Transient annealing of isomeric transition recoils in sodium bromate

INORG.

HUCL.

CHEM.

LETTERS

Vat.

3,

pp.

363-367,

1967.

Pawgamen Press

Ltd.

Printed

in

Great

Britain.

T R A N S I E N T A N N E A L I N G OF ISOMERIC T R A N S I T I O N RECOILS

IN SODIUM BROMATE

C.H.W. Jones Simon Fraser University, Burnaby 2, British Columbia ~Rocelv~ 22 June 1~7~

While m u c h work has been carried out on the thermal annealing

of

(n,y) a c t i v a t e d

lattices,

very little work has been reported

recoils produced in isomeric proposed

in oxyanion containing

transitions.

on a n n e a l i n g of

It has been

(i) that inherent crystal defects are important in

controlling

recoil a n n e a l i n g in sodium bromate.

case then a n n e a l i n g observed

recoils

of isomeric

to occur in this

If this is the

transition recoils

lattice.

However,

Campbell

reported no a n n e a l i n g for 8°Br recoils produced crystals at 175°C while Harbottle

Campbell and Jones

increase

in NaS°aBr03 in

to reach isotopic

(4) in re-investigat-

ing this p h e n o m e n o n were unable to resolve discrepancy

(2)

(3) reported an increase

retention from 35% to 50% for crystals allowed e q u i l i b r a t i o n at 98°C.

should be

the a p p a r e n t

in the two works quoted and observed

in r e t e n t i o n at 300°C in NaS°aBr03

only an 84

labelled crystals

over that observed at room temperature. In a l l the above

investigations

the a n n e a l i n g process

was examined by a l l o w i n g the labelled crystals isotopic e q u i l i b r a t i o n a n n e a l i n g temperature.

over a period of 2-3 hours at the It was of interest to see whether

a n n e a l i n g could be observed by heating the growth

of daughter

to achieve

the crystals

following

B°Br a c t i v i t y into e q u i l i b r i u m with

8°aBr parent. 363

344

ANNEALING OF MO~ERI¢ TRANSITION RECOILS

Yel. S, N*. 9

Experimental Crystals of sodium bromate

labelled with S°'Br were

prepared by neutron irradiation of sodium bromate followed by repeated recrystallisation to remove S°'Br a c t i v i t y as bromide ion (3).

The separation of lower valency forms of S°Br from

bromate was performed by fractional precipitation of bromide and bromate as previously described

(i).

In this work however

the bromate fraction, precipitated as silver bromate, was filtered using a filter-chimney.

The solid sample thus mounted

on a filter paper was counted using a gas-flow proportional detector. The regrowth of 8°Br activity into equilibrium with S°'Br was followed by immediately counting the precipitated bromate fraction for 1 minute intervals over 90 minutes. (Counting was generally started 8 minutes following the separation.)

The eventual decay of the S°'Br - S°Br equilibrium

mixture was followed over three half-lives and the S°Br component present was determined two days later. Results The regrowth of S°Br a c t i v i t y into equilibrium with its parent can be fitted by the standard expression: A2 =k_~A~ k2 -

(e -k:t - e -ket) + A~e -let k,

which can be rearranged to give: A2e k2t = ka A~(e(k2 k2 - kl

- k,)t _ i) + A~

Yd. 3, )4*.9

ANNEALING OF ISOMERIC TRANSITION RECOILS

34|

where A2 is the S°Br a c t i v i t y at any time t, A~ is the initial a c t i v i t y of S°'Br and A~ that for S°Br present as bromate, and ka are the respective decay constants. Ace k2t against ka ~2

Thus a plot of

(e(~e - kl)t _ i) gives a straight line - k,

of slope At and intercept A~.

The total a c t i v i t y measured at

any time was taken as being equal to Aa, 8°Br daughter.

kI

the activity of the

The retention of 8°Br as brom~te was then given

by the ratio of A~ to A~. Crystals were allowed

to reach isotopic equilibration

at room temperature over a period of 3 hours.

Aliquots of

crystals were then heated at 245°C for increasing times t, and the retention of 8°Br as bromate determined as indicated above. The results are shown in figure i.

o z

0

~6o 5O X

4O

TIME (HRS.[ 3O FIG. 1

344

AHHEALIHG OF ISOMERIC TRANSITION RECOILS

Yel. 3, He. 9

Discussion The large increase in retention for short times of heating indicates that isomeric transition recoils in this lattice are extremely sensitive to thermal annealing.

The

decrease in retention for longer times of heating apparently derives from the finite life-time of defects and of annealed recoils in the lattice. recoil annealing,

In terms of the current models

(5) for

defects are emptied out of traps in the

crystal matrix at temperatures corresponding to their activation energies.

In this process recoil annealing is triggered and the

isomeric transition recoils present in the lattice at that time anneal to a bromate yielding species.

The de-trapping process

is a rapid one and is complete in a short period of time.

For

longer times of heating few further defects are liberated and thus few recoils are annealed.

But at the same time the S°Br

species are decaying away and are being replaced by other 8°Br atoms produced in the isomeric

transition.

of 8°Br atoms is only 25 min.

At the end of a one hour anneal,

therefore,

The mean life-time

very few of those recoils which were initially

annealed remain and nearly the whole 8°Br population is now made up of recoils produced after

the initial defect de-trapping

process. Implicit in this explanation is the assumption that inherent crystal defects are operative in promoting annealing. This is consistent with the previous observation that inherent defects appear to be predominant in annealing

(n,¥) recoils and

that such defects can be very rapidly removed from the lattice by heating before the neutron activation

(I).

The above theory

also explains why Campbell and Harbottle failed to observe a p p r e c i a b l e annealing.

Y*l. 3, NO. 9

ANNEALING OF ISOMERIC TRANSITION RECOILS

Perhaps recoil a n n e a l i n g increasing

the most occurs

so rapidly

a total concomittant

y-radiation

on irradiation

y-dose

that recoil a n n e a l i n g

of ~

transition

case,

to yield

recoil a n n e a l i n g

It should be observed during

the trapped daughter

defects

isotope

Isomeric further

is aware

reaction,

are short and where

transition

This

suggests of

of such radiation,

the annealing

reaction

in retention.

the phenomenon

in any system where

is comparable

This is a

by very low doses

has not p r e v i o u s l y

the annealing

the retention

(n,y) recoils

(1).

large changes

As far as this author

is that

using a source with

5,000 rads

is suppressed

very rapidly

produced

for

and that in the complete absence

as in the isomeric

transient

in this system,

than that observed

in sodium bromate

proceeds

observation

from 28.5% to 75.5% in 5 min. at 245°C.

much more rapid process produced

significant

367

of

been reported.

the recoils are being

where

the life-times

the half-life

of

of the

with the time of annealing. effects

in bromates are being

investigated. References

i.

I.G.

Campbell and C°H.W.

Acta.

(June

Jones,

I.G.

Campbell,

3.

G. Harbottle,

J.A.C.S. 82,

4.

C.H.W.

Jones,

Ph.D. Thesis,

(Sept.

1965).

See K.E.

to Radiochim.

1967).

2.

5.

submitted

J. Chim.

Collinsj

p. 421, I.A.E.A.

Proc.

Vienna

Phys.

56, 665

805

(1960).

University

Symp.

(1959).

of Manchester,

Chem. Eff. Nucl.

(1964).

Engl.,

Tr. Vol.

I,