The gamma-ray response of radiochromic dye films at different absorbed dose rates

Rad/at Phys. Chem. VoL 18. No. 5--6, pp. 987-999. 1981 Printed in Great Britain.

0146-5724/81/110987-13S07..00/0 Pergamon Press Ltd,

THE GAMMA-RAYRESPONSEOF RADIOCHROMIC DYE FILMS AT DIFFERENT ABSORBEDDOSERATES W.L. McLaughlin*, J.C. Humphreys*, H. Levine*, A. Miller**, B.B. Radak***, and N. Rativanich****.

ABSTRACT In leucotriphenylmethane radiochromic dye systems, using polynmric host materials, lowintensity rate dependenceof gamma-rayresponse at doses >lO kGy and at dose rates
Center for Radiation Research, National Bureau of Standards, Washington, D.C. 20234, USA

** ***

Accelerator Department, Ris# National Laboratory, DK-4000 Roskilde, Denmark. Department of Solid State Physics and Radiation Chemistry, Boris Kidri~ Institute of Nuclear Sciences, Vin'ca, llOOl Yugoslavia.

**** °

Office of Atomic Energy for Peace, Bangkok, Thailand From Far West Technology, Inc., 330D South Kellogg, Goleta, CA g3017 [2] or from Accelerator Department, Ris~ National Laboratory, DK4000 Roskilde, Denmark [4] 1 gray (abbrev. Gy) ~ l J.kg "~ : I00 rad

°°

987

988

W . L . McLAuGHUN et al.

Experimental Details and Results Phase I In the f i r s t part of this work, the response of three types of radiochromic dye films were investiaated at different gamma-rayabsorbed dose rates and at different r e l a t i v e humidities during i r r a d i a t i o n . The films are listed in Table I. They contain the leucocyanide of hexa (hydroxyethyl) pararosaniline and as plastic base, either nylon or polyvinyl butyral-acetate-alcohol (PVB-PVA). TABLE I RADIOCHROMIC FILMS IN PHASE I Film Type Code

Batch Date

Approximate Thickness Ipm)

Plastic Base

Solvents Used During Formulatior,

FWT-60 Ris~ P-15

3-80 1-79

57 52

nylon PVB-PVA

ethanol (mixture) a

Ris~ P-15

7-79

55

PVB-PVA

2 ethoxy-ethanol

The films were f i r s t dried over PzOs for at least 5 days. They were then conditioned in dessicators at one of three r e l a t i v e humidities provided by saturated aqueous salt solutions for at least 5 days prior to i r r a d i a t i o n . The salts used were NaCl (75% r . h . ) , MgCl2 (34~ r . h . ) , and LiCl (12% r . h . ) [ I l l . The mean temperature during gamma-ray i r r a d i a t i o n was approximately 30°C, and the i r r a d i a t i o n times were 5 hours and 32 hours for 20 kGy at two dose rates: l . l and 0.17 Gy.s-~. The dose rates were measured by means of standard %rrous sulfate (Fricke} dosimeter solutions. The irradiations were ~nade in identical geometries, except that a 3-cm lead shield surrounded the i r r a d i a t i o n chamber in order to obtain the lower dose rate. The results are shown in Fig. I. I t is Seen that for the hydrophilic nylon-base film (FWT-60) the humidity and rate dependence agrees f a i r l y well with that found by Gehringer et at. [8]. I t is also see,; that, for the two PVB-PVA films, the dependence of gamma-ray --

i

:

I ^ f ~ .J

/-~"

4,0

,4

,/

~/

"E

/

/0

D

r.lj /

c

0

~.

E -~

/ / j .

<:

~

F,L~..'

C):'Jr. ~.i )

ORC~r;X:E 5~.','~C~

I t

LE I:T

~

0.17

LEFT

- - ~

• C 17

LE~T

--X----

R!S~ .'=-15

t

J'.'L '?'9

~" O, 17

R:C.t~T "~h~HT

---~---- -~---

JA;;.'7'

;

o

~

IZ

E

~E_~( ~

"~LC~I

t.l

=

I

3~.

