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Chemical state of radiorhenium formed in some inorganic iridium compounds irradiated with 660 MeV protons
J. inorg,nucl.Chem.,1970,Vol.32, pp. 3165to 3175. PergamonPress. Printedin Great Britain
CHEMICAL STATE OF RADIORHENIUM FORMED SOME INORGANIC IRIDIUM COMPOUNDS IRRADIATED WITH 660MeV PROTONS
IN
E U G E N I A IANOVICI* and N A T A L I A ZA1TSEVA Joint Institute for Nuclear Research, Dubna, U.S.S.R. (Received28 October 1969) A b s t r a c t - T h e chemical behaviour of radiorhenium formed by the Ir(p, 3 pxn)Re nuclear reaction with 660 MeV protons has been investigated. The effects of water of crystallization and ammonium ion as well as of the radiation dose on the yield of Re(VII) are described. The thermal annealing of radiorhenium in anhydrous and hydrated sodium hexachloroiridate(IV) are studied. The role of radiation induced defects and inherent crystal defects in the annealing process is also examined. A comparison between the fate of the radiorhenium recoil formed by high energy proton irradiation in lattices containing rhenium and the fate of atoms produced in non-isotopic lattices is reported. INTRODUCTION
THE CHEMICALbehaviour of radiorhenium atoms following the (n, 7) reaction in some rhenium compounds has been studied by Schweitzer and Wilhelm[l], Herr [2], Apers and Maddock [3], Miiller [4] and recently by Aten [5]. The results showed the exceptional behaviour of rhenium in Szilard-Chalmers processes. Thus, it was shown that the nuclear processes in perrhenates at room temperature lead to about 100 per cent retention, i.e. no reduction of radiorhenium takes place. On irradiation of tetravalent rhenium compounds as solid K2ReCIG a part of the radiorhenium (about 30 per cent) was found in the heptavalent state, i.e. an oxidation of rhenium recoils occurs. When the solution was irradiated almost all the radiorhenium was found to be in the heptavalent state. By high energy proton irradiation of perrhenates and KzReCI6 the present authors have obtained the same results [6]. We considered it interesting to compare the fate of the radiorhenium formed by high energy proton irradiation in a rhenium containing lattice to the fate of atoms produced in non-isotopic lattices. The present work was initiated to investigate the chemical behaviour of radiorhenium formed by the 191"193Ir(p, 3pxn)Re* nuclear reaction with 660 MeV protons. The thermal annealing of radiorhenium in anhydrous and hydrated sodium *On leave of absence from the Institute of Atomic Physics, Bucharest, Romania. 1. 2. 3. 4. 5.
W. Herr, Z. Elektrochem. 56, 911 (1952). G. K. Schweitzer and D. L. Wilhelm, J. inorg, nucl. Chem. 3, 1 (1956). D.J. Apers and A. G. Maddock, Trans. Faraday Soc. 56, 498 (1960). H. Miiller, J. inorg, nucl. Chem. 27, 1745 (1965). A. H. W. Aten and J. C. Kapteyn, Radiochim. Acta 9, 224 (1968); however see, A. G. de Kimpe, D. J. Apers and P. C. Capron, Radiochim. A cta 9, 113 (1969). 6. E. lanovici and N. G. Zaitseva, J. inorg, nucl. Chem. 31, 3309 (1969). 3165
3166 hexachioroiridate(IV)
E. I A N O V I C I and N. Z A I T S E V A has been
studied. The
role of radiation-induced
defects
a n d i n h e r e n t c r y s t a l s d e f e c t s i n t h e a n n e a l i n g p r o c e s s is e x a m i n e d . EXPERIMENTAL
Samples The following compounds are used as target material: (i) Tetravalent iridium compounds: Na2IrCIn; N~21rCI6-6H20; (NH4)21rCI6. (ii) Trivalent iridium compounds: Na:~IrCIG; Na:~lrCl6. 12H20; (NH4)31rCI6 • H20 and irCl:~. All compounds used had been prepared according to the methods reported in the literature[7-9], except for IrClz (reagent supplied by "Merck"). All other reagents used in the chemical procedures were of A.R. quality.
Irradiation The irradiations were performed at room temperature in the external beam of the Dubna Synchrocyclotron, the crystals being enclosed in polyethylene containers covered with AI foils except for the anhydrous salts which were sealed into glass ampoules in vacuo. IrC13 suspended in 4M HCI and organic solvents were irradiated in glass ampoules. The proton beam intensity, which was measured by the yield of 'Z4Na formed in the 27A1 (p, 3pn) Z4Na reaction and the duration of irradiations are listed in Tables I and 2.
Thermal annealing experiments The samples were heated in an electric oven. Fluctuations in temperature amounted to less than 1°C.
Analytieal procedures and radioactivity measurements The treatment of samples began about 10 hr after the end of irradiation. The samples were dissolved in 4M HCI in the presence of the Re(VII) and Re(IV) carriers. The Re(VIi) was precipitated with HzS as Re2ST. The sulfide was dissolved and Re(VII) extracted by pyridine from a basic medium. To determine the chemical yield. Re(VII) was precipitated a s ( C I ; H s ) 4 AsReO4. Rhenium found in lower valency-states was separated from the iridium by extraction of the former with tributylphosphate[10]. After its back-extraction into 5M H NO:~ solution, rhenium was oxidised to the heptavalent state and extracted with pyridine and precipitated with tetraphenylarsonium chloride. A check analysis performed with Re(II !) carrier indicated the absence of Re*(111). In the case of lrCl:~ the irradiated material was filtered through a glass filter and Re(VII) was precipitated as sulfide from the filtrate freed of iridium. The chemical procedure was performed using active rhenium and iridium compounds, i.e. '~;ReO4-, '8~Re~CI~2- and 1~IrC16~-. The activity of samples was measured by means of a thin windowed bell-jar type Geiger counter. RESULTS AND DISCUSSION (a) Effects o f chemical composition o f the target on the rhenium f o u n d in higher
valency-state. Effect o f water o f crystallization and a m m o n i u m ion Table 1 shows the yield of radiorhenium found in higher oxidation state in various iridium compounds (crystals and aqueous solutions) irradiated with high-energy protons. The values tabulated are the mean of three independent analyses, the errors d o n o t e x c e e d ___2 p e r c e n t . 7. S. I. Ginsburg, K. A. Gladyshevskaya, i dr. Rukovodstvo po khimicheskomu analizu platinovykh metallov i zolota, str. 98, Izd. Nauka, M. (1965). 8. M. Del6pine and P. Boussi, Bull. Soc. chim. Fr. 23, 278 (1918). 9. I. I. Chernyaev, Sintez komplexnykh soedinenij metallov i platinovykh grupp, str. 207-208. lzd. Nauka. M.-L. (1964). 10. Z. Malek, E. lanovici and N. G. Zaitseva, Radiokhimiya 10, 359 (1968).
