An ephemeral Frenkel pair in iridium

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An ephemeral Frenkel pair in iridium

M.-H. Gély, A. Dunlop, Y. Quéré

To cite this version:

M.-H. Gély, A. Dunlop, Y. Quéré. An ephemeral Frenkel pair in iridium. Journal de Physique Lettres,

Edp sciences, 1983, 44 (6), pp.219-224. �10.1051/jphyslet:01983004406021900�. �jpa-00232183�

L-219

An

_ephemeral

Frenkel

_pair

in iridium

_(*)

M.-H.

_Gély,

A.

_Dunlop

and Y.

_Quéré

Section d’Etude des Solides Irradiés, C.E.N., B.P. 6, 92260

_{Fontenay-aux-Roses,}

France

(Reçu le 9 novembre 1982, _accepte le 28 _{janvier 1983)}

Résumé. 2014 _Après activation sous flux de neutrons, de l’iridium est maintenu à basse

_température

(22

K). La résistivité

_électrique

décroît ou croit au cours du _temps suivant que l’activation a été faible ou forte. Une

_{interprétation}

cohérente est donnée

_{qui implique}

la création, par recul

radio-actif, d’une

_paire

de Frenkel durant _{l’activation,} et son annihilation ultérieure _(avec une

_probabilité

égale

à ~ 0,8) par un second recul sous seuil. Abstract. 2014 After neutron

activation, iridium

_samples

have been

_kept

at low temperature (22 K). The electrical

_resistivity

decreases or increases with time

_according

to whether the activation was

mode-rate or

_high.

A coherent

interpretation

is

given implying

the recoil-induced creation of a Frenkel

pair during

activation and its

_subsequent

annihilation, with a

_{probability ~}

_0.8, due to a second

subthreshold recoil.

LE

JOURNAL

DE

_{PHYSIQUE - LETTRES}

J.

_Physique

- LETTRES 44

(1983) L- 219 - L- 224 15 MARS 1983,

Classification

Physics

Abstracts 23.00 - 61.80

During

a reactor neutron irradiation of a

_solid,

one is

_usually

interested in the effects due to

the elastic collisions of the neutrons with the nuclei :

_production

of isolated or more or less

agglo-merated

_point

_defects,

of dislocation

_loops,

of disorder... We describe here another effect due to

the inelastic collisions

_of

the thermal neutrons with the

_{nuclei ;}

it consists in a _sequence of two

contradictory

events : first the creation of a Frenkel

_pair

due to a radioactive recoil of _energy

higher

than the threshold

_displacement

energy, then a

_rigourously

athermal recombination of

this same

_pair

due to another subthreshold recoil.

1.

_Experiment

and results. - For the

_experiment

iridium is irradiated at a low

_temperature

_To

in a nuclear reactor. After the irradiation the electrical

_resistivity

_p of the metal was measured at

the

_temperature

_To

as a function of time t.

We use an iridium wire

₍₁₅₀

_J.1m

diameter)

of nominal

_purity

99.9

_%.

It has been

_{recrystallised}

in a

good

vacuum

(5

x

10- 8

torr)

at 1 300 ~C

_(sample

heated

_by

Joule

effect).

Its

_resistivity

ratio

P3oo/P4

is 34 ± 1. The irradiation

_facility

is the VINKA _system located in the TRITON reactor

of

_{Fontenay-aux-Roses.}

This

_{facility [1, 2]}

allows us to maintain the

_samples

at constant

tempera-(*) La version française de cet article a été

_proposee

aux

_Comptes

Rendus de F Academic des Sciences.

L-220 JOURNAL DE

_{PHYSIQUE -}

LETTRES

tures around 20

_{K (temperature stability :}

0.02

_K),

the

_samples

_being

in or out of the neutron flux. The

_temperature

and the electrical

_{resistivity (sensitivity :}

10-4)

are measured

[3].

Two

_experiments

_(named

_(I)

and

_(II)

in the

_following)

have been

_successively

_performed

on two

different

_{samples. They mainly}

differ

_by

the fluences of the initial irradiation and

_by

the time

during

which the measurements out of the flux have been made later. The characteristics of the

two

_experiments

are

_quoted

in the

table,

in which a distinction is made between the thermal

(~

1/40

eV)

and the fast

_(~

1

_MeV)

neutrons.

Table. -

Characteristic numbers _describing the two _experiments.

One should note from this table that the fluence is

_{approximately}

10 times

_greater

for

_(I)

than for

_(II).