75

R;rL-~.TIVE

ri~ju~e 1

,.0

HLIMiOITY

<]

0 ~oo

, %

Variation in response (change in op;icai de,;sity per unit thickness for absorbed dose 20 kGy) of three types of radioch,'omic dy-= film~ (see Table I ) , as a function of r e l a t i v ; humidity during i r r a d i a t i o n far t~.1o different gang,a-ray dose rates.

a

2-methoxy ethanol, 2-butoxy ethanol, ethanol, butanol, N, N-dimethyl formamide.

~

Calibration by ferrous sulfatc (~ricke) ~sitnetry [8]. Calibration by g,'apnite calorimetry, checked by Fricke dosimetry [3,14].

The gamma-ray response of radiochromic dye films

989

response on dose rate an~ humidity is not equal. In making the films, two solvents were used (.see Table I ) , and the differences in response sugges~ an influence of solvent concentration. Generally, the s e n s i t i v i t y of the PVB-PVA films increases with increasing r e l a t i v e humidity, while that of the nylon films decreases as the r e l a t i v e humidity increases. This is in agreement with e a r l i e r studies [ ] 0 ] . Note that for the nylon-base film (FWT-60), the rate dependence is considerably diminished at 75% r . h . , and for the PVB film ~w~dewith several solvents, the rate dependence is negligible at the lower r e l a t i v e humidities during irradiation. The l a t t e r result coincides with a private j o i n t investigation carried out by McLaughlin and Gehringer, the results of which are shown in Table II [ ] 3 ] . TABLE I I O__p_~ical Density per unit -

Thickness -

-

~A/mm) Readings for 1.07 kGy Absorbed Dose Absorbed Dose Rate (Gy-s-~l ~A/mm (600 nm).

Forchungszentrum Seibersdorf~

2.5 0.24

0.80 0.81

National Bureau of Standards°°

2.7 0.19

0.80 0.79

In this study, a r e l a t i v e l y thick PVB-PVA film (thickness 0.55 mm) produced by Far West Technology was irradiated at different dose rates at both the U.S. National Bureau of Standards and Forschungszentrum Seibersdorf, Austria. The r e l a t i v e humidity value during the i r r a d i a t i o n was 32% and the temperature was 25~C. Table II shows that, for this thi=k film type, the response was not appreciably affected by differences in absorbed dose rate in this range (o. Ig to 2.7 Gy-s'~). Experimented Details and Results, Phase II On the basis of the results of Phase I of this study, i t was decided to irradiate a large number of available radiochromic dosimeter film types and thicknesses under conditions typical of variation prucessing in a controlled environment (20°C, 60% r.h.~ TABLE I l l PJ~DIOCHROMIC FILMS IN PHASE II Film Type Code

Datch Datea

FWT-30-05 Ris~ P - 1 8 FW%-~VP (coated) FWT-G3-02 FWT-63-02 FWT-b3-02 FWT 5 3 - 0 2 Ris~ P - 1 5 FWT-68-2! Ris~ P - l O Ris~ F-iO F~'~F-.60-O0 FWT-60- O0 FWT-60-O0 FWT-67-20

May'80 July73

0.15

paperb PVP~

Mar'80 Mar SO Aug'79 Feb'80 D~='78 0ct'73 Mar'80 Feb'72 Feb' 72 Mar'RO M~r' ,J Dec' 72 P:ar'85

0.042 0.013 0.053 0.~2 0.5B 0.063 0.045 0.095 0.13 O.OIl 0.044 O. 70 0.047

PVP _ 2-methoxy ethanol PVP-PVAt ethanol PVB-PVA ethanol PVB-PVA ethanol PVB-PVA ethanol PVB-PVA 2-etnoxy eLhanol PVC:PVAg tetrahydrofuran CTA~ cyclohexanone CTA cyciohexanone nylon ethanol nylon ethanol ~ I on ethanol polychlorostyrene benzene-xylene

a~ b) c) d) e~ f) g! h) i) j)

