Production of 128 I Atoms through 127 I ( n , γ ) 128 I Reactions in some Solid Iodocompounds Using an ( 241 AmBe ) Neutron Source

Present work reports the production of I atoms through I (n,γ) I reactions in six iodine containing compounds (viz. NaI, NaIO3, NaIO4, KI, KIO3 and KIO4) in solid state. An Am-Be neutron source has been used for the production of I atoms. The radioactivities produced have been measured by a thin end-window G.M. Counter. The irradiation time and counting geometry were constant throughout the present sets of experiments. Results obtained have been analyzed through time-decay analyses. From the decay curves, the half life of I has been estimated ~25 minutes. The average zero time activities of I from three independent measurements in each of NaI, NaIO3.H2O, NaIO4, KI, KIO3 and KIO4 target, have been found 116±2, 92±4, 88±6, 102±6, 98±1 and 93±5 cpm, respectively. It is interesting to note that the activities show a decreasing trend while passing through either from NaI to NaIO4 or from KI to KIO4. Results are attributed to the quantity of I produced in a specified volume element and, self absorption and self scattering of radiations within the target compounds studied.

of iodine and convenient half life (25 minutes) of 128 I enable researchers to study favorably the high energy iodine chemistry following 127 I (n,γ) 128 I reaction.Moreover, the characteristics of 128 I compound nucleus and its subsequent de-excitation through photon emission or electron conversion and the associated recoil spectra [6] have made the subject interesting.The spin and parity i.e.1 + [7] of the ground state of 128 I is different from the spin and parity of 127 I i.e. 5/2 + [8].These properties are much helpful in explaining and understanding the (n,γ) consequences in iodine containing compounds.
One of the earlier studies on the production of 128 I is due to Benczer et al. [9].Maruyama and Idenawa [10] produced 128 I atoms by small research reactor.Alfassi [11] produced 128 I by neutron activation of KIO 3 .Dissolution effect on recoil 128 I retention in the form of IO 4 -ion following (n,γ) reaction in KIO 4 target has been studied by Sharma and Mishra [12].Sharma and Mishra [13] also studied the behavior of energetic radioiodine atoms arising in NaIO 3 -NaNO 3 solid solutions.Rhode [14] performed a series of measurements of nuclear beta and gamma radiation arising in neutron irradiated NaI(Tl) crystal.Islam et al. [15] studied the 127 I(n,γ) 128 I reaction and observed 248 transitions in the photon energy range 4.5 -6.8 MeV.Zaman et al. [16][17][18][19][20][21][22] studied 128 I reactions in several iodine containing organic compounds in liquid phase under various chemical environment.The present work deals with the production of 128 I atoms through (n,γ) activation of NaI, NaIO 3 .H 2 O, NaNO 4 , KI, KIO 3 and KIO 4 using an ( 241 Am-Be) neutron source in order to study the chemical effects of nuclear transformations in these solid iodocompounds of inorganic origin.

Preparation of paraffin cylinders
Five paraffin cylinders were prepared with the help of commercial paraffin wax bought from local market (Shaheb Bazar, Rajshahi City).The diameters of these cylindrical paraffin blocks were 0.5 cm, 1 cm, 1.5 cm, 2.0 cm and 2.5 cm.

Sample preparation
Requisite quantities of solid iodocompounds were weighed through a digital balance (precision 0.0001gm) so that each sample target contains 3.626 × 10 21 iodine atoms.The calculated weights of these targets were

Thermal neutron irradiation
The encapsulated iodocompounds were irradiated with the help of an 241 Am-Be neutron source for the production of 128 I through 127 I (n,γ) 128 I reaction.Specification of the source and irradiation detail is described elsewhere [23].The source ( 241 Am-Be) was taken out from the hole of the neutron housing and then placed into a paraffin cylinder.The samples were then attached with the wall of the paraffin cylinder in the middle position.In this way, the solid iodocompounds prepared as described above were subjected to thermal neutron activation for 3 hours in dark.After 3 hours of irradiation, the irradiated samples were removed from the wall of the paraffin cylinder and carried to the laboratory for counting with the help of an end window G.M. Counter.Among those five paraffin cylinders, the cylinder having diameter 2.5 cm showed comparatively high activities as compared to remaining other four paraffin cylinders.So, the cylinder of diameter 2.5 cm was used for all subsequent experiments.