Our results can be summarized as follows :

1)

in both

_experiments

the

_resistivity

increases

_during

the

_irradiation,

which is a usual

pheno-menon due to the accumulation of defects. We shall not comment further on this

_point.

The

resistivity

increase

_Apo

at the end of the irradiation

is,

as is the

fluence,

about 10 times

_greater

for

_(I)

than for

_{(II) ;}

2)

then,

out of the

flux,

at a constant

_temperature

_{(To =}

22

_K),

the

_resistivity

_p

_changes

as a

function of time : _{p increases} for

_(I)

and decreases for

_(II)

_(see

the

_I1Pexp.(t)

curves on

Fig.

1).

We shall discuss and

_interpret

this result

2)

and the

_apparent

contradiction.

Fig.

1. - Iridium

samples

have been

_initially

irradiated with reactor neutrons. Their

resistivity

is then

measured at 22 K out of the neutron flux as a function of time

_during

two

_experiments

noted

(I) (Fig. a)

and

_{(II) (Fig. b) :}

2.

_{Interpretation.}

-

2. 1 PRODUCTION OF PLATINUM. - Iridium consists of

38.5 %

191Ir

and 61.5

_{% 1931r.}

In the presence of slow

(and especially thermal)

neutrons it

_undergoes

a series of

Fig.

2. - Activation

diagram

of iridium :

Ti 1 = 1 000 barns, (J 2 = 700 barns, ~3 ~ 130 barns, (J 4 = 90 barns. t, = 74

days,

t2 =12

_days,

_{t3 ~ 20 hrs .}

We see

_immediately

that these reactions lead to the creation

_{of platinum,}

either

_during

or after

the irradiation. The various

_periods

and cross-sections

_being

known

_(see

_[4]),

one can

_easily

calcu-late the atomic concentration of

_platinum

which is created as a function of time :

Cp~).

This can

be done either

_during

the irradiation or

_during

the isothermal measurements after irradiation. The concentration of

_platinum

is small

_(for

_example

_cp, ~ 4.7 x

10-4

at. at the

_beginning

of the

isotherm

_following

irradiation

_(I))

but is easy to reveal

_{through resistivity}

measurements. In a

separate

experiment,

once the

_point

defects had been

_thermally

annihilated

_[4],

we have let that

platinum

accumulate

_during

a

long period (6 months)

and could thus determine

precisely

the

value of the

_{specific resistivity}

of substitutional

_platinum

in iridium :

Using

this number and the calculated

_Cp~),

one can then determine how

platinum

contributes

~4 p~,(t))

to the isotherms measured out of the neutron flux. This contribution is

_quoted

on

figures

1 a and 1 b as

well

as the

_{measured Ap,,,,P. (t).}

,

2.2 RECOMBINATIONS. - One

sees

_immediately

from

_figure

1 that the creation of

_platinum

accounts

_{poorly (Exp. I)}

or

quite badly (Exp. II)

for the measured

_I1Pexp.(t).

In both cases,

_I1Pexp.

is smaller than

_Appt.

The

_{complementary...contribution I1Prec.,}

such that

_(I1Prec.

+

_{I1PPt) is}

_equal

to the measured

_Ap~p,

can be deduced as

_{Ap~ ==}

_{Ap~p 2013}

_Appt.

It is

negative

and is shown on

figures

1 a and 1 b.

Taking

into account the various

_{species ’existing}

in the

_{samples : point}

defects

_{(and especially}

isolated Frenkel

_pairs)

and

_platinum

atoms, we

_postulate

that this

_resistivity

decrease

_ð.Prec.

can

_only

be attributed to a recombination of Frenkel

_pairs,

this recombination

_being

induced

_by

one of the nuclear reactions. Such recombinations have been

_previously

observed

_by

Coltman

et al.

_{[ 10].}

What is known of Frenkel

_pairs

in iridium has been determined

_through

4 K electron irra-diations. It can be summarized as follows :

i)

the threshold for atomic

_displacement

is

_Td

=

(46

±

2)

eV

_[5];

ii)

the

_resistivity

of a concentration c of Frenkel

_pairs

is

₍₆₇₀

_±

_{50) c~ .}

cm

_{[5] ;}

iii)

there is

_rigorously

no thermal recombination of an interstitial and a _{vacancy below} a

temperature

of 35 K

[5].

L-222 JOURNAL DE PHYSIQUE - LETTRES

Let us now consider the evolution of

_I1Prec..

It varies

_{exponentially}

as :

with the

_{following quantities :}

a)

the half life! is

_equal

to 23 hrs for

_(I)

and

_{(II) ;}

b)

the saturation

_{resistivity 0p ~}

is

_equal

to 3.2 x

10

_gfl.

cm for

_(I)

and to 3.7 x

10

_gil.

cm

for

_(II).

