The effects of pre-irradiation treatments on the chemical behavior of 115Cd recoil atoms in neutron irradiated cadmium phthalocyanine

J. inorg,nucl. Chem., 1972,Vol.34,pp. 453-460. PergamonPress. Printedin GreatBritain

THE EFFECTS OF PRE-IRRADIATION ON THE CHEMICAL BEHAVIOR OF ATOMS IN NEUTRON IRRADIATED PHTHALOCYANINE

TREATMENTS 115Cd R E C O I L CADMIUM

HIROSHI KUDO Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken, Japan (Received 10 May 1971)

A b s t r a c t - T h e chemical behavior of 115Cd recoil atoms was studied in cadmium phthalocyanine which had been subjected to pre-irradiation treatments such as quenching, irradiation with y-rays, or irradiation in a nuclear reactor. It was found that the initial retention of the quenched sample was smaller than that of the untreated sample, and that the difference between the initial and the plateau retention, AR, increased when the sample had been quenched from 190°C. Only a small change in the behavior of the recoil atoms was observed in both cases of irradiation prior to the recoil reaction. The results are discussed in terms of the defects; the central metal vacancies introduced in the complex by quenching has a big effect on the subsequent course of the thermal annealing reaction. INTRODUCTION

IT HAS already been reported for ionic crystals that pre-irradiation treatments, such as crushing, quenching from an elevated temperature and irradiation with ionizing radiation, sensitize the material to thermal annealing. These effects are, in many cases, explained by the interaction of the recoil fragments with the defects introduced in the crystals [ 1-6]. In the previous work[7] the chemical consequences of cadmium recoils produced in neutron irradiated cadmium phthalocyanine have been studied, with emphasis laid on the nature of bonding between the central metal and the ligand in the complex. In order to assess the role of the defects in the molecular crystals of the metal phthalocyanine, the present study is extended to the case in which cadmium phthalocyanine has been quenched, irradiated with Co-60 y-rays, or subjected to previous irradiation in a reactor, prior to the recoil processes due to the radiative neutron capture. EXPERIMENTAL Target materials Cadmium phthalocyanine used here was a portion of the material which had been used in the previous work; the preparation and purification of the target have been reported earlier [7]. i. 2. 3. 4. 5. 6. 7.

A. G. Maddock and J. I. Vargas, Nature 184, 1931 (1959). A. G. Maddock, F. E. Treloar and J. I. Vargas, Trans. Faraday Soc. 59,924 (1962). T. Andersen and A. G. Maddock, Nature 194, 371 (1962). T. Andersen and A. G. Maddock, Trans. Faraday Soc. 59, 2362 (1963). J. Shankar, A. Nath and V. G. Thomas, J. inorg, nucl. Chem. 30, 1361 (1968). I. G. Campbell and C. H. W. Jones, Radiochim. Acta 9, 7, 71 (1968). H. Kudo and K. Yoshihara, J. inorg, nucl. Chem. 32, 2845 (1970). 453

454

H. K U D O

Irradiation

The neutron irradiation of cadmium phthalocyanine was performed at dry ice temperature ( - 78°C) in the nuclear reactors of the Japan Atomic Energy Research Institute; in JRR-2 (thermal neutron flux ~m: 5.2 x 10TM n/cm2/sec) or in JRR-3 (thin: 2.0 × 10TMn/cm2/sec). The y-ray dose rates of the reactors were 1.2 x 10a R/hr and 1 x 10a R/hr, respectively. For the y-ray irradiation, the target material was irradiated by means of a 16 KCi 6°Co y-ray source to a dose of 1.1 × 107 R or 1.0x 108 R at a temperature o f - 7 8 ° C and 20°C, respectively. The target was then irradiated in JRR-2 for I min at dry ice temperature. The previous irradiation in the reactor was performed in JRR-2 at the reactor temperature (ca. 60°(2) with nvt of 7.2 × 10TMn/cm 2 or at dry ice temperature (-78°C) with nvt of 1-4 x 10TMn/cm 2. The previously irradiated target was stored in dry ice for 40 days, and then it was re-irradiated in JRR-2 for 1 min (nvt = 3.1 x 1015n/cm 2) at dry ice temperature to produce "sCd recoil atoms in the complex. Chemical treatments and radioactivity measurements