Chemical state of radiorhenium
3167
Table 1. The yield of Re(VI1), percentage in inorganic iridium compounds irradiated with 660 MeV protons
Compound
Crystals
Na21rCl~ NaelrCIn-6H20 (NHq)21rCI~ Na:~IrCl~ Na,~IrCI6.12H20 (NH4)31rCI~.H20 irCl.~(susp.)
45.9 97.9 60.2 39.8 93.2 77.8
Solution (4M HCI)
Alcohol
Dioxane
99.0
2.2 × 2.2 x 2.2 × 1.4 × 1.4 x 1.4 × 1.0×
87.0
90.2
69.3
Proton flux (p/cm 2 sec)
68.4
Irradiation time (hr)
10TM 10~') 101° 1011 1011 1011 10H
8-10 8-10 8-10 0.5 0.5 0.5 -- 0-1
As is seen from Table 1, the chemical composition and the state of aggregation of the irradiated compound affect the yield of radiorhenium found in the higher oxidation state. On irradiation of hydrated sodium salts and ammonium salts (irrespective of iridium valency) the yield of Re(VII) is increased in comparison with the anhydrous salts. Thus, for irradiated hydrated sodium salts of Ir(IV) and Ir(IIl) almost all of the radiorhenium was found in the higher oxidation state in comparison with 45.9 per cent and 39.8 per cent for the corresponding anhydrous salts. In crystalline ammonium complexes of lr(IV) and Ir(IIl) a considerable percentage of Re(VII) was found. When the aqueous solutions of sodium and ammonium hexachloroiridate(IV) were irradiated the yield of Re(VII) increased up to 100 per cent and 87.0 per cent for sodium and ammonium, respectively. In the case of IrCl3 suspended in various media, the results obtained show that the conditions under which the material was irradiated effect the yield of Re(VII). Under acid conditions almost all the radiorhenium atoms were found to be in the higher valency state, using an organic solvent (alcohol and dioxane) an appreciable decrease in the percentage of heptavalent rhenium was found. The results presented in Table 1 show the influence of water of crystallization on the fate of radiorhenium produced in high energy proton irradiation. Concerning this problem the effects of post-irradiation hydration and dehydration are shown in Table 2. The samples were dehydrated and hydrated by the method reported in the literature [11 ]. Table 2. Effect of hydration changes on radiorhenium formed in proton irradiated sodium hexachloroiridate(IV) crystals Yield of Re(VII ), %
Na~lrCl~
Na21rCI6 hydrated
30-2±0-8 33.3±0.6
NazlrCI6.6H20 97.6±0-5
p/cm2sec N ~21rCI6"6H~O dehydrated 95-0±1.2
5-1×10:'
11. J. M. Peixoto, M. Cabral and A. G. Maddock, J. inorg, nucl. Chem. 29, 1825 (1967).
3168
E. I A N O V I C I and N. Z A I T S E V A
As is seen, the dehydration of Na2IrC16 • 6H20 prior to the irradiation leads to stabilization of about 30 per cent of the radiorhenium as Re(VII). It should be noted that the post-irradiation hydration had no effect on the yields of Re(VII). The results show also that post-irradiation dehydration of Na21rCi6 • 6H20 has influence on the higher oxidation form. The influence of water and ammonium ion on the fate of the radioactive species produced in the crystals by the (n, y) and other nuclear transformation has been described for many compounds/l 1-16]. The results showed that the presence of water or ammonium ion can either decrease or increase the retention. Recently Aten[5] has reported that the heptavalent fraction of the radioactive rhenium is higher in (NH4)2ReCIn then in K2ReCI6. Our results for hydrated and anhydrous sodium salts and for ammonium salts show that the yield of the higher oxidation form decreases as follows: NazlrCl6.6H20 > (NH4)2IrCI6 > Na2IrCl6 and Na3IrC16 • 12H20 > (NH4)alrCI6 • H20 > Na3IrCl6. As is seen, our results show the oxidizing role of water of crystallization and ammonium ion. It is known that high energy bombardment produces extensive damage in the crystal lattice. Therefore, it is possible that the radiorhenium oxidation is due to the interaction of rhenium recoils with some lattice defects created by the nuclear transformation or/and with radiation produced radicals. Indeed, the greater the radiation dose, the higher the proportion of oxidized form (Fig. 1). The oxidizing reactions may take place either in the solid or in solution. The results presented in Table 2 suggest that the oxidation of radiorhenium by the water occurs in the crystals. The water of crystallization and ammonium ion may alter the fate of the recoil atom by acting as trapping centers for electrons. Thus Cabral and Maddock [11] have suggested that the radicals OH" and H', NH2" and H" formed by irradiation of hydrated and ammonium salts can provide acceptor and donor sites in the lattice. It is suggested to suppose that some positively charged species formed by the interaction of the rhenium recoil with chloride ions are sufficiently strong reducing agents to react with water in the crystal and become oxidised. The data on the irradiation of aqueous solutions of iridium compounds show that apparently all the rhenium recoils are oxidized by the radiolytic products of water to perrhenate. The differences in the yield of heptavalent rhenium found on irradiation of IrCl3 suspended in various solvents can be explained by the different reaction of 12. 13. 14. 15. 16.