Counting and analysis
A G.M. Counter was used to count the activity (counts per minute) of 128 I in the irradiated samples.

Results and Discussion
The production of 128 I species was effected by an 241 Am-Be neutron source through the reaction 127 I (n, γ) 128 I. Paraffin cylinder was used for thermalizing the fast neutrons in order to produce a substantial amount of 128 I species.The activity of radioiodine produced during the (n,γ) reaction in potassium iodide is graphically shown in Fig. 1.It is evident from the figure that the 128 I species have been produced.It is also apparent from the figure that, the activity of 128 I species is decreasing with time.It has been observed from the figure that the zero time activity is 110.The half life of 128 I measured in this experiment is found 25 minutes.
The half life of 128 I has been calculated with respect to zero time activity.It has been seen from the figure that, activities persisted up to two half lives of 128 I.After first and second half lives the activities are 55 and 27.5 respectively and thereafter attain the background counts.For each target, three independent irradiation experiments and counting were performed.

Searching an optimum irradiation arrangement
The optimum irradiation arrangement was searched with the help of five paraffin cylinders of 0.5 cm, 1.0 cm, 1.5 cm, 2.0 cm and 2.5 cm diameters using potassium iodide.Three independent irradiation experiments were carried out for each of 5 paraffin cylinder.Thus, altogether 15 irradiation experiments have been carried out and the results were computed through time-decay analyses.Average zero time activities (ZTA) in counts per minute (cpm) are arranged in Table 1.It is seen from the table that, the zero time activities are comparatively high in case of paraffin cylinder of diameter 2.5 cm.So, this paraffin cylinder was assumed to be suitable for (n,γ) activation and was used for all the subsequent experiments.The fate of a hot atom depends, initially upon the particular way of deexcitation and, at the end of the process, on its chemical properties.When an iodocompound undergoes thermal neutron activation, the 127 I nucleus becomes activated by neutron capture; the excitation energy is equal to the neutron binding energy.The compound nucleus thus formed de-excites its excess energy through the emission of one or more gamma quanta and a net atomic recoil results.More specifically, atoms produced in solids by neutron capture as in the case under discussion, possess energy in excess of the thermal vibration energy of the crystal lattice.In turn, their chemical properties differ from those of the same atoms under normal temperature conditions.Other than KI and NaI, the other iodocompounds used in the present investigation are oxy-salts of iodine.Since the energies of chemical bonds in the present cases are usually of the order of a few eV, it is apparent that the majority of neutron capture reactions will result in bond breaking, leaving the 128 I atoms in a quasi-free state [24].