The procedures of chemical separation and thermal annealing used in these experiments were virtually identical with those described in the previous paper [7]. In the quenching experiment, samples were sealed in a quartz capillary, heated to the specified temperature in an electric furnace, and cooled quickly in liquid nitrogen. The target was then submitted to neutron irradiation. Radioactivity was measured by a Baird Atomic single channel y-ray spectrometer using a NaI(T1) crystal as the detector. A T M C 1024 channel y-ray spectrometer was also used when necessary. RESULTS

AND DISCUSSION

The effect of quenching In order to assess the role of the defects in the annealing of "sCd recoil atoms in the crystalline cadmium phthalocyanine, the effect of quenching from an elevated temperature was examined first. In Fig. 1 are illustrated the initial reten-

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TIME

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Fig. 1. Initial retention of cadmium phthalocyanine as a function of the heating time before neutron irradiation.

115Cd recoil atoms

455

tions* of cadmium phthalocyanine, which has been irradiated in JRR-3 for 2 min, as a function of the heating time before quenching. The initial retention decreases with increasing time of heating. The rate of decrease at 190°C is faster than that of 80°C, although the initial retention reaches almost the same value at a heating time of about 5 hr in both cases. The temperature of 80°C and 190°C were chosen due to the reason that the thermal annealing process of the neutron irradiated sample is divided into two stages; stage I below 100°C and stage II above 110°C. The initial retention of 26.4_ 2 per cent obtained for the quenched sample is smaller than that of the untreated sample by 8 per cent. Figure 2 shows the isothermal annealing curves of H5Cd recoil atoms produced in cadmium phthalocyanine which was heated for 120 min at 190°C before the

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Fig. 2. Isothermal annealing curves of cadmium phthalocyanine which has been quenched from 190°C before neutron irradiation.

neutron irradiation in JRR-3. In Fig. 3 the plateau value of the retention of the quenched target is plotted against the annealing temperature in comparison with that of the untreated sample. The plateau values of the retention of the quenched sample were found 'to be smaller than those of the untreated sample with the exception of the values at 20°C and 1 10°C. As shown in Fig. 4 plots of the annealable portion AR against the inverse of the absolute temperature are very nearly linear in both stages. When samples are heated to a higher temperature for several hours and then cooled very quickly in liquid nitrogen, the equilibrated number of defects at the elevated temperature is expected to be trapped in the crystals. If the annealing reaction of the recoil atoms depends on the concentration of defects introduced, AR-(1/T) plots should be linear as pointed out for the behavior of 51Cr recoil atoms in potassium chro*In this paper as in the previous one, the initial retention means the retention observed in target material which has been irradiated at dry ice temperature and stored till chemical treatment without allowing it to warm up. However, the value should be called an apparent initial retention, since it might include radiation and thermal effects concurrent with irradiation in a reactor (See Ref.[24]).

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of untreated

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TEMPERATURE (%)

Fig. 3. Comparison of the isochronal plots at a heating time of 200 rain between untrea-

ted and quenched cadmium phthalocyanines. (~-untreated sample; (~-quenched sample.

mate [8]. Thus in the case of cadmium phthalocyanine, the defects introduced in crystals through quenching are also considered to have a strong effect on the subsequent course of the thermal annealing reaction. Furthermore, Fig. 4 indicates that the dependence of AR upon the absolute temperature is different in the untreated sample and the quenched target. This difference is rather clear for stage I but is somewhat reduced for stage II. This may be attributed to a different role of the defects at each stage. Cadmium phthalocyanine is classed in the ionic type of phthalocyanine [9, 10]. The central atom of the complex is supposed to be rather mobile in the matrix when heated, leaving vacancies at the original site. If the defects trapped in cadmium phthalocyanine by quenching are the central atom vacancies, removal of the central metal from the complex raises the concentration of cadmium atoms (or ions) in the interstitial position, so that the initial retention should decrease. On the contrary, the increase of A R is due to the increase of the vacancies. Analysis of the annealing kinetics was also carded out for cadmium phthalocyanine which had been quenched from 190°C and had been irradiated in JRR-3. 8. A.G. Maddock and M. M. de Maine, Can. J. Chem. 34, 275 (1956). 9. F.H. Moser and A. L. Thomas, Phthalocyanine Compounds, Reinhold, New York (1963). 10. A. B. P. Lever, Ado. lnorg. Chem. Radiochem. 7,27 (1965).