G. Harbottle, J. chem. Phys. 22, 1083 (1954). I. R. Bolton and K. I. McCallum, J. chem. Soc. 35, 761 (1957). A. G. Maddock and H. Miiller, Trans. Faraday Soc. 56, 509 (1960). I Shankar, A. Nath and S. P. V. Vaish, Radiochim. A cta 4, 162 (1965). M. Share and S. R. Mohanty, J. inorg, nucl. Chem. 29, 853 (1967).
Chemical state of r a d i o r h e n i u m
I00
~ __ ~ ,
3169
_ _,.~=~ _ _ _ _ ~.~ _.~ .~-..a~ 3
90
90
/
2
70
oJ
60
50
40
30
I
20101:'
10~3
I
i014 (~, p/crn 2
i
i0 IS
Fig. 1. D e p e n d e n c e of the heptavalent r a d i o r h e n i u m yield on the integrated p r o t o n flux (p/cm2); 1 - Na21rCl6; 2 - ( N H 4 ) 2 1 r C I 6 ; 3 - N a ~ I r C I . . 6 H 2 0 .
radiolytic products with the rhenium recoils. The high yield of Re(VII) found on irradiation of IrC13 suspended in an aqueous solution is due to the interaction of radiorhenium with strongly oxidizing radiolytic products and as: H202, C12, H C 1 0 etc. T h e lower yield of heptavalent rhenium for samples suspended in nonaqueous media; alcohol and dioxane show that the reducing radiolytic products (for example CH3, C H , O H ) become involved. (b) Effect of the high energy proton flux In order to find whether the radiation dose has any effect on the behaviour of the radiorhenium, the yield of the high oxidation state as a function of the integral proton flux was studied. T h e results for the hydrated and anhydrous sodium hexachloroiridate(IV) and for ammonium salts are presented in Fig. 1. T h e irradiation time was varied between 30 min and 35 hr. It can be seen that for anhydrous sodium and ammonium salts the higher the integral proton flux the greater the yield of Re(VII), i.e. oxidation of recoils occurs. Unfortunately,'it was not possible to extend our data far enough to obtain complete oxidation of the radiorhenium. In the hydrated sodium salt practically all rhenium recoils are found in the heptavalent state at relatively low fluxes and remain unchanged at higher radiation doses. It can be expected that radiorhenium resulting from such energetic reactions,
3170
E. I A N O V I C I and N. Z A I T S E V A
as in our case, has a large positive charge which may be reduced by interaction with electrons from the surroundings as energy continues to be lost. The experimental results show that on dissolving the moderate flux irradiated anhydrous sodium salt the lower valency state of rhenium is preponderent. This may be evidence that the positive charge of radiorhenium stabilized in the lattice is also lower. The increase in the yield of the heptavalent state with increase in the proton flux may be a consequence of radiation-produced defects with oxidizing character. It is also possible that either the concentration of the defects responsible for the reduction of the rhenium decreases with the increase of the proton flux or they are annihilated when new traps are formed. The oxidation of rhenium with increase of radiation dose may be due to the increase in the positive charge on the rhenium as a result of interaction of recoils with chlorine atoms. This reaction can take place in the solid and also during the solution process. In the case of ammonium salts irradiated at moderate proton fluxes a higher yield of heptavalent rhenium was found than in the anhydrous sodium salt. It may be supposed that ammonium salts are more sensitive to irradiation and so the higher oxidation state can be produced by the products of radiation decomposition of the target. It is shown that in the hydrate sodium salt irradiated at relatively low proton fluxes practically all radiorhenium was oxidized to the heptavalent state and it was not affected by an increase of the radiation dose. This means that the irradiation does not create defects with reducing character. (c) T h e r m a l t r e a t m e n t It was of interest to investigate the effect of dehydration on the annealing of the radiorhenium atoms. The annealing isotherms obtained for hydrated sodium hexachloroiridate(IV) are shown in Figs. 2 and 3. It may be seen from Fig. 2 that the samples annealing at 80°, 120°, 150° and 200°C show a very slight change of radiorhenium yield in the higher oxidation form. The small maxima shown by the isotherms up to 200°C disappear on reannealing of samples at 320°C (Fig. 3). The yield of Re(VII) decreases slowly with increase of the time of heating. The results
Ioo 0
99 o. 9 8 ' 97 96
95
!
I
3
i
4
5 //
410
Time of heating, hr
Fig. 2. The yield of the Re(VII) versus time of the thermal annealing in NazlrCI6.6H20 irradiated with 660 MeV protons (6 = 1.4 × 109 p/cm~sec) A _ 80°; • - - 120°C; © - 150°C; - 200oc.
Chemical state of radiorhenium
3171
98
k k 97
NO
\ \
0
96 0
95
I
I
I
I
I
2 3 Time of heotinq, hr
4
Fig. 3. The yield of R e t V I I ) as a function of the isothermal annealing time at 320°C in Na,2IrCI6"6H20 irradiated for different proton fluxes. © - 4 , = 1 . 4 x l 0 9 p / c m ~ s e c ; • -~b = 3.4 × 10 TM p/cm 2 . sec.
presented in Fig. 3 show that the time of irradiation in the proton beam has no effect on the annealing behaviour of the rhenium. The annealing isotherms at various temperatures, except 80°C, show a slight increase of the yield of Re(VII) followed by a decrease after about one hour of heating. Possibly the annealing process is accelerated by concomitant dehydration. The decrease of the yield of Re(VII) with time of heating and temperature can be explained by the thermal decomposition of the target. Annealing isotherms at 320°C show a slight but continuous decrease in the yield of Re(VII). Annealing isotherms for anhydrous sodium hexachloroiridate(IV) are shown in Fig. 4. As is seen, typical annealing curves are obtained. An increase of the yield of radiorhenium in the higher oxidation form with time and temperature is observed, the isothermal curves each reach a pseudo-plateau which is dependent
70
o_"
6O
50 i
40
I
I
....
