Parameters affecting the intensity of light sources powered by tritium

NUCLEAR

INSTRUMENTS

AND

METHODS

I30

(I975)

231-237;

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NORTH-HOLLAND

PUBLISHING

CO.

PARAMETERS AFFECTING T H E INTENSITY OF L I G H T SOURCES P O W E R E D BY TRITIUM A. K O R I N * , M. G I V O N

Soreq Nuclear Research Center, Yavneh, Israel and D. W O L F

Isotope Separation Plant, Weizmann Institute of Science, Rehovot, Israel Received 5 September 1975 T h e intensity o f light sources powered by gaseous tritium h a s been related to physical p a r a m e t e r s o f the light source, such as gas pressure, tube diameter, coating thickness a n d grain size o f the p h o s p h o r . Empirical relations for the intensity as a function

o f these p a r a m e t e r s have been derived. These relations are generally o f a form similar to those o f the properties o f the screens o f cathode ray tubes.

1. Introduction Light sources powered by radioactive materials have been studied for many years and are nowadays commercially available1). Various radioisotopes such as krypton-85, strontium-90, promethium-174 and tritium have been considered, but tritium is most commonly used2). Since almost no quantitative information can be found on the performance of these light sources, we have undertaken to study the parameters affecting the performance of such sources based on gaseous tritium. Some of the parameters studied are: the dimensions of the tube, the pressure of the tritium gas, the thickness of the phosphor layer, and the grain size of the phosphor.

low energy and of its short biological half-life (about 12 d in its most hazardous form: water). f) Its price is low. g) It is available as a gas. Other radioisotopes are inferior for various reasons: krypton-85 is a gamma emitter, presents a radiation hazard and its radiation darkens glass, which would decrease the light output with time. Strontium-90 is extremely poisonous, and promethium-147 has a shorter half-life (2.5 y).

1.1. THE ISOTOPE Tritium has proved to be the most advantageous radioisotope for powering light sources for the following reasons2-4): a) It is a pure beta emitter with a maximum energy of 19.5 keV and a main energy of 5.7 keV. Therefore, relatively large activities do not present any radiation hazard, since all the radiation is absorbed, and when suitable light materials are used as a protective envelope, the bremsstrahlung is negligible. b) Its physical half-life of 12.3 y is sufficient for preparing relatively long-lived light sources. c) Because of its low energy radiation damage to the phosphor is relatively low. d) By reason of its short range (average range is 0.5 mm in air) all radiation is absorbed in the phosphor, which provides for a high conversion yield. e) Its biological hazard is very low, by reason of its

1.2. LIGHT SOURCES Light sources consist of a glass tube of any desired shape that is coated at the inside with a radiation stable inorganic phosphor5'6). This phosphor is activated by radiation and emits light. When the layer is thin enough it is semitransparent and the light can pass through it. When a gaseous radioisotope is used it is simply admitted to the tube, which is then sealed off. When it is a solid material, it has to be mixed intimately with the phosphor before coating. In the case of tritium, phosphor containing polymers, or polymers containing phosphorescent organic molecules have been used7'8). In all the latter cases the light output decreases because of absorption in the polymer or the isotope-phosphor mixture. The light output decreases with time due to radiation damage to the phosphor, decay of the isotope, and in the case of tritium by the pressure increase on decay 2'3) (two atoms of helium-3 are formed from one molecule of tritium gas).

* Present address: W e i z m a n n Institute o f Science, R e h o v o t , Israel.

