chemical effects of (n, γ) reactions in periodate systems

Journal of Radioanalytical and Nuclear Chemistry, Articles, 91/2 (I 985) 251-25 7

CHEMICAL EFFECTS OF (n, 7) REACTIONS IN PERIODATE SYSTEMS

E. M. BATALHA,* A. V. BELLIDO**

Coordenar dos Programas de Pds-Graduacto de Engenharia, Universidade Federal do Rio de Janeiro,

21941 Rio de Janeiro [Brasil]

(Received August 21, 1984)

Szilard-Chalmers effect in crystalline periodates NalO4, Na3H2IO6 and Na4120 ~ .3HzO containing 12 ~ I and 12 ~ i+12 ~I was investigated. The initial yields and thermal annealing behaviour of the 2 = *I and 1 s~ induced activities were found to be different for each type of periodate; both isotopes kept their own beliaviour in systems having at the same time

~ s i+~ a o I and an isotope effect was evident. The de-ex citation of the "hot atoms" through Auger processes and subsequent relaxation steps in a particular chemical and structural environment seems to explain the results better than nuclear recoil.

Introduction

OWENS has writ ten an excellent review, up to 1976, of the hot a tom chemistry

studies on nuclear reactions induced by neutrons in salts of halogen oxy-anion~. ~

Since that time, not very many papers have been published on iodine oxy-salts 2 -7

and the main questions still remain wi thout appropriate answer. These questions

are related to the way how the compound nucleus gets de-excited by recoil or by

Auger cascades and charge neutralization or both, and to the processes by which the

"ho t a tom" comes to rest in an inorganic crystalline environment.

To contribute more data to the knowledge of these systems, it was decided to

investigate iodate and periodate compounds in which, like in a previous work, s

natural 1271 and 1271 + t 291 systems were studied with respect to the extension

of the molecular disruption and the particular behaviour of the isotopes produced in

the nuclear reactions. Results of (n, 3') reactions in metaperiodate (NalO4), para-

periodate (Na3H2106) and dimesoperiodate (Na412 09 " 3H2 O) systems are presented

*Present address: Empresas Nucleares Brasileiras S.A. - NUCLEBRAS, Av. Presidente Wilson 231, 20030 Rio de Janeiro, Brasil.

**Present address: Instituto de Engenharia Nuclear, Comiss~o Nacional de Energia Nuclear, C.P. 2186, 20001 Rio de Janeiro, Brasil.

1" Elsevier Sequofa S. A., Lausanne Akaddmiai Kiad6, Budapest

E. M. BATALHA, A. V. BELLIDO: CHEMICAL EFFECTS

in this paper and in successive publications results will be reported on (n, 2n) reactions and studies on transfer annealing in search of a correlation which could shed light on

the subject.

Experimental

Samples for irradiation

Meta- and paraperiodate sodium salts containing natural 1271 were A.R. grade chemicals used without further purification. Dimesoperiodate with 1271 and all the periodates containing 1271 + ~ 291 were prepared by oxidation of sodium iodide following procedures based on methods proposed by SMITH, 9 BARNAYS 10 and mentioned by MELLOR, ~ respectively. The Na 1 2 9 I in aqueous solution was provided by New England Nuclear. Purity of the prepared salts was checked by X-ray diffractometry, with exception of the dimesoperiodate for which no standard. card was available.

30 mg samples of metaperiodate and 3 mg samples of para- and dirnesoperiodate (due to their low solubility in ammonia solution) were irradiated in small polyethylene envelopes.

Irradiations

The thermal neutron irradiations were carried out in the Argonaut Reactor at the Nuclear Engineering Institute, Rio de Janeiro. The samples were irradiated for 4 hours under the following conditions: thermal flux density = 1.86 �9 109 n " cm -2 " s-1 ; cadmium ratio = 2.82; gamma radiation level = 5.77 " 104 rad/h.

1so'thermal annealing

The irradiated samples were heated at 90 ~ in a thermostated bath with a stability of + 0.2 ~ and for periods of 10, 20, 40 and 60 minutes.