Approxin~te Thickness {n~)

Base Material

Solvents Used During Formulation acetic acid 2-metho×y ethanol

Leuco Dye HHEV-CNc PR-CNe HHEV-CN HHEV-C~I '!HEV-CN HHEV-CN HHEV-CN HHcV-~*' NF~CN'"' PR-CN HHEV-CN LHEV-C~ HNEV-C~I HHEV-CN MG-OCHJ

{;ate ef r~ceipt of L~atch. pa~er: pure unsized waterleaf paper. hHEV-Cfi leucocyani~e o¢ hexa ~hydroxyethyi) pararosaniline. PVP: polyvinyl pyrro~iuo~e. PR-CN: leucocyanide of pararosaniline. ~V3-PVA: polyvinyl butyr..l-acetate-alcohol. PVC-PVA: polyvi,yI chl.:)ride-acetate-alcohol. ,;7. CN: ~eucocyaniGeof new i;chsin. C(A: cellulose tri~ceta~e. ':G-JC!!3: leuco methoxide of malachite green.

The nos~ materials ~re decreasingly Lydroonilic in the following order: paper, PVg, PV~-PVA, nylon, CTA, ~V~-Pv~, polychlorc ~yrene, ~ith the last two being considered hydropi~obic. All films were stored ~t ]ea
990

W.L. MCLAUGHLII,~et al.

irradiation at that relative humidity. The mean temperature during irradiation was 25°C. The irradiations were madewith 5°Co gammaradiation under the conditions given in Table IV. TABLE IV Approximate Source Activity (kCi)

Irradiation Parameters for Phase II 6°Co source Irradiation Geometry

Absorbed Dose Rate IGy.s"~)

lO

Cylindrical isotropy (central position) inside stainless steel canister [3].

2.7

IO

Same canister 25 cm away in water from axis of annular source array.

0.36

lO

Same canister 51 cm away in water from axis of annular source array.

0.018

5

Vertical gamma-raybeam collimated by 6x6 cm square lead opening [15].

0.0080

All absorbed dose rates were determined by graphite calorimetry and checked by means of both Fricke dosimetry and by a graphite-wall air-cavity ionization chamber of the same dimensions [14,15]. During irradiation the films were held between 0.5 cm polystyrene layers, thick enough to provide conditions of electron equilibrium. Knowingthe approximate photon spectrum, i t was possible to determine values of absorbed dose in water, by applying appropriate cavity-theory corrections. The films were read spectrophotometrically at selected optical wavelengths (bandpass 2 nm) approximately 24 hours after the end of each irradiation. Film storage before, during, and after irradiation was maintained under the environmental conditions described above (dark; ~25°C; ~60% r . h . ) . The results are given in Figs. 2 through 16 in terms of increase in optical density per unit thickness (A~/mm) as a function of absorbed dose, for the different absorbed dose rates listed in Table IV. Fig. 2 shows that for the paper-base system there is no apparent rate dependence when 60% r.h. humidity is maintained. The very hydroscopic PVP base film, on the other hand, shows extensive rate dependence, that is, diminishing sensitivity as dose rate is decreased in the range of interest (see Figs. 3 and 42. A slight rate dependenceoccurs at the lowest dose rate (0.0080 Gy-s" l ) for PVB-PVA and PVC-PVA films (see Figs. 5-I0). Figs ll-13 show that in the case of nylon-base films for the intermediate value of relative humidity during irradiation (60%), essentially no rate dependenceoccurred over the dose rate range 0.008 to 2.7 Gy-s"~. An exception to this is the case of the very thin film, ~O.Ol mm, as shown in Fig. I I , where i t is also seen that lowering the relative humidity to 12 percent during storage and irradiation produces marked low-intensity rate dependence of response at the wavelength of maximumabsorption (605 nm). Figs. 14-16 show no appreciable rate dependence in the case of the r e l a t i v e l y hydrophobic CTA and polychlorostyrenebase films. Summary and Discussion We have shown that under certain environmental conditions, a low-intensity rate dependence of response of hydrophilic plastic radiochromic dye films occurs, in agreementwith the findings of Gehringer et al. [8]. I f the relative humidity during irradiation is lower than ~50%and the d o s e ~ a ~ i s < 4 kGy.h" I , the nylon-base film may give somewhatlower optical density readings for a given dose than at higher dose rates. The rate dependence is prevalent at dose levels > lO kGy. The effect is apparently exaggerated by the extreme preconditioning treatment of thorough dessication and subsequent moisturizing to a given water burden in the film. Under this treatment, a slight low-intensity rate dependenceof response occurs even at the higher relative humidities, In the case of PVB-PVA radiochromic films, the low-intensity rate dependence is more pronounced at the higher relative humidities than at the lower relative humidities. The results of Phase II indicate generally less rate dependencethan those of Phase I. In the Phase II case, the films were not dessicated beforehand, and were simply kept at a given intermediate relative humidity ~60% during storage and during irradiation. Evenwith this less severe pre-treatment, the very hygroscopic films ( i . e . PVP) and the very thin moderately hygroscopic films (i.e. nylon and PVB-PVA films) showedmarked low-intensity rate dependence of response.

