THE TRANSIENT TEMPERATURE DEPENDENCEOF RADIOACTIVE REFERENCE LIGHTS

A. C. Lucas and B. M. KapsarHarshaw Chemical CompanyDiv. Kewanee Oil Company

Solon, Ohio

Summary

Scintillating phosphors intimatelymixed with long lived radionuclides areused to construct stable reference lightsfor many measurement applications. Ifphosphors having long lived traps areused in the construction of such lightsthe temperature dependence of light emis-sion may be a complicated function of thetemperature history of the reference light.The transient temperature dependence hasbeen calculated for several traps rangingin depth from 0.5 to 1.5 ev. Experimentaldata are presented for Harshaw TLD-100,CaF2:Mn, CaF2:Dy, ZnS, and NaI(Tl). Thecalculations and measurements show thatphosphors should be chosen which haveeither very deep traps not likely to pro-duce light at normal operating tempera-tures or those which have shallow trapslikely to equilibrate in a short time.

Introduction

Scintillating crystals may emit lightat a time long after irradiation if thecrystal structure provides for charge trap-ping or, more simply, if the crystal exhi-bits thermoluminescence. If the amount oflight emitted as a result of thermolumines-cence is significant with respect to thatemitted promptly, the time and temperaturestability of a continuously irradiatedcrystal may be adversely affected.

Randall and Wilkinsl have shown thata wide variety of thermoluminescent phe-nomena may be explained by assuming thatthermoluminescence occurs as a result ofcharge untrapping and that the probabilityfor untrapping from a trap of a singlespecies is:

p = se -E/KT (1)

where s is a frequency factor, sec,

E is trap depth, ev

K is Boltzmann's constant,

8.614 x 10 5 ev/°KT is the absolute temperature,OK

In the event that traps of several speciesare present, the light emission may oftenbe predicted by assuming that the trapsact independently and that the light emit-ted by the several species is simply thesum of the light emitted by each.

If a scintillating crystal is contin-uously irradiated so as to emit light at arate P, initially as a result of promptprocesses, and the total light, J, emittedby both prompt processes and thermolumin-escence will be:

J = P + pn (2)

where n is the amount of trapped chargeand p is defined in equation (1) above.At constant temperature n is defined bythe equation:

dn/dt = Q - pn

where Q is the rate of filling of trapsin the continuously irradiated crystal.

Integrating equation (3):

n =1 [Q - ce-ptIp

At t = 0, n = 0, and c = Q

n = Q [1 - e Pt]p

Combining (2) and (4):

J = P + Q [1 - e ptI

(3)

(4)

(5)

(6)

Equation (6) describes the time dependenceof a continuously irradiated crystal heldat constant temperature. The initial andfinal brightness is not dependent upontemperature but only on the relative val-ues of P and Q. The time required to reachequilibrium brightness is, however, a verystrong function of temperature.

If, alternatively, the crystal hasbeen under irradiation continuously for avery long time and equilibrium has been es-tablished at a temperature, T1, an abruptchange in brightness will result if thetemperature is changed to a new value, T2.The value of the discontinuity and its rateof recovery may be determined by consider-ing from (3) that, at dn/dt = 0:

n = Q/p1 (7)

where pl, is the probability of untrappingat temperature T1. The brightness of thelight at the new temperature, T2, is:

J(T2) = P + P2 pQ (8)

where P2 is the probability of untrappingat temperature T2. The time duration ofthe transient may be inferred by lettingn = Q/p1 at t = 0 and evaluating the con-stant of integration, c, in equation (4).

Q = 1 (Q -c)P1 P2

c = Q (1 - P2/P1)Substituting (11) in (4) and (2),

(9)

(10)

95

J(T2) = P + Q [1 - (1 - p2/pl)e-P2t](11)The net effect of equations (6) and (11) isdiagrammed in Figure (1).

Luminescence Measurements

Both prompt and delayed luminescencewere measured by depositing carbon-14 ontothe heating planchet of a Harshaw Model2000A Thermoluminescence Analyzer. Sampleswere placed on the planchet and promptluminescence was observed as a function oftime. The amount of TL was determined byheating the sample at 10°C/sec to 400°C.The total amount of TL resulting from ex-posing the sample to carbon-14 beta rayswas compared to the light which was emit-ted promptly upon irradiation.

The shape of the glow curves is shownin Figures 2, 3, and 4. Each phosphor hasa different glow vs. temperature character-istic important in evaluating its stabili-ty in use as a radioactive reference light.