231

2. Experimental 2.1. MATERIALSAND TUBEPREPARATION The phosphor was coated onto the tube according

232

A. K O R I N et al.

2.2. THE PHOTOMETRICSYSTEM

Fig. 1. Detail of photomultiplier housing. A - Light source guard, B - light source connector, C - hole, D - light pipe, E - plasticine, F - photomultiplier, G - light source, H - locking nuts. to a modified version o f the conventional method o f coating cathode ray tubes, using phosphoric acid dissolved in acetone as a binder. After heating the acid polymerized and a homogeneous p h o s p h o r layer was obtained. The p h o s p h o r used was Radelin P-11, manufactured by the U.S. R a d i u m Corporation, New Jersey. Three grain sizes were used: 15/tm (type 272B), 11/lm (type 1039F) and 2.5/~m (type 3129). It is a hygroscopic powder, based on zinc sulphide, and its emission spectrum is continuous between 390 and 600 nm, with a maximum at 490 nm. The density of the p h o s p h o r layer was measured by weighing a number of tubes before and after coating. The densities o f the phosphor coating as a function of the phosphoric acid concentration are given in Table 1. The error is + 10%. The light absorption of the phosphor layer was never more than 1.5 %. The tubes were made o f cylindrical Pyrex glass and were of the following dimensions: (a) 1 m m i.d. and 6 . 0 m m o.d.; (b) 2 . 0 m m i.d. and 5 . 0 m m o.d.; (c) 3 . 4 m m i.d. and 5 . 2 m m o.d.; (d) 6 . 0 m m i.d. and 8.0 m m o.d. All tubes were 110 m m long.

The photometric system consisted of a hv power supply, a photomultiplier with a voltage divider, a micromicroammeter and a stripchart recorder. The photomultiplier was RCA-5819, whose spectral range is between 300 and 650 nm. The spectral range of the p h o s p h o r lies in the range of highest sensitivity of the photomultiplier. The photomultiplier was placed in a lightproof housing. A transverse tube was built into the housing into which the light-emitting tube was placed. Optical contact was established by means of a short piece of quartz rod, that acted as a light pipe. This quartz rod was suspended by black plasticine in order to avoid mechanical stresses that might break the glass tube. Details on the photomultiplier housing are shown in fig. 1. The photomultiplier was operated at a voltage of 550 V, where the ratio between signal to dark current was maximal. The sensitivity of the arrangement was 3.5 A/lm. The system was checked periodically with a standard light source. The current was measured with a Keithley micromicroammeter with a maximum sensitivity of 10 - 1 2 A.

2.3. THE GAS SYSTEM

The gas system consisted of a p u m p i n g system (rotation and diffusion pump) and a manifold to which a uranium cartridge (which served as a tritium reservoirpump), a manometer, a calibration volume and the light source were connected. It was built o f 3- stainless steel tubing, and connections were made with -83" G y r o l o k seals (Hoke Inc). A diagramatic representation of the gas system is given in fig. 2. Since pressures higher than atmospheric were used in the system, care had to be taken that the glass tube is not blown out. The open end of the tube was therefore shaped into a flange, which was passed through a stepped metal tube of ~" o.d. The seal was made with Torr-Seal (an epoxy cement manufactured by Varian Inc.) which gave a rigid vacuum-tight connection. The

TABLE 1 Density of the phosphor coating (mg/cm2).

Conc. of HsPO4 in acetone (%) Phosphor 272B Phosphor 1039 F Phosphor 3129

0.01

0.05

0.1

1.0

2.0

5.0

10.0

2.0 1.2 0.6

3.0 1.8 0.8

3.5 2.2 1.4

4.0 2.6 2.0

6.4 5.0 2.9

10.5 8.5 3.3

11.2 9.1 4.1

INTENSITY C \\

/ /D v-I --I><]"

OF LIGHT

\

v-2

v-6

(~

,-7 v-8

~v - 4

F

Fig. 2. T h e gas s y s t e m . A - U r a n i u m c a r t r i d g e , B - f u r n a c e , C - m a n o m e t e r , D - c a l i b r a t i o n v o l u m e , E - l i g h t source, V - valves.

tubes were tested at twice the maximum pressure before the experiment. 2.4. E X P E R I M E N T A L P R O C E D U R E After the tube had been connected, the system was evacuated, and the dark current of the photomultiplier was measured. Tritium was added to the system by heating the uranium cartridge, its pressure was read off and the photocurrent was measured. This process was repeated in steps of 200 torr up to a maximum pressure of 2000 torr. At the end of the experiment the tritium was reabsorbed on the cold uranium, and the tube was disconnected.

233

SOURCES

3. Results The dependence of the brightness on the pressure is approximately linear at low pressures (up to about 600 torr), and flattens off at higher pressures. Table 2 shows the results for the phosphor of 2.5/Lm grain size and a tube of 3.4 m m i.d. The coating density is a parameter in the table. In all other cases the general behaviour of the brightness vs pressure curves was the same. The effect of the tube diameter was studied for the phosphor with grainsize of 11/zm and coating density of 2.6 mg/cm 2. The experimentally measured quantity was the brightness at the outside of the tube, which had to be multiplied by Do/Di (Do is the o.d., and D i is the i.d.), in order to obtain the brightness at the inner surface of the glass tube. The results for a grainsize of 11/~m and a coating density of 2.6 mg/cm 2 are summarized in table 3 and in fig. 3. 3.1.

