Indian Journal of ChemistryVol. 25A, April 1986, pp. 362-364

Effect of Radiation Dose on Formation& Annealing of Different DamageFragments in Potassium Nitrate

B M MOHAPATRA & D BHATTA*

Nuclear Chemistry Laboratory, Utkal University.Bhubaneswar 751 004

Received 27 May 1985; revised and accepted 20 September 1985

It is observed that at all absorbed doses (0.05 to 2.00 MGy), theconcentration of equivalent nitrite. (NOl")' is higher than that ofnitrite, NOl" in Y-irradiated potassium nitrate. With increasing dose,though the initial damage induced in the crystals increases, G-valuesand the fraction annealed (fIl) for the species decrease.

Despite extensive work 1 - B on the radiolyticdecomposition of alkali metal nitrates 1 - 11, whereinNO;, 0 and O2 are the primary radiolytic products, aclear cut mechanism is yet to be established. It is seenthat oxygen remains trapped':" in the lattice asmolecular oxygen upto about 22 % decomposition onirradiation at room temperature. Further the G-valuefor the decomposition of potassium nitrate varies withtemperature and also with % decomposition at aparticular temperature+":". The slope of the plot ofnitrite ion formation as a function of dose changesabruptly at some point below 2 % decomposition 1 andupto 22 % decomposition, the yield of the nitrite ion isindependent of the intensity but dependent upon theamount of nitrate present.: Many workers2.4,B,J 0.11have reported an increase in apparent nitrite yield,determined by eerie method of analysis as compared tothat determined by conventional Shinn method. Thisdiscrepancy is attributed to the presence of smallamount of reducing species other than NO;. Thepresent note deals with the effect of radiation dose onthe formation and thermal recovery of total reducingspecies, (NO; r and nitrite, NO; in v-irradiatedpotassium nitrate.

Crystals of potassium nitrate (AR, BDH) were driedat 383K and stored over phosphorus pentoxide beforeuse. Glass ampoules containing the samples and sealedin vacuo were irradiated with Co-60 Y-rays at roomtemperature. The dose rate was 0.102 MGy hr - 1, asdetermined 12 using Fricke dosimeter, taking G (Fe3 +)= 15.5. Crystals were exposed to different dosesbetween 0.05 and 2.00 MGy.

Nitrite was estimated by the diazo colour reaction ofShinn 13as modified by Kershaw and Chamberlin+f.Tneerie analysis, irradiated crystals were dissolved in 2x 10-4 M Ce4+ in 0.4 M H2S04 and the absorbance at330 nm was compared with that of a reagent blank

362

Notes

containing the unirradiated nitrate at the sameconcentration. Isothermal recovery was carried out at423K in an oil-bath thermostatically regulated to± 0.1K of the desired temperature.

The colours of the crystals were not much affectedeven at high irradiation doses and at all absorbed dosesthe nitrite yield i.e. the equivalent nitrite, (NO;)' asestimated by its ability to reduce the eerie ion, is higherthan the nitrite, i.e. NO; determined colorimetricallyafter diazotisation and coupling. The difference,d(NO;) may be attributed to presence of non-nitritereducing species2.B.15 -IB, such as NO, N02, N03, 0 -,0;, 0; and e,-, generated by irradiation. Thesuggested mode of radiolysis are as follows:

At room temperature irradiation, nitrite ion andoxygen are the primary radiolytic products '" (Eq. I)followed by reactions (2) and (3).NO; --NO; +0 (I)

NO; +O .....•NO; +02 (2)

NO; + 0""" NO; (3)According to Cunningham", NO; decomposes to giveeither NO+O; or NO+O-+O. The presence ofozonide ion (NO;) in irradiated crystals have beensuggested by Zdansky and Sroubek 16 and the speciesmay be formed as a result of reaction: NO; + N03.....•N203+O;. Din Muhammad and MaddockBsuggested the following modes of decomposition (Eq.4-7):

2NO;

3NO;

2NO;

NO;

.....•N03+NO~-

--N03 + NO~- + NO; +02

"""2 NO; +02

--N03 +e,-

... (4)

(5)

(6)(7)

The difference in yield cannot be attributed/ to surfaceeffects, particle size or prior treatment of the salt.