Results on sodium containing iodocompounds have been arranged in
Assuming that the target molecule is initially torn down [25] there are the following general stages during which reactions might occur between the radioactive atoms and various fragments and molecules to give the observed products: i) Hot reactions take place with recoil atoms either in flight [26] or at the end of their trail, while the immediate surrounding is still thermally or electronically excited [27,28], ii) Reactions occur in the target after initial thermal excitation has been dissipated and iii) Reactions may also occur during the chemical work up of the sample.In our case, we have taken equal number of 127 I atoms to undergo (n,γ) activation; the difference is that 127 I atoms exist under different chemical environment.After production of 128 I species in the target compounds, radiations from β-decay (electron capture) and γ-transitions cause various reactions to occur mostly towards the end of the track.In our solid iodotargets, initial recoil energy would not be so important and the decisive region for chemical reaction might be terminal 'hot zone' in which energy would be dissipated in a small volume, causing melting, chemical decomposition and vacancy production around the recoiling species.Partial intermingling of the material is speculated in a volume comprising ~3 × 10 6 atoms; the molten region exists for a time of about 10 -9 s, and this is sufficient for thermal equilibrium to be established and for many chemical processes to occur in the molten region, since the normal lattice vibration period is ~10 -13 s [26,29].As 128 I nuclei are normally produced with a very high energy in the present target compounds they lose excess energy in successive interactions with the medium and should become stabilized through various reactions.Reactions between a recoil fragment and the parent molecule (i.e. the ionmolecule reactions), exchange reactions, and diffusional jumps by interstitial or vacancy mechanisms are possible.The 128 I atoms or ions produced in our iodo-targets loss its kinetic energy and is stabilized in various chemical forms within the solid matrices.Following propositions can be made for the present sets of compounds; after bond rupturing, the energetic atoms may re-enter the parent molecule similar to the thermal exchange: Na 127 I (n,γ) 128  Various types and varied amount of diverse iodine species are produced during the activation process (see later).Energetic atoms may possess varying degrees of excitation that determines their subsequent chemical fate.In gaseous or liquid phases, hot atoms generally react with the intact molecules of the medium.However in solid systems, as in the present case, the displacement of the recoiled atoms is much shorter and the lifetime of lattice defects, excited states and molecule fragments is longer, radiation damage (randomly distributed chemical changes) plays an important part [30].It is reported [31] that neutron irradiation of iodate and periodate solutions produce IO 4 -, IO 3 -, IO 2 -, IO -, I - species with the distribution as shown in Table 3. Presence of water of crystallization as in the case of hydrated iodate (NaIO 3 .H 2 O) may have remarkable influence on the product yield [32].The difficulties of determining the chemical nature and environment of a recoil atom arise because of the very low concentrations of such atoms.Batalha and Bellido [33] investigated the state of 128 I hot atoms in the crystalline periodates of NaIO 4 , Na 2 H 2 IO 4 and Na 4 I 2 O 9 .3H 2 O containing 127 I.
They postulated that the excitation of hot atoms through Auger processes and subsequent relaxation steps in a particular chemical and structural environment seems to explain the results better than recoil in the de-excitation of 128 I atoms.In our case we must take into consideration two other factors that can affect the relative abundance of the recoil fragments: a) mass of the iodine atom that influences the recoil energy, the number of atoms lost and the extent to which the recoil fragment is reduced; b) the enthalpy of formation of the IO n -ion that governs the tendency towards reduction or breakdown of parent molecule.The probability of regenerating the parent compound, partly disintegrated by the radioactive recoil, decreases with increasing structural complexity since the number of possible chemical forms produced by the decomposition of a few molecules by recoil energies might be quite large.In the case of iodates, a restricted distraction and disarray would justify the fact that each oxy-salt reveals its own behavior during and after each nuclear event.Our present results ratify the fact that the lattice surrounding the radioiodine species appears to be an important aspect reported earlier for iodate-periodate crystalline mixtures [32,34].Crystal structure of sodium periodates has been reported [35].Evidently, the explanation for the results observed in the present study can be worked out viewing to the direct participation of the medium in the changes occurring during the nuclear transformation and, in particular, in the final stages when the process ends up by chemical reactions.Near the end of the track, the recoil species loses energy predominantly by ejecting atoms from the medium forming interstitials [24].It is reported [36] that in hard ionic solids containing a simple molecular ion, such as the salts of the oxy-anions, the process probably requires about 25 eV.The recoil atom may then react with their parent fragments or interact with a neighboring parent molecule or may remain trapped in the lattice site.
From Table 4, one can also note that the activities decrease while passing from NaI to NaIO 4 .This result points out that as 128 I atoms experience relatively complex and structurally different chemical environment, the activities of radioiodine decrease.Results indicate that self absorption of radiations might be important.Self absorption and self scattering depend not only on β-particle energy, but also on the chemical form of the sample and on the geometrical arrangement of sample and detector [37].As the counting in the present work has been performed under constant geometry, chemical composition of the sample and the generated radionuclides in a specified volume element are accordingly responsible for the observed effects.Uniformity is also important insofar as the specification of maximum surface density, set by a given experimental situation, applies not only to the source as a whole but to any small portion of it.Self scattering and self absorption become negligible for samples << 1 mg cm -2 [37].Conversely, our targets are thick enough (401-577 mg cm -2 ) in which the incident bombarding particles are significantly degraded in energy.The thickness values (mg cm -2 ) of sodium containing iodocompounds in our cases are considerable and cannot be disregarded (cf.Table 4).In emission sources of finite thickness where radiant energy quanta emitted by atoms (or molecules) are absorbed by atoms of the same kind present in the same source, radiation level reduces.The absorbed energy is usually dissipated by collisional transfer, or through emission of radiant energy of the same or other frequencies.In consequence, the observed radiant intensity of a spectral line (or band component) emitted by a source may be less than the radiant intensity would be from an optically thin source having the same number of emitting atoms.Self-absorption may occur in all emitting sources to some degree, whether they are homogeneous or not.Reduction of the intensity of the center of an emission line occurs by selective absorption by the cooler portions of the source of radiation [38,39].