A N A L Y S I S OF THE RESULTS

The general shape of the curves brightness vs pressure as measured by the output current of the multiplier phototube is caused by the absorption of electrons in the gas itself. The expression for the current was found to be of the form: I = Is(l--e-elk),

(1)

where I s is a saturation current for infinite pressure; P is the pressure and k is a constant.

TABLE 2 L i g h t i n t e n s i t y as a f u n c t i o n o f the gas p r e s s u r e for different c o a t i n g d e n s i t i e s ( g r a i n size 11 p m , i n n e r d i a m e t e r 3.4 m m ) a. d = 0.55

d = 0.84

d = 1.38

d = 2.02

d = 2.94

d = 4.14

P (torr)

I (10 - 7 A)

P (torr)

I (10 -7 A)

P (torr)

1 (10 -7 A)

P (torr)

I (10 -7 A )

P (torr)

I (10 -7 A )

P (torr)

100 200 400 600 800 1020 1210 1460 1610 1810 2030

1.43 2.60 5.25 7.90 9.95 11.70 13.50 15.30 16.20 17.40 18.60

100 220 400 600 800 1000 1200 1440 1600 1810 2030

2.10 4.00 6.80 9.60 12.00 14.00 16.00 17.70 18.90 20.20 21.00

110 200 400 600 800 1000 1200 1400 1600 1800 2000

2.50 4.25 7.80 10.90 14.10 16.20 18.30 20.40 22.20 23.70 25.00

100 200 400 600 800 1000 1200 1420 1600 •800 2000

2.24 4.30 7.90 11.10 14.10 •6.50 18.70 21.00 22.50 24.00 25.40

120 200 400 600 800 1000 1200 1400 1600 1800 2020

1.85 3.06 5.88 8.54 10.80 12.75 14.35 16.00 17.65 19.15 20.50

100 200 400 600 800 1000 •200 1400 1600 1800 2000

1 (10 -7 A )

1.70 3.10 5.80 8.30 10.40 12.30 13.80 15.30 16.50 17.70 18.60

Is = 28.52

IS = 28.00

I s = 35.46

I s = 35.83

I s = 32.51

18 = 25.21

k = 1870

k = 1500

k = 1640

k = 1620

k = 1500

k = 1530

a d = phosphor coating density (mg/cm2).

234

A. K O R I N et al.

TABLE 3 Light intensity as a function o f tube d i a m e t e r (grain size 11/~m, c oa t i ng density 2.6 mg/cm'2). 1 in units o f 10-7 A.

Di

P

120 215 400 615 800 1000 1200 1400 1600 1800 2000

=

1

mm, Do

=

6

mm

Di

=

1

(l(Do/Oi)

P

0.21 0.34 0.69 0.96 1.29 1.56 1.84 2.13 2.40 2.65 2.85

1.26 2.04 4.14 5.76 7.74 9.36 11.04 12.78 14.40 15.90 17.16

100 205 420 600 800 1000 1200 1400 1600 1800 2000

2

mm, Do = 5.0 m m

1

0.85 1.65 3.20 4.55 5.90 7.20 8.30 9.20 10.30 11.10 12.00

Di = 3.4 mm, Do = 5.2 m m

l(Oo/DO

2.12 4.12 8.00 11.37 14.75 18.00 20.75 23.00 25.75 27.75 30.00

P

I

100 210 420 600 800 1000 1200 1400 1600 1800 2000

Di = 6 mm, Do = 8 m m

l(Do/Di)

1.90 3.95 7.20 10.00 13.00 15.00 17.00 19.00 20.00 22.00 23.20

2.88 8.0 10.94 15.2 19.76 22.80 25.8 28.8 30.4 33.44 35.26

P

1

l(Do/Di)