The growth of damage fragments with dose isgraphically shown in Fig. I. The plots are linear upto0.30 and 0.50 MGy respectively for (NO;), and NO;,beyond which deviations occur. Slopes of the initiallinear parts correspond to G (NO;), = 1.53 (lower thanthe value" 1.7) and G(NO;)= 1.3 [lower than thereported values2.S.B.17 - 20, 1.70, 1.60, 1.61, 1.57 but ingood agreement with the data (125), obtained by Chenand Johnson:']. As lattice changes+':' occur inpotassium nitrate at a dose of about 0.66 MGy, G-values for the entities decrease with increase in doseand at doses of 0.05 and 2.00 MGy, G(NO;)' = 1.49and 1.00; and G(NO;)= 1.37 and 0.95 respectively.

200 .. ----- T -----.-.- -- r-----·

ISO -

~ INZ 0~ 100 -~w~o>:

"-POTASSIUM NITRATE

o INO,!'6 (NO,)

~------~/O----~'------~2~~0-ABSORBED DOSE 1 MGY

Fig. I-Growth of total reducing species, (NO;)' and nitrite, NO;with dose in )I-irradiated potassium nitrate at 300K

The recovery data (Fig. 2) are a combination of fastphase change annealing+' - 23 and thermal annealingprocesses. The annealing isotherms show that there isan abrupt recovery of the damage followed by apseudo-plateau. The fraction of the damage fragments(NO;)' and NO; recombine rapidly+ -24 (foundin t = 0 axis) due to crystal transition followed by alimited amount of annealing. Lattice mobility alsoplays an important role in determining the reactions incrystalline solids during phase change by liberating theneutral damage species, possibly O2' an~ 0, aproportion of which rapidly recombines with N02.' togive the targe.t material''" - 24. As the crystaltransformation leads to loss of most of the damage O2from the lattice, very little further annealing can occur.The data show that though the extent of recovery of thefragments increases with increase in the period ofheating, it decreases with increase in dose of r-rays andis practically insignificant at high doses, e.g. at 1.00-2.00 MGy (Fig. 2).

Under the present experimental conditions, thespecies present+"! 9.22 in appreciable concentrationare NO;, O2 and O. At low and moderate doses':",molecular oxygen remains trapped in the crystal latticewhereas at high doses it diffuses out of the lattice due tohigh percentage of decomposition', and radical"concentration reach saturation where recovery processinvolves secondary radicals or non-radicals pre-dorninently. The suggested mechanisms for fast phasechange annealing, first and second order processes areas follows:

The fast phase change recovery and the first orderannealing may be due to rapid reaction of NO; and 0pairs.NO; +O-+N03' ... (8)

whilst the more significant second order process is

NOTES

~

' • POTASSIUM N)~

S.0.8i, r ·····(NOi{

~

• O.OS MGy.0.IS~f0.6 • 0.2S

- " O. SO~! 1 1.0,1.S,2.0zz«

(NO,io O.OS MGYo 0 ISo 0.2SI:> 0.'01 1.0,1.S,2.0r-o-u-O

~~=____ ~~r-~~.~~~~~ -.-+--- ---r-----t100 200 )00 0 100

TIME OF HEATING, min200

2 S

Fig. 2-Annealing of total reducing species, (NO;), and nitrite,NO; in potassium nitrate irradiated with different doses

2NO; +02-+2N03' ... (9)Potassium nitrate undergoes lattice expan-

sion 1.25.26during irradiation resulting in an increase infree space where second order reaction (Eq. 9) is morefavoured over the first order process (Eq. 8). But withincreasing dose, more and more molecular oxygen,diffuses out of the crystal lattice resulting retardation inthe recovery process and at sufficiently high doses, e.g.at 1.00 to 2.00 MGy, almost no annealing is observed.

One ofthe authors (BM) is grateful to the UniversityGrants Commission, New Delhi for a fellowship and tothe Government of Orissa in the Department ofEducation for leave of absence.

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INDIAN J. CHEM., VOL. 25A, APRIL 1986

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364

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