991

The gamma-ray response of radiochromic dye films

0.8

!

0

0.6

+ CI

I

,

2.7 Gy- s-~ 0.36 G y 0 . 0 0 8 Gy's "l

.

I

s

-

1

t

/

I

~

~ZO.5

N

~ 0.4

g n~

,~ 0 . 2 U

fo

0.I-

0

I0

Figure 2

0.7/

20 ABSORBED DOSE IN HzO. kGy

30

Response curves of FWT dye impregnated paper for three gamma-ray dose rates and measured in terms of reflection optical density in the red part of the spectrum.

FWT-PVP (Cooled|, 0.044 mm ~

.

Io ,o ~o.~P

/

+-°

o.4

ABSORBED OOSE~N HzO.~G~ Figure 3

Responsecurves of 0.041 ram-thick FWT-PVP film (coated on polyester base) for three gamma-ray dose rates and at 600 nm wavelength.

992

MCLAUGHLIN et al.

W.L.

~.0

~

,

i

l

~

R:SO P-IB, PVP, 0.'15mm 2.5-

2.7

o

/o//

Oy.s-~

I

/....,ET" /

/

///

,4- / / 1

o I~

o

I

I

'

I

i

30

Response curves of 0.15 mm-thick Rise P-18 PVP f i l m f o r three gamma-ray dose rates and at 550 nm wavelength.

Figure 4

14

I

IO 20 4BSORBED DOSE IN HzO, kGy

i

i

J

•

i

i

/

i

FWT-63-O2, PVS-PVA ,O.O13m m r ~ 12

o

2.~ G,.,-,

+ o

0.36 G,.,-' o oo~ o~ < ,

I

7

-~

JY~S-~ / / . ~

/~JS

I0

~

5

o 8 o ~D

2

E 46

4

c

2

½

I

Oo ~, Figure 5

: I0

t

I

T

2:0 30 ABSORBED DOSE IN HzO, kGy

I

?

Response curves of 0.013 mm-thick FEr-63-02 PVB-PVA f i l m f o r three g a l a - r a y dose rates and at two optical wavelengths,

The gamma-ray response of radiochromic dye films

t

993

FwT- 6 3 - 02, PV8 -PVA, 0.053 m m

':~1-

O

L

+

10|

2.7 Gy.S -I 0.36

"

8

Gy" s "t

O 0.008 Gy" $-i

~

7

r ~a

5 Z

+- +F

°+u~

E

O0

IO

Figure 6

i 20 ] I

,5~

20 30 ABSORBED DOSE IN HzO. kGy

40

Response curves of 0.053 mm thick FWT-63-02 PVB-PVA film for three gamma-ray dose rates and at three optical wavelengths.