100 200 400 600 810 1000 1195 1400 1600 1800 2000

3.50 6.40 11.40 15.60 19.00 22.00 23.40 25.20 26.55 27.60 28.40

4.66 8.53 15.20 20.80 25.33 29.33 31.20 33.60 35.39 36.80 37.87

Is(Oo/Oi) = 56,16

Is(Do/DO = 53.89

~(Oo/Oi) = 50.22

Is(Do/Oi) = 41.37

k = 5500

k = 2480

k = 1650

k = 810

When put in a logarithmic form the experimental points can be fitted with great accuracy to a straight line as shown in fig. 4. The values of I s were calculated by an iteration process. The effect of the tube diameter is illustrated in fig. 5, which shows the dependence of the brightness at the interior of the glass tube as a function of tube diameter for various pressures. At low pressures this

4o /

,

,

,

, Di:6,01

relationship is practically linear. This is to be expected since the total activity per unit length is proportional to the square of the radius, and the total area of phosphor is proportional to the radius only. At higher pressures absorption of the beta particles becomes significant, and for large tube diameters the electrons emitted in the innermost part cannot anymore reach the phosphor, and will not contribute to the brightness.

jlO0 1.0

0.5 20

Di =1.0

50

7

0.2

I0

O,I

o

IO

20

30

P (tort)

Fig. 3. Brightness at the inside o f the glass tube as a function o f pressure for four tube sizes,

0.05

~

Di ~ LOmm

Di = 2.0ram

Oi :5.4ram

Di:6.0mm

'

,o'oo

i

i

o'oo

3000

PCtorr) Fig. 4. Dependence of 1 - I / I s on the pressure for four tube diameters (grain size ~b = 11 #m, density = 2.6 mg/cm2).

235

INTENSITY OF LIGHT SOURCES

The shape o f the curves becomes m o r e a n d m o r e t h a t o f a s a t u r a t i o n function. k is f o u n d to d e p e n d on the t u b e d i a m e t e r according to k = a D~-",

I00


(2)

o

where n = 1.08 ~ 1, a n d a = 5500 t o r r / m m . The same value o f a is f o u n d for all three kinds o f p h o s p h o r a n d for all c o a t i n g densities. This dependence is illustrated in fig. 6. T h e p a r a m e t e r I s d e p e n d s on the t u b e d i a m e t e r as

Is = bDT",

20

.....

=

21

1.0

i

.0

i 5,.0 . . . . .

I0.0

Oi (ram)

(3)

where m -- 0.316 ~½, a n d b = 57 × 10-7 A, i n d e p e n d e n t o f the gas pressure. W e can therefore write eq. (1) in the following form:

I

I= = 57 Oi-0"am

bD7, 1/a [1 - exp(-PDi/a)],

I00

, ,,,

.

.

.

.

,

, ,,,,

50

(4) o o

where a d e p e n d s on the gas only, a n d b on the p h o s p h o r only.

lO

3.2. THE FACTORS AFFECTINGPARAMETERSb W h e n we plot the intensity (which is p r o p o r t i o n a l to the parameter b) against the coating density, a curve that exhibits a m a x i m u m is obtained, as shown in fig. 7. The coating density for which this m a x i m u m occurs is

5 =

. . . .

i

i

i

i

I

0.5

,

,

I,i

I0

5 Oi (mm)

Fig. 6. Dependence of parameters Is and k on the tube diameter. 40

40

,

,

P = 2 0 0 0 torr P = 1600

',

,

\

¢

30

\

/

/
/

20

P = 800

',

P=

,,

600


'T

20 P = 2 0 0 0 torr

o

P=

/ /

400

"

I0 ,~

P = 200

"

P--

"

I O0

~

P I

I'.0

2.0

3'.4

610 Di (ram)

Fig. 5. D e p e n d e n c e o f the brightness o n the t u b e d i a m e t e r f o r d i f f e r e n t pressures ( g r a i n size ~ = 11/~m, d e n s i t y = 2.6 m g / c m 2 ) .

0

P=1600

.

P=1200

.

P= 8 0 0

,,

o p= 4 0 0

,,

= I

2

I

I

4

200

-

I

I

6

I

8

d (mcj I c m 2 )

Fig. 7. Brightness vs coating density of the phosphor (grain size ~b= 2.5 mg/cm2).