Chemical separation

Three fractions were separated from the irradiated samples: reduced fraction (I ~ and I , because of their practically Instantaneous isotope exchange), iodate fraction (IO~)

252


and periodate fraction (IO~). The fraction separation was performed according to the procedure published earlier s with the following modifications to get liquid samples for gamma counting: (1) the silver iodide precipitates (of the reduced and periodate fractions) were dissolved with sodium thiosulfate after addition of 1 ml of concentrated ammonia solution and (2) the silver iodate precipitate (of the iodate fraction) was dissolved with 3M ammonia solution. Precision of the method: 2%.

Counting

The gamma rays of 12SI (25 m) and 13~ (12.4 h) were measured, in the three

separated fractions, by means of a 3" X 3" well-type scintillation detector coupled to a single channel analyzer with its window adjusted from 400 to 800 keV. In case of having the two activities together, the total gamma activity was determined first and, after enough time to allow the decay of 12sI, the 13~ activity was counted. Corrections for decay and extrapolation of the counting rates to zero time were made.

Results

Sodium metaperiodate systems

Figure 1 shows the initial yields and behaviour of the ~ 281 and 130I activities in isothermal annealing at 90 ~ in the irradiated metaperiodates Na 127 IO4 and Na ~ 27 - 129 IO4. The activity distribution of the two radioiodine isotopes is not the same in the three fractions. The 12 s I has the same initial yields and behaves in the same way in both metaperiodate systems, not being influenced by the presence of

100

"6 8O 20

o

, 1o;

L v ~ 3 II �9

r,1 ~ 0 ~

Time, rain

Fig. 1. Isothermal annealing of 12 s I and 1 a o I at 90 ~ in irradiated metaperiodate systems: o l 2 S l i n N a l 2 7 I O 4 andNa 127- '29104;e13~

253


a301 in the irradiated samples of Na t 27- t29 IO4. In the annealing processes, the

plateau is reached within the first 20 minutes of heating and the iodate fraction is

favoured with an increase of 3.5% for ~ 2 s I and 5% for ~ 3 o I.

Sodium paraperiodate systems

Distributions of t 281 and t 3 o I induced activities after irradiation and in isothermal

annealing at 90 ~ in Na.a H2 t 27 IO6 and Na3 H2 x 27 - a 29 IO6 are presented in Fig. 2.

6C

.>_

o 6O

4o

20

~, ~ ~o~

10 20 30 40 60 Time~ rain

Fig. 2. Isothermal annealing of ~ = s I and ~ 3 o I at 90 ~ in irradiated paraperiodate systems: o 12Sl inNa3H21~ ~IO6 and Na3H2127-t29IO6;e t3~ inNa3H212 ~-t 29106

The same facts already pointed out for the metaperiodate systems are also evident

in these systems, but in this case the favoured fraction in annealing is the periodate

with an increase of 12% for 1281 and 8.5% for 13~

Sodium dimesoperiodate systems

Results of the annealing treatment of 1281 in irradiated Na4127 I2 09 " 3H20 are

given in Fig. 3. I t was not possible to study the behaviour of the 1 ao I because of the

small activity in the dissolved sample due to the very low solubility of the dimeso-

periodate compound in ammonia solution. The plateau is also reached within the first

20 minutes of heating but the favoured fractions are now two: iodate and periodate.

The increases are 4% and 3%, respectively.

254


60 "6 - 0 ~

o

!zo~ ~to o ~- o ~o I t 30 40

Time~ rain Fig. 3. Isothermal annealing of i 2 s I at 90 *C in irradiated dimesoperiodate system

lnitial yieMs

Table 1 shows the initial yields of the neutron induced activities ~ 2sI and 13~ in

the different periodate systems under the conditions of irradiation already mentioned.