TLD-100

Lithium fluoride (Harshaw TLD-100)has been shown to have a total of sixseparate traps operating to produce ther-moluminescence. The shallower traps areknown to fade rapidly at room temperatureand the deeper traps are depopulated at arate of only a few percent per year atroom temperature. Grant, et al, have de-termined trapping constants shown in Table1 for each of the five predominant traps.2

CaF2:Dy

Normally used as a TL dosimeter,CaF2:Dy crystals (Harshaw TLD-200) exhibitsix separate trapping levels grouped close-ly to form a nearly rectangular band. Trap-ping constants for the material have beenestimated from glow curves and are tabula-ted in Table 2.

ZnS

Commonly used as a scintillator andfluorescent screen in x-ray analysis, ZnShas a continuum of trapping levels. Ingeneral filled traps exist in an irradia-ted sample which may be depopulated by anyrise in temperature.

NaI(Tl)

Designed primarily for use as a scin-tillator, NaI(Tl) exhibits little TL. Sev-eral samples were examined and found to besimilar in that they formed glow peaks inthe 100-200°C region when heated. The sam-ples are typified by two trapping levels:

1. s = 1011 sec. -1 E = 0.9 ev

2. s = 10ll E = 1.0

CaF2:Mn

While the TL glow curve for CaF2:Mnis dominated by a single peak, that peakappears to be constituted by two or moretraps interacting with each other. As apractical matter, the emission of lightas a result of heating to moderate temper-atures can be approximated by assuming thetrapping constants, s = 8x106 and E = .90for the main peak and s = 5x108 and E= .71for a small low temperature peak which hasonly one percent the population of themain peak.

The ratio of total TL to prompt emis-sion for the five phosphors is shown inTable 3. The phosphors are ranked in orderfrom that which has the smallest amount ofTL to that which has the greatest amountof TL. The five differ markedly in TLcharacteristics and serve to illustratethe effect which TL has on the time andtemperature stability of radioactive ref-erence lights. These data have not beencorrected for the effect of thermal quen-ching on the efficiency of producing lightas a result of TL.

Temperature Dependence at Zero Time

The temperature dependence of a refer-ence light which has been recently fabri-cated may be inferred by heating the sampleto remove the effect of TL and then measur-ing the light emitted at different tempera-tures while the sample is in contact withthe beta ray source. Such data are shownin Figures 5 and 6 for each of the samples.CaF2:Mn, ZnS, and NaI(Tl) have a negativetemperature dependence, CaF2:Dy has a posi-tive temperature dependence and TLD-100goes through a minimum.

These data are the result of the com-bined effects of intrinsic efficiency chan-ges and the prompt emission of TL at highertemperatures. The latter effect introducessome non-uniqueness with respect to timeinto the function, but it is generally re-producible to within a few percent forsettling times of a few minutes.

Initial Time Dependence at Room Temperature

After heating to remove residual TL,the light emission of each sample was ob-served periodically. Figures 7 and 8show the resulting measurements for ZnSand CaF2:Dy respectively. In each casethe measurement is normalized to the lightemitted at approximately one minute afterheating. LiF, NaI(Tl) and CaF2:Mn showednegligible change in light emission duringthis early period.

Using the estimates given above forQ/P, E, and s, equation (6) may be evalu-ated to compare with this experimental da-ta and to provide estimates of the extra-polated time dependence. Such calculations

96

are shown in Figures 7 and 8 for ZnS andCaF2:Dy. Considering the simplicity of themodel, the calculation compare well withthe experimental data.

Temperature Dependence at Extended Times

For small excursions in temperatureafter the TL has equilibrated in time, thevariation of brightness with temperatureis predicted by Equation (8). Table 4 showsvalues of P2/Ph assuming that the crystalhas equilibrated at 20°C and that the crys-tal is not held at the new temperaturessufficiently long for the equilibrium popu-lation of trapped change to be altered sig-nificantly. Under these conditions thevariation in TL with temperature is depen-dent only upon E and the temperatures in-volved in the change. It is clear thatlarge variations in light emission can beobserved under some conditions. Table 4also shows values for p at 20°C for trapshaving differing depths. The equilibratingtime is significant for E greater than 0.75ev.

NaI(Tl) is an attractive crystal formaking light sources since it has littleTL. CaF2:Mn is also attractive since asignificant amount of time is required forthe buildup of TL to become significant.

Figure 9 shows the calculated temperaturedependence of a CaF2:Mn light for severaltimes after its initial construction. Whenused in the temperature range 0 to 500C,the effect of TL would not become a prob-lem for the first two years after its con-struction.

