the emission of thermoluminescence; curve C, change in the absorption during the interval thermoluminescence was emitted. FIG.6. Change in absorption during the emission of luminescence. Curve A, 2537 d.;curve B, 2654 A,; curve C, 2804 A,; curve D, 2967 d.
s
density during the emission of luminescence for wave len ths 2537 A. (curve A), 2654 A. (curve B), 2804 A, (curve C), and 2967 . (curve D). The maximum density for each wave length is given a value of 100 for the purpose of comparison, and therefore the ordinates are indicated as per cent density change. This has been done to eliminate the intrinsic difference of the intensity of the mercury spectrum lines from the comparison. The change of absorption maximum with the wave length is quite evident. In figure 7 curveoA represents the change in absorption density at the wave length 2537 A. during the time luminescence was emitted, whereas
COLOR EXCITED IN GLASSES BY RADIUM
449
curve B represents the luminescence which had its origin in the excitation due to exposure to radium. The densities and the luminescence have both been computed in such a way that an arbitrary value of 100 represents the maximum value of either curve. A similarity between the two curves is apparent. I n figure 8 curve A represents the change in absorption at 2804 A., while curve B represents the luminescence emitted when the luminescence is excited by ultraviolet light (see figure 1). The correspondence is again quite evident. From these results it appears that changes in the optimum concentration can be caused by the variation of the absorption of the exciting energy with wave length. It can be predicted that
FIG.7 FIQ.8 FIG. 7. Comparison curves of absorption change and luminescence. Curve A, absorption change at 2537 b. (figure 6, curve A); curve B, luminescence emitted due t o radium (figure 1, curve A). FIG.8. Comparison curves of absorption change and luminescence. Curve A, absorption change a t 2804 b. (figure 6, curve C); curve B, luminescence emitted, excited by mercury arc (figure 1, curve B).
a change in the wave length of an ultraviolet source used for the purpose of excitation of thermoluminescence would shift the optimum concentration for an activator. LIGHT ABSORPTION IN THE VISIBLE REGION
It has been indicated that there appears to be a definite correspondence between the absorption changes in the ultraviolet region and the luminescence which is emitted from zinc borate glasses which contain manganese as an activator. It was desirable also to investigate quantitatively the changes in the visible transmission of these samples. Unfortunately, the change in the visible transmission was SQ small that photographic measurements could not give the necessary accuracy for the determination of the
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BYRON E. COHN AND S. C. LIND
difference in density. For this reason a direct measurement of the transmission of light confined to given spectrum regions was obtained by means of filters. The densitometer used consisted of a light source, filter, Icns, aperture, photronic cell, and galvanometer. The light source was a 40a a t t tungsten-filament lamp which was supplied with current from the secondary of a regulating transformer. During the nieasurements the voltage across the lamp remained a t 112.4 volts &0.1 volt. The density (D) for each sample was computed from the photometric measurement by
D
=
loglo I0,’I
where I o is the galvanometer reading corresponding t o t h r initial intensity and I is bhe galvanometer reading after the light has traversed the sample. The values for the changes in density are given by D, - D,, where D, is the value of the density after light has travrrsed the sample which has been exposed t o radium and D,is the density obtained nhen the light has traversed the original ample.^
F I G 9 Absorption change in visible region due t o exposure t o radium. Curve A, density change in violet portion of spectrum; curve B, density change in red region of spectrum.
The results obtained in this way are presented in figure 9. The values of the manganese concentration are again plotted on a non-uniform scale to spread the values for the lower concentrations Curve A shows the change in density before and after exposure t o radium for the violet portion of the spectrum. T o isolate this region Corning Glass Code 554 and Glass Code 306 filters were used in combination. Curve B gives the change in densityjvhen the light transmitted was red of wave lengths greater than 6100 A. The red region was isolated with the aid of a Corning Glass Code 224 filter. I t is observed that the change in density in the red region is greater than that in the violet portion of the spectrum. It would appear from these curves that the blue-violet tint TT hich results from the exposure t o radium of zinc borate glasses which contain man3 The value of the visible absorption is given in terms of the density, because the ultraviolet absorption a as expressed in somewhat similar terms. T h e t n o are related in t h a t if the density differences are taken as indicated the values of the changes in density on the photographic plate mhen divided by gamma are comparable with the density obtained by the direct method (see note 2)
COLOR EXCITED IK GLASSES BY RADIUM
45 1
ganese is to be attributed to a greater relative change of the absorption in the red region of the spectrum. More important, these curves show that the maximum absorption in the visible region is in the vicinity of 0.5 per cent manganese. As the thermoluminescence optimum is at 0.05 per cent manganese concentratiori, it appears that there is no direct rrlationship between the visible color and the thermoluminescence emission. Curves similar in form to those of figure 9 were obtained when the glasses had been subjected to heat treatment. The results show that the visible color is greatly reduced by prolonged heat treatment, but that any residual color which remains has a maximum in the region of 0.5 per cent manganese and therefore the change in color intensity bears no direct relation to the emission of thermoluminescence. I t was also found that both the ultraviolet transparency and the visible transparency of the glasses were restored to approximately their initial values by prolonged heat treatment at 300°C. SUMMARY
1. The luminescence produced by the exposure to radium of zinc borate glasses which contain manganese indicates that the ordinary rules which govern activators are applicable when radium is the source of the excitation. 2. The optimum concentration of manganese in zinc borate glasses when the luminescence is excited by exposure to radium is 0.05 per cent of manganese. 3. Exposure to radium of zinc borate glasses which contain manganese produces a change in visible color which reaches a maximum a t approximately 0.5 per cent of manganese. 4. The violet tint developed by exposure to radium of zinc borate glasses which contain manganese is caused by a greater relative absorption in the long wave-length region of the visible portion of the spectrum. 5 . Exposure to radium of zinc borate glasses which contain manganese results in a general increase of absorption in the ultraviolet portion of the spectrum. 6. For each wave length in the ultraviolet region there is a curve which represents the change of absorption as a function of the manganese concentration, and each of these curves has a maximum. 7. The maxima of absorption in the ultraviolet region occur at the same concentrations as luminescence optima. 8. From the results it is inferred that the absorption in the ultraviolet region is closely related to the luminescence which is emitted when the specimens are heated. 9. The initial visible and ultraviolet transparencies are almost completely restored by heating for prolonged periods at temperatures a t or above 300°C.
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BYRON E. COHN AND S. C . LIND
One of the authors (B. E. C.) gratefully acknowledges the financial aid given by the van’t Hoff Foundation to complete this work. The authors wish to express their thanks to Drs. Robert Livingston and R. E. Hull for technical assistance and constructive criticism. REFEREKCES
CLARKI,J. R. : Phil. Mag. 46,735 (1923). COHN,B. E . : J. Am. Chem. SOC.66,953 (1933). COHN,B. E., AND HARKINS, W. D. : J. Am. Chem. SOC.62, 5146 (1930). D. H.: Phys. Rev. 44, 618 (1933). KABAKJIAN, D. H . : Phys. Rev. 61,368 (1937). KABAKJIAN, LIND,S. C. : J. Phys. Chem. 24, 442 (1920). R. E., AND COHN,B. E. : J. Optical SOC.Am. 20, 131 (1930). KYGWANDER, NYSWANDER, R . E . , AND LINU,S. C.: J. Optical SOC.Am. 13,651 (1926). RUTHERFORD, E. : Radioactive Substances and their Radiations, p. 307. Cambridge University Press, Cambridge (1913). (IO) WIEDEMANN, E.: Physik. Z. 2, 269 (1901). (1) (2) (3) (4) (5) (6) (7) (8) (9)