FIw T - 6 3 - 0 2 , PVB -PVA, I 0.12 mm 2.7 Gy-'~ -t 0.36 Gy" s-I x O.O18 G y ' ~ - t "~ O.OOB GY"s-'

5

0 +

I

~

'

4

c o

o

E E

0

Figure 7

5 Io ABSORBED. DOSE ~N HzO.wGy

IL

Response curves of 0.12 mm thick FWT-63-02 PVB-PVA film for four gamma-ray dose rates and at two optical wave]engths.

994

W.L. McLAuGXL~ et al.

' '

0 + x Q

J | 4~-

0

"1

I

I

[

I

FWT-63-02

5

I

I"

'l

PVB-PVA, 0.58 mm

~.7 G y - s -~ " 0.36 Gy • s-' 0.018 G y - s "t 0.008 Gy" s"

+ / "

f

,

"~----FJi

o

Figure 8

2

t

I

,

~

~

4 6 ABSORBED DOSE IN HzO, kGy

t

l

8

IO

Response curves o f 0.58 mm-thick FWT-63-02 PVB-PVA film f o r four gamma-ray dose rates and at three optical wavelengths.

RiSO P- 15. PVB - P V A , O . Q 6 3 ~ ~ m

• O~-~-

o

2.7 Gy-$-'

~

~

T$ c O

-

"E" ~

,SOD nm

LO O

~t

E

°

I-

v2:~~" 5

O

0 Figure 9

10

i

_

L

I

z0 30 ~BSORBED DOSE tN ~i=O, WGy

'

.

40

Response curves of 0.063 mm-thfck Ris~ P-15 PVB-PVA film for three gamma-ray dose rates at three optical wavelengths.

The gamma-ray response of radiochromic dye films

' 0

FWTIIBB-2,. pIvc_PVA IO.04, mm ' 2.7 Gy.$ -i "

'

995

J J'/

"~ I.O O ¢3 w3 E

~O,5

i

,,1

I

IO

I

I

i

I

I

20 30 ABSORBED DOSE IN H=O, kGy

40

Response curves of 0,045 m-thick F~-68-21 PVC-PVA film f o r three gamma-ray dose rates and at 550 nm wavelength.

Figure 10

i

i

--'

i

i

l

:

i

[

i

FWT-60-OO, NYLON,0.O11mm 60

o

~"

2.7 Gy' s-~ 0.36 G:/-s -I 0.0(}8 Gy' s -I

4C

30

~

} 60Snm,60°/"t.h2O

o

g o

2¢

~0

F i g u r e 11

S

IO

,0 ~

~" - - - ~-~-'"

20 30 ABSORBED DOSEIN HzO, wGy

40

Response curves of 0,011 mm thick FW'F-60-00 nylon film f o r three gamma-ray absorbed dose rates and measured at three optical wavelengths. Irradiation made at two relative humidities as indicated.

996

W.L. MCLAUGHLINet al.

FWT-60-O0, Nylon, 0.044 mm ..

O

,

2.7 Gy's-'

'.

,,,,,.Q/ /

o.o,~ ~y.,-,

7~ ~ 1=5

~

/

f

i

"~40 nm E

IOt-

,+

o

tO

5

30

2o

ABSORBED DOSE !N H20, kGy Response curves of 0.044 mm-thick FWT-60-00 nylon f i l m for four

Figure 12

gamma-ray dose rates and at three optical wavelengths.

i

i

I

I

1

i

4

F w T - 6 0 - O O , N Y L O N , O,70 m m

~

2.0

2,7 Gy.s-I + 0.36 Gy-s "4 x 0.018 Gy-s "~ 0 0.008 Gy's'l

I

O

605.m//

S

I I

I

O

O

I

1 4 i

I

,sL

I

I.O O E E

0.5

OO

Figure 13

I

! 2

!

; 1 ' 4 6 ABSORBED DOSE tN HzO,kGy

;

B

I

IO

Response curves of 0.7 mm thick FWT-60-00 nylon film for four gamma-ray absorbed dose rates and at three optical wavelengths.

T h e g a m m a - r a y response of radioch~omic dye films

I

l

I

i

RISO P - I O , C T A , O . 0 9 5

Q OOO8 Gy s t

I ....