Point

_a)

is

_strongly

in favour of a same mechanism for recombination in

_(I)

and

_(II).

Point

_b)

is more

_{striking :}

the total

resistivity I1poo

of the defects which are

likely

to recombine is of the same order for both

experiments

in which however the fluences differ

by

a factor of ~ 10.

We are thus

_naturally

inclined to see

_if,

_{among all the different}

_isotopes

created

_during

the

irra-diation,

there is one which would attain its

_{quasi-equilibrium sufficiently rapidly}

so as to exist with similar concentrations at the end of

_(I)

and at the end of

(II).

The calculation of the

concen-trations

_{(see [4])}

shows that it occurs for

194Ir :

at the end of irradiations

_(I)

and

_(II)

one finds

respectively

c4~

= 3.4 x

10 - 5

and

C4I~

= 1.4 x

10 - 5

for

194Ir.

Moreover the half life _{t 3} of this

isotope (see Fig.

2)

is known with poor accuracy

(17.4

t3 20.7 hrs

_[6

to

_8]),

but is close to the

period of ( 1 )

(see a)

above).

2.3 MECHANISM OF CREATION-RECOMBINATION. - We are now

_naturally

led to the

_following

scheme :

The

_isotope

194Ir

is created

_by

activation

_during

the irradiation

_through

the reaction

193 Ir - 19 4 Ir.

The iridium activation y

spectrum

is shown on

_figure

3. From this

_spectrum

and

taking

into account

_{[11] the}

fact that _{y rays} are emitted in

cascades,

one can evaluate the number of Frenkel

_pairs

created

_by

efficient recoils

_(i.e.

of _energy

_higher

than the threshold _energy

Fig.

3. -

y spectrum

following

the _capture of thermal neutrons

_by

iridium, established from

_[9].

For details about determination of Frenkel

_pair

creation

_by

recoils, see

_[12].

Td =

46 eV from

_i)

above).

From this evaluation we conclude that when 100

194Ir

nuclei are

formed,

55 Frenkel

_pairs

are

_{simultaneously}

created. Each of these Frenkel

pairs,

named below

(I

-

’)41

consists of a _vacancy

(l)

located near

the 194Ir

atom and of an interstitial

_(i)

located outside

Fig. 4. -

Configuration of the

_ephemeral

_{(I - i)4} Frenkel pair in iridium. The activation 193 Ir -+ 194Ir

has

_produced

a _vacancy at the initial

_position

of the 19 3Ir atom, which has been

displaced

to a

_neighbouring

lattice site _(black circle is 194Ir _atom). A

_split

interstitial (see arrow) has been

_produced

out of the recombi-nation volume Y _{(continuous line)} of the vacancy. The

subsequent disintegration

of the 194Ir atom annihi-lates this Frenkel

_pair.

defects

_through

recoil

_phenomena

has been observed in

_quite

a few metals

[10].

At the end of the

irradiation,

we are thus left with a concentration

_{c’j 4}

= 0.55

c’4

of

(I

- i)4 pairs,

where

c’4

is the

concentration

of 194Ir

atoms

_U

=

(I) or

(II)).

During

the

_subsequent

out _{of flux measurements, the}

_temperature

is held constant

(To =

22

_K),

so that these interstitials will not

_undergo

_any thermal recombination

_(see

_iii)

above).

On the other

hand 194Ir

transforms

into 194pt

with a

half-life t3

(see

_Fig.

2).

The average

recoil energy

(~

18

eV)

is now

_inferior

to the threshold _{energy. This energy,}

_dissipated

in the volume

V,

is too small to create a new Frenkel

_pair

but can be sufficient to allow the recombination

of the

(I

- i)4

pair.

This

_ability

to induce the above recombination

_depends

on the exact recoil energy

E4

and on the direction of the

_impulse

of the recoil. When

_averaged

over all the

_possible

events it defines a recombination

probability

_p.

_{c’ 4 changes}

then

according

to :

which

_corresponds

to a recombination

_{resistivity :}

This evolution is that which we found

experimentally

(1),

the half-life T = 23 hrs

being

close _to

t 3

(-

20

_hrs).

The

_{knowledge of Apoo}

in

₍₁₎

and that of

_c4

in

₍₂₎

_{(see §}

_2.2)

leads to the

experi-mentally

measured

_{probabilities :}

The

_periods

_agree

_fairly

well

especially

when one

_keeps

in mind that the radioactive

_half-lives,

particularly

t3, are known with bad accuracy. The two

_experimental

values

_{for p}

differ

noti-ceably

(1).