I
t
I
2
3 Time of heoting, hr
4
// 15
Fig. 4. The yield of R e ( V I I ) as a function of the thermal annealing time in Na21rCl6 ( ~b = 8.9 × 109 p/cm 2 . sec). • - 200°; ( 3 - 250°C; G - 320°C; I ~ - 350°C.
3172
E. I A N O V I C I and N. Z A I T S E V A
on temperature. The higher the temperature the faster the oxidation of radiorhenium. Figure 5 presents the isotherms at 320°C for the anhydrous sodium salt irradiated at different fluxes. As is seen, a higher flux led to more radiorhenium oxidation. The results show that the irradiation-induced defects have a marked influence on subsequent recoil annealing. It was of interest to examine if the inherent crystal defects initially present, a bulk property of the lattice, were involved in the recoil annealing process. Therefore, the effect of heating the crystals before proton irradiation on the subsequent recoil annealing was studied. Figure 6 presents the isotherms at 200°C corresponding to a sample heated 2 hr at 200°C before the irradiation curve 1 and for material not pre-heated (curve 2). 90
80
70
{£ 6O
j
50
40
30 '
I
i
I
2
I 4
I 5
Time of heoting, hr
Fig. 5. The yield of R e ( V I I ) as a function of the isothermal annealing time at 320°C in Na2IrCl~ irradiated for different proton fluxes l - q 5 = 8 . 9 × 109p/cm 2. sec; 2 - ~ = 2.12 × 101°p/cm 2 . sec.
It can be seen that pre-irradiation heating decreased the initial yield of Re(VII) and lowered the plateau value of the annealing isotherms. It has been suggested that the oxidative annealing reaction comprises a step involving removal of an electron from the radioactive fragment, followed by a recombination process involving the radioactive fragment and its surrounding [ 17]. As by thermal annealing of anhydrous sodium salt an oxidation takes place, release of an electron from the radiorhenium recoils can be expected. The defects normally present in the crystallites as well as those induced by irradiation and formed in the track of the recoil atom can form acceptor sites. 17. F. Baumg~irtner and A. G. Maddock, Trans. Faraday Soc. 64, 714 (1968).
Chemical state of radiorhenium
3173
]E--a
5Z 50 48 46 44
o.
42 4O
38 ~ 36 34 321 I
I I
I
I 2 Time of heating,hr
I
I 3
Fig. 6. T h e y i e l d o f R e ( V I I ) as a f u n c t i o n o f the i s o t h e r m a l a n n e a l i n g t i m e in Na~lrCl6 (th = 1-7 × 10'9 p / c m 2 . sec). 1 - s a m p l e s u n t r e a t e d b e f o r e i r r a d i a t i o n , 2 - s a m p l e s h e a t e d
at 200°for 2 hr beforeirradiation. As in the case of NazIrCl6 the oxidative annealing reaction of radiorhenium was partially annihilated by heating, it can be supposed that the defects inherent of the crystal and in part responsible for the annealing, were removed by thermal treatment before irradiation. However, some oxidative annealing was still observed. This means that the defects and/or radiolytic products take part in the annealing process. (d) Comparison of result obtained in isotopic and nonisotopic lattices The chemical similarity of the rhenium(IV) and iridium(IV) hexachlorocomplexes offers a possibility of comparing the recoil radiorhenium behaviour in isotopic and non-isotopic lattices. Figure 7 shows the effects of the radiation dose upon the yield of Re(VII) in Na2IrCl6 and K2ReCI6. It is interesting that at a relatively low integral flux 4)--- 10'z-5 × 10'3p/cm 2 the yield of heptavalent rhenium is quite similar in both compounds. An increase in dose in the rhenium complex produced a decrease of the yield of Re(VII), i.e. reduction takes place. On the contrary in the iridium salt the higher the proton flux the greater the yield of the heptavalent-state, i.e. an oxidation reaction occurs. It is seen that the iridium salt is more sensitive to the irradiation conditions. Figure 8 shows the annealing isotherms at 200°C for K2ReC16 and Na2IrCl~. As is seen, the annealing of radiorhenium in the two compounds involves processes of a different nature. The annealing in K2ReC16 shows the trends described by Herr[l], Apers and Maddock[3] for the same compound irradiated with neutrons, i.e. a reduction of radiorhenium takes place. By contrast in Na2IrCl6 an oxidation of rhenium is observed.
3174
E. IANOVICI and N. ZAITSEVA
60 *
J
50
n
I
...z /
40
30
20 I
I
I
I IIIII
I
I
I
I I IIII
1013
~,
I
I
I
1014 p/cruZ
I
tlltl
I
1015
Fig. 7. Dependence of the heptavalent radiorhenium yield on the integrated proton flux (p/cm2), 1- Na21rC16; 2 - K2ReC16.
5O
i
2
::i,ol ' I
0
I
2
I
3 4 Time of heatin G, hr
I
I
5
6
Fig. 8. The yield of Re(VII) as a function of the isothermal annealing time at 200°C in K2ReCI6 (1) and Na21rCl6 (2) irradiated with high energy protons. T h e different b e h a v i o u r o f r a d i o r h e n i u m f o r m e d b y R e ( p , pxn)Re a n d I r ( p , b e d u e to different t y p e s o f d e f e c t s a n d r a d i o l y t i c p r o d u c t s i n d u c e d in i s o t o p i c a n d n o n - i s o t o p i c c r y s t a l l i n e l a t t i c e b y t h e p r o t o n i r r a d i a t i o n . T h e a n n e a l i n g b e h a v i o u r o f r a d i o r h e n i u m in an i s o t o p i c m e d i u m c a n b e d u e to t h e r m a l e x c h a n g e r e a c t i o n s . R a d i o r h e n i u m r e c o i l s c r e a t e d in n o n - i s o t o p i c l a t t i c e c a n b e r e g a r d e d as s o m e k i n d o f i m p u r i t y .