I

.

997

I

mm

~ . I : ~

3 C

,Y

O E E

2

<3 I

O r~

I0

Figure 14

2.~

20 30 ABSORBED DOSE IN H20, kGy

Response curves o f 0.095 mm-thick Ris~ P-IQ CTA film f o r three gamma-ray dose rates and at 550 nm wavelengths.

i

I

I

/

I

RtS{~ P-IO, CTA, 0.126 mm

2.C

O 2.7 Gy-s -I ~. 0.36 Gy's "~ I~ 0.008 C,y" s"l

./

E

0 0

!0

I , 20

I

L

30

£

ABSORBED DOSE IN HzO, WGy Figure 15

Responsecurves o f 0.126 mm thick Ris~ P-10 CTA film f o r three gamma-ray dose rates and at 600 nm wavelength.

998

W . L . MCLAUGHLIN et aL

'°I

t

I

i

t

[

i

/~-~'---I ~

F'WT- 57-20~POLYCHLOROSTYRENE,O.O47m~//

O 2.7 Gy -s-~ •" O. 36 G y " s "l G 0 0 0 8 Oy s f

/ /

" ~

6O 0 r,3 ,.p E

40

<3 20

I

t

l

tO

i

I

20

i

30

L

,b

ABSORBED DOSE IN HzO, kGy Figure 16

Responsecurves of 0.047 mm-thick FWT-67-20 polychlorostyrene film for three gamma-raydose rates and 630 nm wavelength.

The disparity in the results of the two phases of the present work may be at least partly explained by the differences in environmental pretreatment. I t is suggested that a more thorough study be made with various film types of well-determined degrees of moisture a f f i n i t y , using long-term storage before, during, and after irradiation at a number of relative humidities, rather than dessication before moisture treatment. Also studies should be made of the effects of plastic type, temperatures, acidity, and absorbed dose level, as well as the possible role of radiolytic bleaching in competition with dye formation. There is indication that bleaching is more pronouncedat low dose rates than at high dose rates and also is more prevalent at low relative humidities than at high relative humidities. By taking this effect into account, i t may be possible to correct for the low-intensity rate dependence of hydrophilic films. The findings of Chappas are also significant in the case of electron-beam irradiated nylon-base films of 0.05 mm thickness [9]. He finds that in raising the relative humidity from 20% r.h. to 80% r . h . , the slow transfer from an intermediate absorption species of the dye (415 nm) to the final one (605 nm) is accelerated from time scales of hours to seconds. In liquid phase, the same effect can be promoted by gradually adding small amounts oxygen or a weak oxidizing agent to the solution [16,]7]. The mechanism for the rate-dependenceeffect is s t i l l open to conjecture. Absorbed water is apparently a c r i t i c a l factor, and oxygen may also play a role. Since oxygen serves an electron acceptor, the diffusion-controlled donor-acceptor interaction could in solid phase be slow enough to require longer times for f u l l formation of dye carbonium ions from a free-radical intermediate species. This would be especially prevalent in the absence of water. Unstable free radicals of dye intermediates in such films have been observed and are more transitory in the presence of oxygen and water[18].

REFERENCES Ill W.L. McLaughlin (1977) Radiation measurements and quality control. Radiation Processing, Volume I (J. $ilverman and A. Van Dyken, Eds.) Trans. I s t Int. ConY. Puerto Rico, T~-~-~6~_adiat. Phz_sz Chem. 9, 117-181. [2] A.D. Kantz and K.C. Humpherys (1979) Quality assurance for radiation processing, Advances in Radiation Processinq, Volume I I (J. Silverman, ed.) Trans. 2nd Int. Conf., Miami 19-'~; Rp~{at. Phys. Chem. 14, 575-584. [3] W.L. McLaughlin (1980) The measurement of absorbed dose and dose gradients, Radiation S t e r i ] i z a t i o n of Plastic Medical Devices, Proc. Seminar, Univ. Lowell MA, USA 1979 (H.K. Mann,~, ~a-aTa~h~,s. Chem. 15, "9138.