236

A. K O R I N et al. I

I

I

I

I

I

'

I

I00

,5

\\\

2.5,~

-

8O

J H

6O

40

I

I

I

I

51

I

I

i

I

I [0

I

d{mg/cm2 ) Fig. 8. Relative brightness as a function o f coating density for different grain sizes (~.

independent of the pressure, and does not change when we extrapolate to infinite pressure, where I in eq. (1) becomes Is, the saturation brightness. The optimum density, for which I is maximum, depends on the grain size, as illustrated in fig. 8. Here the brightness has been divided by the optimum coating density, and the ratio plotted against the coating density. We see that the optimum coating density decreases with increasing grain size. The absolute value of the maximum brightness decreases with increasing grain size as can be seen in fig. 9. The decrease is larger the higher the pressure.

density increases the geometrical cross-section for the luminescence increases, but after a certain density the phosphor layer becomes less transparent and the brightness starts to decrease again. Larger grain sizes require a larger density for forming a continuous coverage of the tube. The mean range of beta particles in ZnS is 0.94/~m 9) which compare favourably with a grainsize of 2.5/Lm, but when a phosphor of larger grain size is employed, some of the emitted light in absorbed in the outer part of the phosphor grain. This explains the dependence of the optimum coating density on the grain size. The 40

,

4. Discussion

The above results show that the brightness of a cylindrical light source based on gaseous tritium depends on a number of parameters. The intensity depends on the pressure according to a simple exponential expression. The factor a appearing in the exponent is simply related to the attenuation coefficient of the beta-particles in tritium gas. The appearance of the diameter in the exponent becomes self-evident according to this interpretation of the pressure dependence. The appearance of the factor D-~/3 in eq. (4) is less clear and is of empirical value only. The dependence of the maximum brightness of a certain light source on the diameter could be interpreted in terms of the mean energy of the beta-particles reaching the phosphor, but no good figures for the energy dependence of the interaction between phosphor and electrons are available. The dependence of the brightness on the coating density and the grain size can be interpreted in the following way: When the coating

50

",, \

\

=.

~0

P: 2000

I0

~ o

0

215

~

torr

P= 1 6 0 0

-

P= 1 2 0 0

"

P= 8 0 0

,,

P:

600

"

P:

400

P= 2 0 0

" ,,

L

lll.O

15.0

Fig. 9. Dependence o f the m a x i m u m brightness o n the grain size ~b.

INTENSITY OF LIGHT SOURCES decrease of the absolute value with increasing grain size can be explained in the following way. The ratio area/ v(~lume of the smaller grains is larger than that of larger grains. The active centers at the surface of the grain are less effective than those at the inside, since they cannot transfer energy in all directions and the probability for luminescence decreases. It should be pointed out that a behaviour very similar to that in our experiments has been found in the study of the behaviour of cathode ray tubes. The brightness as a function of the coating density passes through a maximum, and the coating density at which the maximum occurs increases with increasing grain size. 5. Conclusions F r o m these results one can conclude as follows: There is no point in increasing the tritium pressure indefinitely, since both the price of tritium and the difficulties in sealing tubes with a pressure higher than atmospheric will make the resulting increase in brightness uneconomical. It is also pointless to increase the amount of tritium by increasing the diameter of the tube, since the innermost part of the tube will be inoperative

237

(the beta particles will be absorbed in the gas and will not reach the phosphor). Coating should be of optimum thickness in order to obtain maximum efficiency, and the particle size should be on the small side. It may however be possible that the smallest size is not the most economical, since it is conceivable that the effect of radiation damage is greater on small grains, which might disintegrate more easily, which would shorten the useful life of the light source. References 1) j. D. Ault, Light without power, Sounders Roe and Nuclear Enterprises, Ltd. 2) A. Breccia and E. Lazzarini, Energia Nucleare 5 (1962) no. 4. z) E. J. Wilson and J. D. H. Hughes, Light sources using radioisotopes. 4) E. A. Evans, Tritium and its compounds (Butterworths, London, 1966). 5) U.S. Pat. 2,953,684 (1960). 6) U.S. Pat. 3,478,209 (1969). 7) Radiation protection standards for radioluminous timepieces, Safety Series no. 23, IAEA, Vienna (1967). s) S. M. Kim and L. Vaughan, Radioanal. Letters 2 (1971) 115. 9) W. Espe, Materials of high vacuum technology (Pergamon Press, Oxford, 1968).