The thermal release of krypton from thin layers of gold

International Journal of Applied Radiation and Isotopes, 1969, Vol. 20, pp. 447-452. Pergamon Press. Printed in Northern Ireland

The Thermal Release of Krypton from Thin Layers of Gold O. GAUTSCH and H. WAHL J o i n t N u c l e a r R e s e a r c h Centre, Ispra Establishment, I t a l y

(Received 15 December 1968) T h e release of K r from t h i n layers of A u was investigated by t h e r m a l a n n e a l i n g of K r b o m b a r d e d Au-discs. L a r g e fractions of the gas were released below 9 0 0 ° K within 200 min. T h e gas emission occurred w i t h a s p e c t r u m of activation energies, a n d it was m a i n l y controlled by a b o u t 30 kcal/mole for 1 keV of b o m b a r d i n g energy, 50 kcal/mole for 5 keV, a n d for I0 keV. T h e release b e h a v i o u r showed t h a t t e m p e r a t u r e m e a s u r e m e n t s w i t h k r y p t o n a t e d Ausamples are sufficiently reliable only for those t e m p e r a t u r e s w h i c h correspond to the ascending p a r t of the spectrum. LA LIBI~RATION THERMIQUE DU KRYPTON MINCES D'OR

HORS

DES COUCHES

O n a examind la libdration d u kr hors des couches minces d ' o r p a r la recuite t h e r m i q u e des disques d ' o r b o m b a r d d s p a r kr. De grandes proportions d u gaz furent lib4rdes 5 moins de 900°K en 200 m i n a u plus. L'4mission d u gaz cut lieu avec u n spectre d'dnergies d ' a c t i v a t i o n et p o u r la p l u p a r t le contr61e dtait d ' e n v i r o n 30 kcal/mole p o u r 1 keV d'dnergie de b o m b a r d e m e n t , 50 kcal/mole p o u r 5 keV, et p o u r 10 keV. Le c o m p o r t e m e n t de la libdration i n d i q u a que les mesures de t e m p d r a t u r e avec des dchantillons d ' o r kryptonnds sont suffisamment sflres seulement p o u r les t e m p 4 r a t u r e s qui corres p o n d e n t ~k la partie m o n t a n t e d u spectre. T E I I J I O B O E B M ~ E J I E H I / I E H P H I I T O H A I'13 T O H H H X CJIOEB 3 O J I O T A I/IccJIe~oBazIocb Teg~onoe BL~eJieHne KpIInTOHa n3 TOHKtIX cJIoeB 30~ZOTa goepe~cTBOM OT~nra 30JIOTbIX ~I,IACHOB50M6ap)~gpyeMHX HpIIIITOHOM. I~ TeqeHHe 200 MIIHyT 5bIJIH BbI~e~eH~I 50abLuHe napwH~ r a a a n p n TeMnepaType n n m e 900 ° HeabBnHa. Bbi~e~eHr~e r a 3 a I~poncxoxn:~o co crteHTpOM DHeprgg anTnear~nu ~ peryanpona~oc~, FJIaBHLIM 06pa30M HOJIIIqeCTBaMH OHOJIO 300 HgJIOHa~IopHfi/MO~Ib ~Jig 1 HI/f~O-OJIeHTpOHBO~I~Ta oHepI']Hl, 50 Hg:JIoHa~IopI4fi/MOJIt, 7UIIt 5 I4 9~IIt 10 HIIJIO-DJIeHTpOH-IIOJIbT. IIporiecc B~i;teaeHng norcaaa~, qTO rmuepeHng TeUl~epaTyp Hpu 6oM6ap~r~pyei~ix I~prInTOIt0M o6paaIIax 30~OTa ~00TaTOqItO Ita~'leH~HI,I TO~bHO ~JIIt TeMnepaTyp, COOTBeTCTByIOH~IIX BOCXOAttlII[e~ qaeTH CHeHTpa. DIE THERMISCHE

FREIGABE VON KRYPTON

AUS DUNNEN

GOLDSCHICHTEN

Die Freigabe v o n K r aus d i i n n e n Schichten yon A u w u r d e d u r c h thermisches Ausgliihen von mit K r beschossenen A u - S c h e i b e n untersucht. Grosse Anteile des Gases w u r d e n i n n e r h a l b 200 M i n u t e n bei weniger als 9 0 0 ° K freigegeben. Die Ausgasung fand m i t e i n e m S p e k t r u m v o n Aktivierungsenergien statt u n d w u r d e haupts/ichtlich b e s t i m m t d u r c h etwa 30 k c a l / M o l f a r 1 keV Beschiessungsenergie, 50 kcal/Mol ftir 5 keV u n d 10 keV. Das Freigabe v e r h a l t e n zeigte, dass T e m p e r a t u r m e s s u n g e n m i t kryptonierten A u - P r o b e n n u r for solche T e m p e r a t u r e n gentigend zuverl/issig sind, die d e m ansteigenden Teil des S p e k t r u m s ent sprechen. 447

448

O. Gautsch and H. Wahl

1. I N T R O D U C T I O N THxs WPER describes experiments to investigate the release of K r injected into An-discs by ion b o m b a r d m e n t . After the b o m b a r d m e n t with (Kr + SSKr)-ions of 1, 5 and 1 0 k e V the kryptonated Au-samples were thermally annealed and the corresponding gas release measured. T h e release data were expected to show : (1) whether there are annealing processes with an initial distribution of activation energies, a-4~ (2) whether in such thermally activated processes the correlation between gas release and temperature is sufficiently well defined to allow reliable temperature determinations by measuring the a m o u n t of released gas. 2. E X P E R I M E N T A L

PROCEDURE

Au-discs of 10 m m dia. and 1 m m thickness were b o m b a r d e d to saturation with (Kr +SSKr)-ions, having an energy of 1, 5 and 10 keV ~). After b o m b a r d m e n t the initial beta activity of the SSKr was measured in a v a c u u m of 1 . I0 -5 m m Hg, when the b o m b a r d e d sample was m o u n t e d in the release assembly, described previously. (~) Each target was annealed during successive temperature steps, the v a c u u m being

better than I . 1 0 - 4 r a m IIg. T h e desired temperatures were attained within a few min (max. error ~ 10°C). T h e counting time for the aaKr-beta activity was sufficiently long to give a m a x i m u m statistical error of _-~z3 per cent. T h e SaKr-activity which remained in the target was recorded as a function of time and temperature, and the fraction of K r which was left in the Au-discs was plotted against log time. After 200-300 rain annealing the next temperature step was started. Typical release curves for different b o m b a r d i n g energies are shown in Figs. 1-3. T h e slopes of these curves were practically constant during the measured time intervals. 3. R E S U L T S To obtain the activation energies for the release, a m e t h o d outlined by PRIMAKTM was used. T h e characteristic activation energy, which contributes most to the annealing processes at a given temperature is defined by: n 0 = R T l n (Bt)

where R = 2 eal/mole degree, T is the temperature in °K, t the annealing time at T i n see, and B a frequency factor (constant in first approximation). I f po(Eo) is the initial distribution of the

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FIO. 1. Release of Kr from Au. 1 keV; dose = 1.6. 1017 Kr-ions/cm 2 X = 151°C, • = 199°C, O = 252°C, ~ = 303°C.

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F,o. 2. Release of K r from Au. 5 keV; dose = 9 . 4 . ]0 n Kr-lons/cm 2 • = 244°C, × 3 5 9 ° 0 , (3 = 506°C, A = 6 0 0 ° 0 , [] = 733 °C, ~7 = 862°C, -}- = 960°C.