The gamma-ray response of radiochromic dye films

999

[4] W.L. McLaughlin, A. Miller, S. Fidan, K. Peitersen, and W. Batsberg Pedersen (1977) Radiochromic plastic films for accurate measurement of radiation absorbed dose and dose distributions, Radiat. Phys. Chem. lO, II9~127. [5] W.L. McLaughlin, P.E. Hjortenberg, and W. Batsberg Pedersen (1975) Low energy scanned electron-beam dose distributions in thin layers, Int. J. Appl. Radiat. Isotopes 26, g5-lO6. [6] A. Miller, E. Bjergbakke, and-W.L. McLaughlin (1975) Some limitations in the use of plastic and dyed plastic dosimeters, Int. J. Appl. Radiat. Isotopes 26, 611-620. [7] W.L. McLaughlin, J.C. Humphreys, B.B. Radak, A. Miller, and T.A. Olejnik (Ig79) The response of plastic dosimeters to gammarays and electrons at high absorbed dose rates, Advances in Radiation Processing, Volume II (J. Silverman, Ed.) Trans. 2nd Int. Meeting, Miami, 1978; Radiat. Phys. Chem. 14, 535-550. [8] P. Gehringer, H. Eschweiler, and E. Proksch (1980) Dose-rate and humidity effects on the gamma-radiation response of nylon-based radiochromic film dosimeters, Int. J. Appl. Radiat. Isotopes 31, 595-605. [9] W.J. Chappas (lg81) Accelerated color development in irradiated radiochromic dye films, Bull, Am. Phys. Soc. 25, (7); Applications of Accelerators in Research and Industry, Proc. 6th Conf., North Texas State University, Nov. 1980. [lO] H. Levine, W.L. McLaughlin, and A. Miller (1979) Temperature and humidity effects on the gamma-ray response and s t a b i l i t y of plastic and dyed plastic dosimeters, Advances in Radiation Processin9, Volume II (J. Silverman, ed.) Trans. 2rid Int. Meeting, Miami, I~78; Radiat. Phys. Chem. 14, 551-574. [ l | ] A. Wexler and S. Hasagawa (1954) Relative humidity-temperature and relationships of some saturated salt solutions in the temperature range of 0° to 50°C, J. Res. NBS 53, 19-26. [12] E. Bjergbakke and E. Engholm Larsen (1973) The lO,OOO Ci 6°Co and the 300 Ci 6°Co gamma cell, Ris~ Report M-1651, Ris# National Laboratory, Roskilde, Denmark. [13]

W.L. McLaughlin and P. Gehringer (Ig7g), private investigation.

[14] B, Petree and P.M. Lamperti (1967) A comparison of absorbed dose determinations in graphite by cavity ionization measurements and by calorimetry, NBS J. Res. 71C, lg-27. [15] J.S. Pruitt, S.R. Domen, and R. Loevinger (1981) Calorimetric ion chamber calibration in water for cobalt-60 gamma radiation, (to be published in Medical Physics). .I

[16] W.L. McLaughlin, M.M. Kosanlc, V.M. Markovi~, M.T. Nenadovi~, K. Sehested, and J. Holcman (1980) The kinetics of dye formation by pulse radiolysis of pararosaniline cyanide in aqueous or organic solution, Ris~ Report 2202, Ris# National Laboratory, Roskilde, Denmark. [17] W.L. McLaughlin and M.M. Kosani~ (1974) The gamma-ray response of pararosaniline cyanide dosimeter solutions, Int. J. Appl. Radiat. Isotopes 2.5, 249-262. [18] R. Uribe, W.L. McLaughlin, K. Miller, T.S. Dunn, and E.E. Williams (1981) Possible use of electron spin resonance of polymer films containing leuco dyes for dosimetry, Radiation Processin9, (J. Silverman, Ed.) Trans. 3rd Int. Conf. Tokyo, 1980; Rad. Phys. Chem__Inln~ess)