3pxn)Re c a n
Chemicalstate of radiorhenium
3175
It should be noted that the excitation and ionization induced by the nuclear transformation may be important in determining the subsequent chemical reaction of the recoil species. It can be expected that the Re(p, pxn)Re* and Ir(p, 3pxn)Re* nuclear transformations may present different degrees of excitation and ionization which may result in different charge distributions on the recoil fragment and in population of the inherent crystal traps to different levels with electrons and positive holes:
CHEMICAL STATE OF RADIORHENIUM FORMED SOME INORGANIC IRIDIUM COMPOUNDS IRRADIATED WITH 660MeV PROTONS
IN
E U G E N I A IANOVICI* and N A T A L I A ZA1TSEVA Joint Institute for Nuclear Research, Dubna, U.S.S.R. (Received28 October 1969) A b s t r a c t - T h e chemical behaviour of radiorhenium formed by the Ir(p, 3 pxn)Re nuclear reaction with 660 MeV protons has been investigated. The effects of water of crystallization and ammonium ion as well as of the radiation dose on the yield of Re(VII) are described. The thermal annealing of radiorhenium in anhydrous and hydrated sodium hexachloroiridate(IV) are studied. The role of radiation induced defects and inherent crystal defects in the annealing process is also examined. A comparison between the fate of the radiorhenium recoil formed by high energy proton irradiation in lattices containing rhenium and the fate of atoms produced in non-isotopic lattices is reported. INTRODUCTION
THE CHEMICALbehaviour of radiorhenium atoms following the (n, 7) reaction in some rhenium compounds has been studied by Schweitzer and Wilhelm[l], Herr [2], Apers and Maddock [3], Miiller [4] and recently by Aten [5]. The results showed the exceptional behaviour of rhenium in Szilard-Chalmers processes. Thus, it was shown that the nuclear processes in perrhenates at room temperature lead to about 100 per cent retention, i.e. no reduction of radiorhenium takes place. On irradiation of tetravalent rhenium compounds as solid K2ReCIG a part of the radiorhenium (about 30 per cent) was found in the heptavalent state, i.e. an oxidation of rhenium recoils occurs. When the solution was irradiated almost all the radiorhenium was found to be in the heptavalent state. By high energy proton irradiation of perrhenates and KzReCI6 the present authors have obtained the same results [6]. We considered it interesting to compare the fate of the radiorhenium formed by high energy proton irradiation in a rhenium containing lattice to the fate of atoms produced in non-isotopic lattices. The present work was initiated to investigate the chemical behaviour of radiorhenium formed by the 191"193Ir(p, 3pxn)Re* nuclear reaction with 660 MeV protons. The thermal annealing of radiorhenium in anhydrous and hydrated sodium *On leave of absence from the Institute of Atomic Physics, Bucharest, Romania. 1. 2. 3. 4. 5.
W. Herr, Z. Elektrochem. 56, 911 (1952). G. K. Schweitzer and D. L. Wilhelm, J. inorg, nucl. Chem. 3, 1 (1956). D.J. Apers and A. G. Maddock, Trans. Faraday Soc. 56, 498 (1960). H. Miiller, J. inorg, nucl. Chem. 27, 1745 (1965). A. H. W. Aten and J. C. Kapteyn, Radiochim. Acta 9, 224 (1968); however see, A. G. de Kimpe, D. J. Apers and P. C. Capron, Radiochim. A cta 9, 113 (1969). 6. E. lanovici and N. G. Zaitseva, J. inorg, nucl. Chem. 31, 3309 (1969). 3165
3166 hexachioroiridate(IV)
E. I A N O V I C I and N. Z A I T S E V A has been
studied. The
role of radiation-induced
defects
a n d i n h e r e n t c r y s t a l s d e f e c t s i n t h e a n n e a l i n g p r o c e s s is e x a m i n e d . EXPERIMENTAL
Samples The following compounds are used as target material: (i) Tetravalent iridium compounds: Na2IrCIn; N~21rCI6-6H20; (NH4)21rCI6. (ii) Trivalent iridium compounds: Na:~IrCIG; Na:~lrCl6. 12H20; (NH4)31rCI6 • H20 and irCl:~. All compounds used had been prepared according to the methods reported in the literature[7-9], except for IrClz (reagent supplied by "Merck"). All other reagents used in the chemical procedures were of A.R. quality.
Irradiation The irradiations were performed at room temperature in the external beam of the Dubna Synchrocyclotron, the crystals being enclosed in polyethylene containers covered with AI foils except for the anhydrous salts which were sealed into glass ampoules in vacuo. IrC13 suspended in 4M HCI and organic solvents were irradiated in glass ampoules. The proton beam intensity, which was measured by the yield of 'Z4Na formed in the 27A1 (p, 3pn) Z4Na reaction and the duration of irradiations are listed in Tables I and 2.
Thermal annealing experiments The samples were heated in an electric oven. Fluctuations in temperature amounted to less than 1°C.
Analytieal procedures and radioactivity measurements The treatment of samples began about 10 hr after the end of irradiation. The samples were dissolved in 4M HCI in the presence of the Re(VII) and Re(IV) carriers. The Re(VIi) was precipitated with HzS as Re2ST. The sulfide was dissolved and Re(VII) extracted by pyridine from a basic medium. To determine the chemical yield. Re(VII) was precipitated a s ( C I ; H s ) 4 AsReO4. Rhenium found in lower valency-states was separated from the iridium by extraction of the former with tributylphosphate[10]. After its back-extraction into 5M H NO:~ solution, rhenium was oxidised to the heptavalent state and extracted with pyridine and precipitated with tetraphenylarsonium chloride. A check analysis performed with Re(II !) carrier indicated the absence of Re*(111). In the case of lrCl:~ the irradiated material was filtered through a glass filter and Re(VII) was precipitated as sulfide from the filtrate freed of iridium. The chemical procedure was performed using active rhenium and iridium compounds, i.e. '~;ReO4-, '8~Re~CI~2- and 1~IrC16~-. The activity of samples was measured by means of a thin windowed bell-jar type Geiger counter. RESULTS AND DISCUSSION (a) Effects o f chemical composition o f the target on the rhenium f o u n d in higher
valency-state. Effect o f water o f crystallization and a m m o n i u m ion Table 1 shows the yield of radiorhenium found in higher oxidation state in various iridium compounds (crystals and aqueous solutions) irradiated with high-energy protons. The values tabulated are the mean of three independent analyses, the errors d o n o t e x c e e d ___2 p e r c e n t . 7. S. I. Ginsburg, K. A. Gladyshevskaya, i dr. Rukovodstvo po khimicheskomu analizu platinovykh metallov i zolota, str. 98, Izd. Nauka, M. (1965). 8. M. Del6pine and P. Boussi, Bull. Soc. chim. Fr. 23, 278 (1918). 9. I. I. Chernyaev, Sintez komplexnykh soedinenij metallov i platinovykh grupp, str. 207-208. lzd. Nauka. M.-L. (1964). 10. Z. Malek, E. lanovici and N. G. Zaitseva, Radiokhimiya 10, 359 (1968).