( 1 )

Let us remark here that the above mechanism could also be considered when 19 2Ir

_{disintegrates}

(see

Fig.

2). This

_isotope

is indeed created in much

_larger

_quantities than 194Ir

_(C2I~

= 1.8 x 10 ~ 3 and

cgl)

= 1.7 x 10 - 4 at the end of irradiations (I) and _(II)

_{respectively).}

192Ir has a

_larger

radioactive half-life

(74 days) than 194Ir (- 20 _hrs). Moreover the

_{corresponding}

average recoil is only 3 eV which is much smaller than that of 194 Ir _{(~18 eV).} The above _{results prove that when the recoil is of}

_{only ~}

3 _eV, the

corresponding

recombination

_probability

p is

negligible.

The recoil

_{energy (~}

0.02 _eV)

_{corresponding}

to the 193mIr -. 193Ir reaction, is thus

_certainly

not sufficient

L-224 JOURNAL DE _{PHYSIQUE -} LETTRES

2.4 COMMENT UPON THE RECOMBINATION PROBABILITY _p. - The

_experimental

value

of p

is

higher

for the short

_(II)

than for the

_long

irradiation

_(I).

These differences in irradiation fluency

imply

that defect recombination under irradiation is much

_higher

at the end of

_(I)

than at the end of

(II).

At the end of

_(I)

the

_production

rate

_{Ap (= dA p/dt)}

is but 0.45 times its initial value. A _{great number} of the

_(I

_{- i)4 pairs}

which were formed no

_longer

exist at the end of this irradiation and thus cannot

_participate

in the recombinations described

_{in §}

2.3. This leads to an

under-estimation

_{of p.}

The correction is much smaller at the end of irradiation

_(II)

_{for which the}

pro-duction rate is still 0.83 times

_its

initial value.

The true value

_{of p, noted po,}

is that which would

correspond

to an

ideally

short irradiation.

When we

_{linearly extrapolate}

the

_experimental

values to zero

_resistivity

increase we obtain :

3. Conclusion. - We have shown here that the irradiation of iridium

_by

slow neutrons creates

an

_ephemeral

Frenkel

_pair,

which is created

_during

the

activation 193 Ir

-+ 194Ir

and

_destroyed

later

_(with

a

_probability

_po)

_by

subthreshold events

_occuring

when 194 Ir

transforms

into 194Pt.

After these two successive events we end up with a substitutional atom

of 194pt

in a

_perfect

lattice.

When the mean recoil _{energy is 18}

eV,

the recombination

_probability

_{po is}

_equal

to 0.8.

Acknowlegments.

- We thank very

sincerely

D. Lesueur for

_interesting

and

_helpful

discussions.

The French version of this paper has been submitted to «

_Comptes

_{Rendus de l’ Académie des}

Sciences ».

References

[1]

CONTE, R. R., Adv. _{Cryog. Eng.} 12 ₍₁₉₆₇₎ 673.

[2]

S.E.S.I., J. Mat. Nucl. 108, 109

₍₁₉₈₂₎

162.

[3]

BOUFFARD, S.,

Rapport

C.E.A.-R-5015 _(1979).

[4]

GELY, M. H., C.E.A.-N, in

_{print (1982).}

[5]

DUNLOP, A., GELY, M. H. _(to be

_published

in Radiat.

_Eff.).

[6]

PANNETIER, R., Vade-mecum du technicien

_(imprimerie

_Maisonneuve) 1966.

[7]

LEDERER, C. _{M., HOLLANDER,} J. M., PERLMAN, I., Table

_{of isotopes}

_(John

_Wiley

_ed.) 1978.

[8]

SEREN, L., FRIEDLANDER, H. _{N., TURKEL,} S. H., Phys. Rev. 72

₍₁₉₄₇₎

888.

[9] GROSHEV, L. V., DEMIDOV, A. M., LUTSENKO, V. N., PELEKHOV, V. I., Atlas

_of

_03B3-ray

_{Spectra from}

Radiative _Capture

_of

Thermal Neutrons _(Pergamon Press-New _York) 1959.

[10]

COLTMAN, R. R., KLABUNDE, C. _{E., REDMAN,} J. K., SOUTHERN, A. L., Rad.

_Eff.

16

₍₁₉₇₂₎

25.

[11]

WALKER, R. M., J. Nucl. Mater. 2

₍₁₉₆₀₎

147.