Table I Initial yields after (n, 7) reactions in periodate systems

System Activity Percentage of activity

I ; I0 I0~ I0~

Na t 2 ~ IO4 i a I I 3.5 89.0 7.5

Na 12 ~-129iO 4 12ai 3.1 90.1 6.8 I s o I 6.9 82.4 1 0 . 7

Na3H~ + ~ ~IO+ 12'I 9.5 73.4 17.1

Na3H2' ~ ~-129IO6 1~1I 10.1 71.9 18.0 t 3 o I 8.1 69.6 22.3

Na, + 2 ~ I 20 9" 3H20 t , a I 19.6 71.2 9.2

Discussion

The data obtained for the periodate systems lead to the following conclusions:

(1) the chemical effects of (n, ~) reactions are different for each type of periodate

since the initial yields and thermal annealing behaviour differ in each case; (2) the

1 2a I and ] 3~ produced in the nuclear events, follow their own behaviour when

255


they are together in the systems; (3) there is an isotope effect, the retention being

always higher for 1301 and the rate of annealing different for each isotope; (4) in

thermal annealing at 90 ~ the favoured fractions are different for each periodate.

To explain the above results it seems reasonable to assume a substantial cancella-

tion of recoil due to the emission of a complex neutron capture gamma-ray spectrum

by the compound nucleus 12 and to consider the Auger processes a dominant way of

de-excitation of the hot atom. Auger cascades will be the consequence of internal

conversion of low-energy 7-rays from the neutron capture 7-ray spectra as well as of the low 7-rays from isomeric transitions in 12 81 and 13 oi.13,14 An immediate charge

neutralization process will follow the Auger cascades before a coulombic explosion

takes place and before the nucleus moves away from the original site in the parent

ion, but at least a partial molecule fragmentation will occur due to the high energy

liberated in the process. The ultimate fate of the de-excited atoms would be reached

by chemical reactions between the fragments and being dependent, of course, on the

characteristics of the environment. I n this particular case of the periodates, a restricted disruption and disorder would

justify the fact that each isotope keeps its own behaviour during and after the nuclear transformation as was already reported for iodate-periodate crystalline mixtures, 8

the isotope effect would be explained by the particular modes of de-excitation of 1281 and 13 o I since the neutron capture gamma-ray spectra and the isomeric transi-

tions are different; finally, the fragments (I and I + O combinations) would react to

form stable chemical species, although unstable entities like IO-and IO~ may remain

trapped in the crystals but they would disproportionate on dissolution and tend to

give stable forms in thermal annealing through a stepwise oxidative process:

I- ~ I ~ ~ IO- --> IO~ ~ IO~ --> IO~, in a fashion already suggested 1 s for the thermal

steps in transfer annealing.

References

1. C. W. OWENS, Chemical Effects of Nuclear Transformation in Inorganic Systems, North Holland Publishing Co., Amsterdam, 1979.

2. H. J. ARNIKAR, S. K. PATNAIK, B. S. M. RAO, M. J. BEDEKAR, Radiochim. Acta, 23 (1976) 121.

3. R. K. BERA, B. M. SHUKLA, Radiochim. Acta, 25 (1978) 27. 4. R. N. SINGH, B. M. SHUKLA, Radiochim. Acta, 27 (1980) 11. 5. R. N. SINGH, B. M. SHUKLA, Radiochim. Acta, 27 (1980) 135, 6. R. K. BERA, B. M. SHUKLA, Radioehim. Aeta, 29 (1981) 121. 7. R. K. BERA, B. M. SHUKLA, Radiochim. Aeta, 30 (1982) 29. 8. A. V. BELLIDO, Radiochim. Aeta, 7 (1967) 12l 9. G. F. SMITH, Analytical Applications of the Periodic Acid and lodic Acid, The Frederick

Smith Chemical Co., Columbus, Ohio, 1950.

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10. P. M. BARNAYS, Inorganic Synthesis, Vol. II, McGraw-Hill Book Co. Inc., New York, 1946. 11. J. W. MELLOR, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. II,

Longmans, Green and Co. Ltd., London, 1961. 12. R. L. AUBLE, Nuclear Data Sheets, 9 (1973) 157. 13. C. M. LEDERER, V. S. SHIRLEY, Table of Isotopes, 7th. ed., John Wiley and Sons Inc., New

York, 1978. 14. H. R. HIDDLESTON, C. P. BROWNE, Nuclear Data Sheets, 13 (1974) 133. 15. A. V. BELLIDO, D. R. WILES, Radiochim. Acta, 12 (1969) 94.

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chemical effects of (n, γ) reactions in periodate systems

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