Chemical state of radiorhenium
3167
Table 1. The yield of Re(VI1), percentage in inorganic iridium compounds irradiated with 660 MeV protons
Compound
Crystals
Na21rCl~ NaelrCIn-6H20 (NHq)21rCI~ Na:~IrCl~ Na,~IrCI6.12H20 (NH4)31rCI~.H20 irCl.~(susp.)
45.9 97.9 60.2 39.8 93.2 77.8
Solution (4M HCI)
Alcohol
Dioxane
99.0
2.2 × 2.2 x 2.2 × 1.4 × 1.4 x 1.4 × 1.0×
87.0
90.2
69.3
Proton flux (p/cm 2 sec)
68.4
Irradiation time (hr)
10TM 10~') 101° 1011 1011 1011 10H
8-10 8-10 8-10 0.5 0.5 0.5 -- 0-1
As is seen from Table 1, the chemical composition and the state of aggregation of the irradiated compound affect the yield of radiorhenium found in the higher oxidation state. On irradiation of hydrated sodium salts and ammonium salts (irrespective of iridium valency) the yield of Re(VII) is increased in comparison with the anhydrous salts. Thus, for irradiated hydrated sodium salts of Ir(IV) and Ir(IIl) almost all of the radiorhenium was found in the higher oxidation state in comparison with 45.9 per cent and 39.8 per cent for the corresponding anhydrous salts. In crystalline ammonium complexes of lr(IV) and Ir(IIl) a considerable percentage of Re(VII) was found. When the aqueous solutions of sodium and ammonium hexachloroiridate(IV) were irradiated the yield of Re(VII) increased up to 100 per cent and 87.0 per cent for sodium and ammonium, respectively. In the case of IrCl3 suspended in various media, the results obtained show that the conditions under which the material was irradiated effect the yield of Re(VII). Under acid conditions almost all the radiorhenium atoms were found to be in the higher valency state, using an organic solvent (alcohol and dioxane) an appreciable decrease in the percentage of heptavalent rhenium was found. The results presented in Table 1 show the influence of water of crystallization on the fate of radiorhenium produced in high energy proton irradiation. Concerning this problem the effects of post-irradiation hydration and dehydration are shown in Table 2. The samples were dehydrated and hydrated by the method reported in the literature [11 ]. Table 2. Effect of hydration changes on radiorhenium formed in proton irradiated sodium hexachloroiridate(IV) crystals Yield of Re(VII ), %
Na~lrCl~
Na21rCI6 hydrated
30-2±0-8 33.3±0.6
NazlrCI6.6H20 97.6±0-5
p/cm2sec N ~21rCI6"6H~O dehydrated 95-0±1.2
5-1×10:'
11. J. M. Peixoto, M. Cabral and A. G. Maddock, J. inorg, nucl. Chem. 29, 1825 (1967).
3168
E. I A N O V I C I and N. Z A I T S E V A
As is seen, the dehydration of Na2IrC16 • 6H20 prior to the irradiation leads to stabilization of about 30 per cent of the radiorhenium as Re(VII). It should be noted that the post-irradiation hydration had no effect on the yields of Re(VII). The results show also that post-irradiation dehydration of Na21rCi6 • 6H20 has influence on the higher oxidation form. The influence of water and ammonium ion on the fate of the radioactive species produced in the crystals by the (n, y) and other nuclear transformation has been described for many compounds/l 1-16]. The results showed that the presence of water or ammonium ion can either decrease or increase the retention. Recently Aten[5] has reported that the heptavalent fraction of the radioactive rhenium is higher in (NH4)2ReCIn then in K2ReCI6. Our results for hydrated and anhydrous sodium salts and for ammonium salts show that the yield of the higher oxidation form decreases as follows: NazlrCl6.6H20 > (NH4)2IrCI6 > Na2IrCl6 and Na3IrC16 • 12H20 > (NH4)alrCI6 • H20 > Na3IrCl6. As is seen, our results show the oxidizing role of water of crystallization and ammonium ion. It is known that high energy bombardment produces extensive damage in the crystal lattice. Therefore, it is possible that the radiorhenium oxidation is due to the interaction of rhenium recoils with some lattice defects created by the nuclear transformation or/and with radiation produced radicals. Indeed, the greater the radiation dose, the higher the proportion of oxidized form (Fig. 1). The oxidizing reactions may take place either in the solid or in solution. The results presented in Table 2 suggest that the oxidation of radiorhenium by the water occurs in the crystals. The water of crystallization and ammonium ion may alter the fate of the recoil atom by acting as trapping centers for electrons. Thus Cabral and Maddock [11] have suggested that the radicals OH" and H', NH2" and H" formed by irradiation of hydrated and ammonium salts can provide acceptor and donor sites in the lattice. It is suggested to suppose that some positively charged species formed by the interaction of the rhenium recoil with chloride ions are sufficiently strong reducing agents to react with water in the crystal and become oxidised. The data on the irradiation of aqueous solutions of iridium compounds show that apparently all the rhenium recoils are oxidized by the radiolytic products of water to perrhenate. The differences in the yield of heptavalent rhenium found on irradiation of IrCl3 suspended in various solvents can be explained by the different reaction of 12. 13. 14. 15. 16.