HsCd recoil a t o m s

457

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0

t

i 2.0

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i

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x 103

Fig. 4. Difference between initial and plateau retention, AR, as a function of the inverse

of the absolute temperature.

The reaction of stage I as well as stage II was found to proceed by the first order rate law. The activation energies of the reaction determined from the slope of the Arrhenius plots are in the neighborhood of 0.25 eV in both stages; 0.26_ 0.05 eV for stage I and 0.23 ___0.05 eV for stage II. The "stage" has been observed in some of the annealing reaction of recoil atoms [7, 11-18] and more frequently in the recovery processes of cold-worked, quenched, or irradiated metals[19-21]. The phenomenon is explained by the threshold to produce a displacement of a particular kind. The thermal annealing process of the HsCd recoil atoms taking place in two stages can, however, hardly be interpreted in this way since the estimated activation energies of both stages are of the same order of magnitude. K. E. Collins and G. Harbottle, Radiochim. Acta 3, 21 (1964). T. Andersen and K. Olesen, Trans. Faraday Soc. 61, 781 (1965). K. Yoshihara and H. Ebihara, J. chem. Phys. 45, 896 (1966). S. M. Milenkovi~ and S. R. Veljikovi~, Radiochim. Acta 8, 146 (1967). R. O. Marquds and R. A. Wolshrijin, Radiochim. Acta 12, 169 (1969). M. H. Yang, RadiochimActa 12, 167 (1969). H. R. Groening and (3, Harbottle, RadiochimActa 14, 109 (1970). S. J. Yeh, N. Shibata, H. Amano, K. Yoshihara, M. H. Yang, T. F. Chen, C. T. Chen and H. Kudo, J. Nucl. Sci. TechnoL 7, 300 (1970). 19. Himmel (Ed.), Recovery andRecrystallization of Metals, lnterscience, New York, London (1963). 20. A. Bishay (Ed.) Interaction of Radiation with Solids, Plenum Press, New York (1967). 21. T. Federighi, S. Ceresara and F. Pieragostini, Phil. Mag. 12, 1093 (1965).

11. 12. 13. 14. 15. 16. 17. 18.

458

H. K U D O

An entropy term might play an important role in determining the reaction kinetics in the solid stage. When an entropy term is introduced the Arrhenius formula becomes k = A exp (AS~R) exp (--EdRT),

(1)

where AS is the entropy change, Ea the activation energy of the process[22]. Considering the vibrational entropy contribution to a crystal, in which the oscillators are changed from frequenty v to v', the entropy change is AS = 3R In (v/v').

(2)

A defect is known to cause a local disturbance in the lattice and will lead to a reduction of v, giving a positive vibrational entropy change. The contribution of this effect must be large especially for stage I, since AR of the quenched sample is larger than that of the untreated sample in stage I as shown in Fig. 4. The stage I process of the thermal annealing of 115Cd recoil atoms in cadmium phthalocyanine is presumably the re-entry of a recoil atom into the vacant central position, which has been prodtrced either by nuclear recoil or thermal treatment, whereas the stage II process is possibly the reformation reaction limited by diffusion.

The effect o f the previous irradiation with 6°Co y-rays or in the reactor The ionizing radiation can be expected to produce point defects, trapped electrons, and some kinds of radicals in solids. The investigation was carried out with cadmium phthalocyanine which had been subjected to irradiation with 6°Co y-rays before the recoil process took place. The initial retentions obtained in this experiment are listed in Table 1. One finds almost the same initial retentions* for the untreated and the target irradiated previously with y-rays at - 78°C. A difference is observed only for the sample irradiated at 20°C. It is pointed out that the initial retention decreases with increasing radiation dose, though the difference is only small. In Fig. 5 are illustrated the isothermal annealing curves of cadmium phthalocyanine which has been subjected to a previous irradiation with y-rays to a dose of 1.0 × 108R, compared with the untreated sample. The results showed similar annealing behavior for these targets, although the magnitude of AR seems to be larger in the case of previous irradiation at 20°C. It seems that ionizing radiation has only a little effect on the behavior of the 115Cd recoil atoms in cadmium phthalocyanine. Consequently trapped electrons or radicals may not necessarily be significant in determining the chemical fate of the recoil atoms in this complex. When cadmium phthalocyanine is irradiated in the reactor for a longer period, *The initial retention observed for the target irradiated in JRR-2 is slightly different from that of the target irradiated in JRR-3. In this paper, however, only the relative difference affected by the preirradiation treatment is discussed, the difference in the initial retention caused by the difference of the nuclear reactor will be mentioned elsewhere (see Ref. [24]). 22. R.A. Swalin, Thermodynamics of Solids. John Wiley, New York (1962).

"sCd recoil atoms

459

Table 1. Initial retention of cadmium phthalocyanine which has been previously irradiated with 6°Co )'-rays or in the nuclear reactor before the (n,~,) recoil processes Pre-irradiation

Initial retention (%)

Untreated

31.3 ± 0-8

Irradiated with e°Co ),-rays at-78°C, - 78°C, 20°C, 20°C,

l'l x 1.0 × 1' l × 1.0 ×

107R l0 s R l0 TR l0 a R

31.0±0.8 31.3 ± 1.0 29.9 ± 0.8 27-5 ± 0.7

Irradiated in JRR-2; a t - 78°C, nvt = 1"4 × l0 TMn / c m 2 at reactor temp. (ca. 60°C), nvt = 7"2 × 10le n / c m 2

29.1±1"0 28'7±1"0

the contribution of the internal radiation caused by the 1]3Cd(n, y ) l 1 4 C d reaction is expected to play a significant role in producing defects in the complex, because of its large cross section for thermal neutrons (o- = 20,000 barn) [23, 24]. For this reason the effect of a previous irradiation in the reactor was examined. The initial retentions obtained are summarized in Table 1. Although the difference in the 1OO

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Fig. 5. Isothermal annealing curves of cadmium phthalocyanine which has been irradiated with e°C )'-rays before the (n,T) recoil processes. (~-untreated sample; ~ - i r radiated at dry ice temperature with T-ray dose of 1.0× l0 s R. 0 - i r r a d i a t e d at 20°C with T-ray dose of 1.0 × 10SR. 23. C. M. Lederer, J. M. Hollander and I. Perlman, Table o f Isotopes, 6th Ed. Wiley, New York

(1968). 24. H. Kudo and K. Yoshihara, R a d i o c h i m . A eta 15, 167 ( 197 i).

460

H. K U D O

initial retention was not quite obvious, the initial retention appeared to decrease after a previous irradiation in JRR-2. The trend is similar to that observed in the case of previous irradiation with ~°Co y-rays. As shown in Fig. 6, little difference in the isothermal annealing curves was observed between the untreated and the previously irradiated sample, contrary to expectation. This may be because the amount of vacancies produced by the 114Cd recoils in the previous irradiation with n vt of 7.2 x 1016n / c m 2 w a s not sufficient to affect the behavior of the tlsCd recoil atoms. 100

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Fig. 6. Isothermal annealing curves of cadmium phthalocyanine which has been submitted to irradiation in a reactor before the (n, 7) recoil processes. (~-untreated sample; ~ - p r e v i o u s l y irradiated in JRR-2 at dry ice temperature with nvt of 1.4 x 1016n/cm2; 0 - p r e v i o u s l y irradiated in JRR-2 at reactor temperature with nvt of 7.2 × 10TM n/cm ~.

To elucidate the effect of the defects on the chemical fate of the 115Cd recoil atoms, the recoil process was studied for samples of complex previously quenched or irradiated with ~°Co y-rays or in the reactor. The central metal vacancies introduced in the complex by quenching seemed to have a large effect on the subsequent course of the thermal annealing reaction. Through the analysis of the thermal annealing kinetics it was revealed that an entropy term might play an important role in characterizing the annealing reaction of recoil atoms in solids. Only a small effect of previous ionizing irradiation was observed. This suggested that trapped electrons or radicals might not be very significant factors in such processes in cadmium phthalocyanine. A c k n o w l e d g e m e n t s - T h e author wishes to express his sincere thanks to Dr. K. Yoshihara of Tohoku University for his kind guidance throughout this study. He also wishes to thank Dr. H. Amano and Dr. H. Baba of the Japan Atomic Energy Research Institute for their useful suggestions and discus-

sions.