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F r o m the release curves (such as s h o w n in Figs. 1-3) t h e v a l u e s o f Po(Eo) w e r e o b t a i n e d for different b o m b a r d m e n t energies a n d release

O. Gautschand H. Wahl

450

the single points of the "initial distribution curve of activation energies" are to be determined. (~) Therefore release curves obtained by successive step annealings will give approximately correct results only if the temperature steps are sufficiently large and the annealing times sufficiently long. T h e influence of the preceding heat treatment on the following temperature step should be small. This implies that dF/d(ln t) must have about the same value after a long annealing time, regardless whether the sample was brought to a given temperature instantly, or via previous heat treatments. T o obtain the predominating activation energy E~ for the release of K r from Au, the relation : E~ =- 60 Tmax (6)

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FIG. 4. Release of Kr from Au. 1 keV. temperatures. T h e results are plotted in Figs. 4-6. From the equations (1)-(5) only approximately correct values can be expected for po(Eo), since the above expressions require that po(Eo) be the same for all samples from which

was used, (~) where E~ is given in cal/mole, and Tm~_~ is the temperature corresponding to the m a x i m u m value ofP0(E0), e q u a t i o n (6) yields for the experimental values in Figs. 4-6 about 30 kcal/mole for 1 keV of bombarding energy, 50 kcal/mole for 5 keV, and for 10 keV. A method to measure temperatures by using kryptonated solids has been reported by CHLECK.(7) A kryptonated source, heated to a given temperature, loses a given fraction of its

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Fro. 6. Release of Kr from Au. 10 keV. initial activity, and further heating at or below this temperature results in no significant additional loss of the SaKr-activityJV) After exposure the sample is re-heated and the activity is measured as a function of temperature The temperature at which a new gas emission is observed during re-heating is taken as the maximum temperature reached by the source during exposureJ 7) The existence of thermally activated processes, which in case of the Kr-release from Au occur with a spectrum of activation energies, may, however, in certain cases complicate the temperature measurements with kryptonated Au-sources. At temperatures which are above those corresponding to the maximum values of Po(Eo), the release rates decrease with increasing temperature, (Figs. 4-6), and on re-heating of the sample a new gas emission is difficult to observe with precision, because even a large increase of temperature is followed only by small release rates (Figs. 4-6). It can also be deduced from Fig. 1 (1 keV) that at 303°C it takes about 150 rain to obtain a decrease of 4 per cent of the remaining activity in the

target. Roughly the same time is needed for 5 keV samples (Fig. 2) to observe a decrease of 6 per cent at 960°C. The irreproducibility of the release rates at given temperatures is larger than 10 per cent, therefore during re-heating of the source variations below 10 per cent may not be detected. At temperatures around the peaks of the spectra the release rates are large, therefore the time of exposure is important. After long exposure times at these temperatures the S5Kractivity which remained in the source may be too small to be detected during the re-heating procedure, since after 300 min less than 10 per cent of the initial activity remains in the target at elevated temperatures, as shown in Figs. 1-3. It seems that only those temperatures which are below the "peak" temperature can be measured with sufficient precision because in this range the release rates increase with increasing temperature• 4. C O N C L U S I O N S The release of K r injected with 1, 5 and I0 keV into Au-discs, occurred with a spectrum of activation energies. The spectrum was more

452

O. Gautsch and H. Wahl

pronounced for samples bombarded with 1 keVions than for 5 and 10 keV. Large fractions of the gas were released below 900°K within 200 min. T h e obtained release data indicate that temperature measurements with kryptonated Au-sources can be expected to give correct values only for those temperatures which correspond to the ascending part of the activation energy spectrum.

REFERENCES 1. AUSKERNA. J. Am. Ceram. Soc. 47, 390 (1964). 2. ERENTSK. and CARTERG. Vacldu, m 16, 523 (1966). 3. JECH C. and KELLY R. J. nucl. Mater. 20, 269 (1966). 4. PRIMAKW. Phys. Rev. 100, 1677 (1955). 5. GAUTSCHO., MUSTACCHIC. and WAHLH. Report EUR 2515.e (1965). 6. GAUTSCHO., MUSTACCHIC. SCHORENKAMPERA.

and WAHL H. Report EUR 3267.e (1967). 7. CHLECK D. In Radioisotope Traces in Industry and Geophysics, p. 49, Proceedings of IAEA, Prague 1966, edited by IAEA, Vienna (1967).