G. Harbottle, J. chem. Phys. 22, 1083 (1954). I. R. Bolton and K. I. McCallum, J. chem. Soc. 35, 761 (1957). A. G. Maddock and H. Miiller, Trans. Faraday Soc. 56, 509 (1960). I Shankar, A. Nath and S. P. V. Vaish, Radiochim. A cta 4, 162 (1965). M. Share and S. R. Mohanty, J. inorg, nucl. Chem. 29, 853 (1967).
Chemical state of r a d i o r h e n i u m
I00
~ __ ~ ,
3169
_ _,.~=~ _ _ _ _ ~.~ _.~ .~-..a~ 3
90
90
/
2
70
oJ
60
50
40
30
I
20101:'
10~3
I
i014 (~, p/crn 2
i
i0 IS
Fig. 1. D e p e n d e n c e of the heptavalent r a d i o r h e n i u m yield on the integrated p r o t o n flux (p/cm2); 1 - Na21rCl6; 2 - ( N H 4 ) 2 1 r C I 6 ; 3 - N a ~ I r C I . . 6 H 2 0 .
radiolytic products with the rhenium recoils. The high yield of Re(VII) found on irradiation of IrC13 suspended in an aqueous solution is due to the interaction of radiorhenium with strongly oxidizing radiolytic products and as: H202, C12, H C 1 0 etc. T h e lower yield of heptavalent rhenium for samples suspended in nonaqueous media; alcohol and dioxane show that the reducing radiolytic products (for example CH3, C H , O H ) become involved. (b) Effect of the high energy proton flux In order to find whether the radiation dose has any effect on the behaviour of the radiorhenium, the yield of the high oxidation state as a function of the integral proton flux was studied. T h e results for the hydrated and anhydrous sodium hexachloroiridate(IV) and for ammonium salts are presented in Fig. 1. T h e irradiation time was varied between 30 min and 35 hr. It can be seen that for anhydrous sodium and ammonium salts the higher the integral proton flux the greater the yield of Re(VII), i.e. oxidation of recoils occurs. Unfortunately,'it was not possible to extend our data far enough to obtain complete oxidation of the radiorhenium. In the hydrated sodium salt practically all rhenium recoils are found in the heptavalent state at relatively low fluxes and remain unchanged at higher radiation doses. It can be expected that radiorhenium resulting from such energetic reactions,
3170
E. I A N O V I C I and N. Z A I T S E V A
as in our case, has a large positive charge which may be reduced by interaction with electrons from the surroundings as energy continues to be lost. The experimental results show that on dissolving the moderate flux irradiated anhydrous sodium salt the lower valency state of rhenium is preponderent. This may be evidence that the positive charge of radiorhenium stabilized in the lattice is also lower. The increase in the yield of the heptavalent state with increase in the proton flux may be a consequence of radiation-produced defects with oxidizing character. It is also possible that either the concentration of the defects responsible for the reduction of the rhenium decreases with the increase of the proton flux or they are annihilated when new traps are formed. The oxidation of rhenium with increase of radiation dose may be due to the increase in the positive charge on the rhenium as a result of interaction of recoils with chlorine atoms. This reaction can take place in the solid and also during the solution process. In the case of ammonium salts irradiated at moderate proton fluxes a higher yield of heptavalent rhenium was found than in the anhydrous sodium salt. It may be supposed that ammonium salts are more sensitive to irradiation and so the higher oxidation state can be produced by the products of radiation decomposition of the target. It is shown that in the hydrate sodium salt irradiated at relatively low proton fluxes practically all radiorhenium was oxidized to the heptavalent state and it was not affected by an increase of the radiation dose. This means that the irradiation does not create defects with reducing character. (c) T h e r m a l t r e a t m e n t It was of interest to investigate the effect of dehydration on the annealing of the radiorhenium atoms. The annealing isotherms obtained for hydrated sodium hexachloroiridate(IV) are shown in Figs. 2 and 3. It may be seen from Fig. 2 that the samples annealing at 80°, 120°, 150° and 200°C show a very slight change of radiorhenium yield in the higher oxidation form. The small maxima shown by the isotherms up to 200°C disappear on reannealing of samples at 320°C (Fig. 3). The yield of Re(VII) decreases slowly with increase of the time of heating. The results
Ioo 0
99 o. 9 8 ' 97 96
95
!
I
3
i
4
5 //
410
Time of heating, hr
Fig. 2. The yield of the Re(VII) versus time of the thermal annealing in NazlrCI6.6H20 irradiated with 660 MeV protons (6 = 1.4 × 109 p/cm~sec) A _ 80°; • - - 120°C; © - 150°C; - 200oc.
Chemical state of radiorhenium
3171
98
k k 97
NO
\ \
0
96 0
95
I
I
I
I
I
2 3 Time of heotinq, hr
4
Fig. 3. The yield of R e t V I I ) as a function of the isothermal annealing time at 320°C in Na,2IrCI6"6H20 irradiated for different proton fluxes. © - 4 , = 1 . 4 x l 0 9 p / c m ~ s e c ; • -~b = 3.4 × 10 TM p/cm 2 . sec.
presented in Fig. 3 show that the time of irradiation in the proton beam has no effect on the annealing behaviour of the rhenium. The annealing isotherms at various temperatures, except 80°C, show a slight increase of the yield of Re(VII) followed by a decrease after about one hour of heating. Possibly the annealing process is accelerated by concomitant dehydration. The decrease of the yield of Re(VII) with time of heating and temperature can be explained by the thermal decomposition of the target. Annealing isotherms at 320°C show a slight but continuous decrease in the yield of Re(VII). Annealing isotherms for anhydrous sodium hexachloroiridate(IV) are shown in Fig. 4. As is seen, typical annealing curves are obtained. An increase of the yield of radiorhenium in the higher oxidation form with time and temperature is observed, the isothermal curves each reach a pseudo-plateau which is dependent
70
o_"
6O
50 i
40
I
I
....
I
t
I
2
3 Time of heoting, hr
4
// 15
Fig. 4. The yield of R e ( V I I ) as a function of the thermal annealing time in Na21rCl6 ( ~b = 8.9 × 109 p/cm 2 . sec). • - 200°; ( 3 - 250°C; G - 320°C; I ~ - 350°C.