Conclusions

The effect of long lived thermolumi-nescence is significant with respect to thetime and temperature stability of radio-active reference lights. The magnitude ofthe effect can be estimated for a particularmaterial using the Randall-Wilins model forthermoluminescence if the trapping constants,E and s are known. Of the materials studied,ZnS and CaF2:Dy are shown to be undesirablematerials and NaI(Tl) and CaF2:Mn to be de-sirable materials for the construction ofradioactive reference lights.

References

J. T. Randall and M. H. F. Wilkins, Proc.Roy. Soc. A184, 366 (1945).

2R. M. Grant, et al., Proc. of the SecondInternational Conference on LuminescenceDosimeters, 1968.

TABLE I

Trapping Characteristics of TLD-1002

-l

s, sec.

5 x 10ol

2 x 1013

5 x 1012

1.2 x 1012

2.2 x 1013

RelativeEP1ulation

.07

.26

.20

.29

1.00

TABLE II

Trapping Characteristics of CaF2:Dy

s, sec. 1(assumed)

loll10 11

1011

loll

97

RelativePopulation

.25

.25

.25

PeakNumber

0

1

2

3

4

E, ev

.82

1.08

1.13

1.15

1.36

PeakNumber

1

2

3

4

E, ev

.87

.92

1.06

1.15

TABLE III

Yield of Thermoluminescence Relative to Scintillation

CEystal Q/L

NaI (Tl)

CaF2:Mn

TLD-l00(LiF)

CaF2 :Mn

ZnS

.001

.04

1.1

2.0

10.

TABLE IV

Calculated Temperature Dependence for Hypothetical Traps

P20 decay constant at 200C

ev P50/P20 PO/P20 for s = loll

sec. -1 hr. -1 r.-

S 6.29 .334 249. --

.012 43.2

- .0036

TIME -

Figure 1. The effect of time and tempera-ture on light output of a hypo-thetical reference light.

TEMPERATURE, °C

Figure 2. Thermoluminescence of TLD-100heated at 10°C/sec. Severaldiscrete traps are identifiedby the glow maxima.

98

0.

0 .75

1.0

1.25

1.5

15.79

39 .64

99.48

249.6

.113

.054

.026

.012

31.5

9.4xlO44.7xlO 8

Pa se-E/KT

-Tj- told T2 >Ta-T <T

J-Q Pi +P

I

El

100 200 300 400TEfMPERATURE, °C

Figure 3. Thermoluminescence of CaF2:Dyand CaF2:Mn when heated at10°C/sec. Four discrete trapsare obvious in CaF2:Dy and adistribution of traps is pre-sent in CaF2:Mn.

1.0 ZnS

0

1-CD

.4_j_w

0 100 200 300

TEMPERATURE, 0CFigure 5. Temperature dependence of light

emitted from ZnS and NaI(Tl)during continuous irradiationby Carbon-14 beta rays. Thedata were taken within a fewminutes of placing the sourcein contact with the crystals.

400TEMPERATURE, °C

Figure 4. Thermoluminescence of ZnS andNaI(Tl) when heated at10°C/sec. ZnS has a continuumof trapping levels and NaI(Tl)shows a pair of traps.

.0CaF2: Mn

a..

01 .6 TLD-I00(LIF)W,.

-J

> .2

0 100 200 300TEMPERATURE, °C

Figure 6. Temperature dependence of lightemitted from CaF2:Mn, CaF2:Dy,and TLD-100 during continuousirradiation by Carbon-14 betarays. The data were taken with-in a few minutes of placing thesource in contact with thecrystals.

99

--

I-

a.

0:3I-

w

4w

8r

6

4

2

ZnS/- EXPERIMENT- CALCULATION

nI_ __ 200 400

MI

n0

0

w

4-JuJ

600 S00TIME, MINS

Figure 7. Time dependence of light emit-ted from ZnS after being placedin contact with Carbon-14 betaray source. The phosphor washeld at room temperature. Alsoshown is the time dependencecalculated from values of E, s,and Q/P determined from thermo-luminescence data.

2.0

1.5

1.0

TEMPERATURE, C

Figure 9. Temperature dependence for aCaF2:Mn reference light calcu-lated for several times afterinitial fabrication. The lightwas assumed to be held in atroom temperature (20°C) duringthe period indicated.

_ o _R--, _

-0 ~ ~~~-

// CaF2: Dy-CLCL EXPERIMENT- CALCULATION

0.5 h

200 400TIME, MIN

600 800

Figure 8. Time dependence of light emit-ted from CaF2:Dy after beingplaced in contact with Carbon-14 beta ray source. The phos-phos was held at room tempera-ture. Also shown is the timedependence calculated fromvalues of E, s, and Q/P deter-mined from thermoluminescencedata.

100

I I - - I

2.0

%0 0