(n,γ) activation of potassium containing solid iodocompounds
Similar results have been recorded in case of potassium containing iodocompounds viz.KI, KIO 3 and KIO 4 .Results are shown in Table 3. Results of Table 3 also show a decreasing trend in the 128 I activities as seen in sodium containing solid iodocompounds considered above.Radioactive 128 I recoils formed by neutron capture in crystalline KIO 3 and KIO 4 appeared as iodide, iodate and periodate ions on analysis of alkaline aqueous solutions of the irradiated solids.With KIO 3 more than two-thirds of the radioiodine was retained as radioiodate and small amounts of radioperiodate.In the KIO 3 crystal, neutral radicals of the type IO and IO 2 are most likely to be formed [23].In KIO 4 , 90% of the recoils appeared as radioiodate (K 128 IO 3 ) while periodate was less than 10%.The radioactive iodide ion was readily oxidized to IO 3 -and IO 4 -thermally and by γ irradiation [31].Potassium iodo-salts used in the present investigation undergo similar reactions as speculated for sodium containing iodocompounds.The question of complex chemical environment again comes.When iodine atoms are bound to structurally more complex chemical environment, the radioactivities would be less due to self absorption of γ-rays.Here the crystalline lattice seems to ensure a higher stability of the primary products.In solids, the Mossbauer effect has been used to study various unstable complexes formed during β disintegration by a daughter atom in the crystal lattice [24].It is reported that I*O 2 -is a stable recoil product in the solid state, and is probably the precursor of I * O 3 - [40,41].Neutron activation of oxygen containing rhenium compounds have been investigated [42,43]; the importance of composition and structure has been dealt in some detail.It is worth noting here that as molecular weight of the substances used in the present investigation is increased, the activities get decreased.Reactions between a recoil fragment and the parent molecule, exchange reactions and diffusional jumps by interstitial or vacancy mechanisms are highly likely.

Conclusion
It is evident that 128 I atoms have been produced in the neutron irradiation of some solid iodo oxy-salts of sodim and potassium.Chemical nature and environment of the recoil atom govern the radioactivity produced in these oxygen containing solid iodocompounds.
A good deal of information on possible exchange reactions will be needed before detailed statements could be made regarding the mechanisms of the radiochemical reactions in these solids.

Fig. 1 .
Fig.1.In this way, the yields of 128 I species due to each sample were computed for zero time activities.

Fig. 1 .
Fig. 1.Time decay analysis of the activity of 128 I species.

Table 1 .
Thermal neutron activation of KI in paraffin cylinders of different thickness.

Table 3 .
Species produced in the neutron radiation of iodate and periodate solutions.However, it is difficult to ascertain with the present sets of experiments, in what chemical forms are hot atoms stabilized in the crystals of NaI, NaIO 3 .H 2 O and NaIO 4.