3172
E. I A N O V I C I and N. Z A I T S E V A
on temperature. The higher the temperature the faster the oxidation of radiorhenium. Figure 5 presents the isotherms at 320°C for the anhydrous sodium salt irradiated at different fluxes. As is seen, a higher flux led to more radiorhenium oxidation. The results show that the irradiation-induced defects have a marked influence on subsequent recoil annealing. It was of interest to examine if the inherent crystal defects initially present, a bulk property of the lattice, were involved in the recoil annealing process. Therefore, the effect of heating the crystals before proton irradiation on the subsequent recoil annealing was studied. Figure 6 presents the isotherms at 200°C corresponding to a sample heated 2 hr at 200°C before the irradiation curve 1 and for material not pre-heated (curve 2). 90
80
70
{£ 6O
j
50
40
30 '
I
i
I
2
I 4
I 5
Time of heoting, hr
Fig. 5. The yield of R e ( V I I ) as a function of the isothermal annealing time at 320°C in Na2IrCl~ irradiated for different proton fluxes l - q 5 = 8 . 9 × 109p/cm 2. sec; 2 - ~ = 2.12 × 101°p/cm 2 . sec.
It can be seen that pre-irradiation heating decreased the initial yield of Re(VII) and lowered the plateau value of the annealing isotherms. It has been suggested that the oxidative annealing reaction comprises a step involving removal of an electron from the radioactive fragment, followed by a recombination process involving the radioactive fragment and its surrounding [ 17]. As by thermal annealing of anhydrous sodium salt an oxidation takes place, release of an electron from the radiorhenium recoils can be expected. The defects normally present in the crystallites as well as those induced by irradiation and formed in the track of the recoil atom can form acceptor sites. 17. F. Baumg~irtner and A. G. Maddock, Trans. Faraday Soc. 64, 714 (1968).
Chemical state of radiorhenium
3173
]E--a
5Z 50 48 46 44
o.
42 4O
38 ~ 36 34 321 I
I I
I
I 2 Time of heating,hr
I
I 3
Fig. 6. T h e y i e l d o f R e ( V I I ) as a f u n c t i o n o f the i s o t h e r m a l a n n e a l i n g t i m e in Na~lrCl6 (th = 1-7 × 10'9 p / c m 2 . sec). 1 - s a m p l e s u n t r e a t e d b e f o r e i r r a d i a t i o n , 2 - s a m p l e s h e a t e d
at 200°for 2 hr beforeirradiation. As in the case of NazIrCl6 the oxidative annealing reaction of radiorhenium was partially annihilated by heating, it can be supposed that the defects inherent of the crystal and in part responsible for the annealing, were removed by thermal treatment before irradiation. However, some oxidative annealing was still observed. This means that the defects and/or radiolytic products take part in the annealing process. (d) Comparison of result obtained in isotopic and nonisotopic lattices The chemical similarity of the rhenium(IV) and iridium(IV) hexachlorocomplexes offers a possibility of comparing the recoil radiorhenium behaviour in isotopic and non-isotopic lattices. Figure 7 shows the effects of the radiation dose upon the yield of Re(VII) in Na2IrCl6 and K2ReCI6. It is interesting that at a relatively low integral flux 4)--- 10'z-5 × 10'3p/cm 2 the yield of heptavalent rhenium is quite similar in both compounds. An increase in dose in the rhenium complex produced a decrease of the yield of Re(VII), i.e. reduction takes place. On the contrary in the iridium salt the higher the proton flux the greater the yield of the heptavalent-state, i.e. an oxidation reaction occurs. It is seen that the iridium salt is more sensitive to the irradiation conditions. Figure 8 shows the annealing isotherms at 200°C for K2ReC16 and Na2IrCl~. As is seen, the annealing of radiorhenium in the two compounds involves processes of a different nature. The annealing in K2ReC16 shows the trends described by Herr[l], Apers and Maddock[3] for the same compound irradiated with neutrons, i.e. a reduction of radiorhenium takes place. By contrast in Na2IrCl6 an oxidation of rhenium is observed.
3174
E. IANOVICI and N. ZAITSEVA
60 *
J
50
n
I
...z /
40
30
20 I
I
I
I IIIII
I
I
I
I I IIII
1013
~,
I
I
I
1014 p/cruZ
I
tlltl
I
1015
Fig. 7. Dependence of the heptavalent radiorhenium yield on the integrated proton flux (p/cm2), 1- Na21rC16; 2 - K2ReC16.
5O
i
2
::i,ol ' I
0
I
2
I
3 4 Time of heatin G, hr
I
I
5
6
Fig. 8. The yield of Re(VII) as a function of the isothermal annealing time at 200°C in K2ReCI6 (1) and Na21rCl6 (2) irradiated with high energy protons. T h e different b e h a v i o u r o f r a d i o r h e n i u m f o r m e d b y R e ( p , pxn)Re a n d I r ( p , b e d u e to different t y p e s o f d e f e c t s a n d r a d i o l y t i c p r o d u c t s i n d u c e d in i s o t o p i c a n d n o n - i s o t o p i c c r y s t a l l i n e l a t t i c e b y t h e p r o t o n i r r a d i a t i o n . T h e a n n e a l i n g b e h a v i o u r o f r a d i o r h e n i u m in an i s o t o p i c m e d i u m c a n b e d u e to t h e r m a l e x c h a n g e r e a c t i o n s . R a d i o r h e n i u m r e c o i l s c r e a t e d in n o n - i s o t o p i c l a t t i c e c a n b e r e g a r d e d as s o m e k i n d o f i m p u r i t y .
3pxn)Re c a n
Chemicalstate of radiorhenium
3175
It should be noted that the excitation and ionization induced by the nuclear transformation may be important in determining the subsequent chemical reaction of the recoil species. It can be expected that the Re(p, pxn)Re* and Ir(p, 3pxn)Re* nuclear transformations may present different degrees of excitation and ionization which may result in different charge distributions on the recoil fragment and in population of the inherent crystal traps to different levels with electrons and positive holes: