The effects of neutron irradiation in the NRX reactor on the order-disorder alloy Cu3Au

THE EFFECTS OF NEUTRON IRRADIATION IN THE NRX ON THE ORDER-DISORDER ALLOY CusAu*

REACTOR

L. G. COOK and R. L. CUSHINGf The irradiation of Cu3.Au with neutrons causes two separable effects. Disordering is caused by fast neutrons, and is negligible with thermal neutrons; the effect is independent of temperature at least up to 100°C. \‘olume disordering rates of 28.9 per cent/day have been observed with fission spectrum neutrons. A secondary effect, probablv ordering, is caused by thermal neutrons: the effect is dependent on temperature even below 100°C. The simultaneous formation of substantial amounts of mercury is nrobahlv the direct cause of the effect. hRX ~iieactor spectrum” neutron irradiation produces both effects which mutually interfere. Provided the thermal component of the neutron spectrum is screened out, it seems likely that the disordering effect on CusAu can be used as a measure of neutron damage potential to other materials. LES EFFETS DE L’IRRADIATION SRX SUK LA TRANSFORMATION

AVEC DES NEUTRONS DANS LE REACTEUR ORDRE-DESORDRE D-An-S L’ALLIAGE CLQ;\LI

L’irradiation de Cu3.Au avec des neutrons cause deux effets s¶bles. Le d&o&e est cause par des neutrons rapides, mais est n&gligeable avec des neutrons thermiques; cet effet est indepcndant de la temperature, en tout cas jusqu’a 100°C. Des vitesses de mise en desordre de 28,9 pour cent par jour, en volume, ont Pte observees avec des neutrons du spectre de fission. Un efIet secondaire, probablement une mise en ordre, est cause par des neutrons thermiques; cet effet depend de la temperature, mCme en dessous de 100°C. La formation simultanee de quantites appreciables de mercure est probablement la cause directe de cet effet. L’irradiation avec des neutrons du “spectre du rCacteur” NRX produit tous les deux effets, qui S’entremClent. I1 parait que, pourvu qu’il soit possible de rendre mop&ante la composante thermique du spectre des neutrons, l’effet de mise en desordre sur Cu3Au puisse &tre utilise comme une mesure du potentiel de destruction des neutrons sur d’autres matieres. DIE

\fIRKUNG

VON

NEUTRONENBESTRAHLUNG DEN ORDNUNGSGRAD

IM CuaAu

NRX

“REACTOR”

AUF

Die Bestrahlung von Cu3Au mit Neutronen ruft zwei verschiedene Effekte hervor. Schnelle Neutronen rufen eine \‘erminderung. der Ordnung hervor, wahrend dieser Effekt bei thermischen Neutronen zu vernachl~ssrgen 1st. Drese Effekt ist, zum mindesten bis zu lOO”C, unabhangig von fission spectrum” Neutronen wurden Ordnungsvermindcrungsgrade \.on der Temperatur. Mit 28.9 prozent/Tag beobachtet. Ein sekundarer Effekt, wahrscheinlich ein Ordnen, wird von thermischen Neutronen hervorgerufen. Selbst unter 100°C hangt dieser Effekt von der Temperatur ah. Die gleichzeitige Bildung von einer nicht unerheblichen Menge Quecksilbers ist wahrscheinlich eine direkte Ursache dieses Effekts. Die NRX “reactor spectrum” Neutronen rufen beide Effekte hervor, die sich gegenseitig beein flussen. Unter der Voraussetzung, dass der thermische Anteil. des Neutronenspectrums ausfiltriert wird, erscheint es mijglich, die \-erminderung des Ordnungsgrades des Cu3A11 zur >lessung des Sch:~digungspotential der Neutronen fur andere Substanzen ZLI benutzen.

Introduction When crystals are irradiated with energetic heavy particles, radiation damage may occur in three ways. Atoms may interchange positions on lattice sites more rapidly than usual; they may be removed entirely from lattice sites and located interstitially; or they may be removed from lattice sites and recombine in a different way entirely. In this paper the first type of damage is considered -the magnitude and mechanism of the interchange of atoms on lattice sites when a solid is irradiated with neutrons. An order-disorder alloy seemed the most promising type of system to study; large changes in observable properties occur when the atoms interchange sites on the lattice, since by

*Received April 2, 1953. tchemistry Branch, Atomic Chalk River, Ontario, Canada. ACTA

METALLURGICA,

Energy

VOL.

of Canada

1, SEPT.

1953

Limited,

this process an initially ordered alloy becomes disordered, or a disordered alloy becomes ordered. CusAu was selected for the first study because: (1) its thermal behaviour and the mechanism of its transition have been thoroughly studied; (2) the “freezing in” temperature is above 2OO”C, well above NKX reactor ambient temperature; (3) chanies in the unit cell dimensions due to the orderdisorder transition are small; (4) the alloy is readily workable, and obtainable in fine wire. Following the work of Sykes and collaborators [t ; 21 it was decided to observe: (i) changes in the electric resistance, since these could be measured during irradiation in the reactor; (ii) changes in the X-ray diffraction pattern, since the behaviour of the superlattice lines would confirm the interpretation of the electric resistance changes; (iii) changes in the specific heat and in subsequent specific heat-temperature curves, since these detect minute and early mechanisms of ordering which leave the

540

ACTA

METALLURGICA,

electric resistance and super-lattice lines relatively unchanged. This paper presents the results of observations on electric resistance and super-lattice lines only, since these give a consistent picture without the support of specific heat data. During the progress of these experiments, results of somewhat similar experiments carried out at Harwell (U.K.) [3] and Oak Ridge (U.S.A.) [4; 5; 61 have appeared. Most of these appear to refer neutron irradiations, the to “reactor spectrum” effects of thermal and fast neutron irradiation not being separated, nor the effects of temperature studied. These results are referred to and their probable connection with the present results explained.

Experimental

Techniques

Cu&u wire, 49.1 per cent Cu, 50.9 per cent Au by weight, 0.015 in. dia., was supplied by Johnson, Matthey and,Mallory in the cold drawn condition. All samples were annealed at 800°C in vacua, or in argon, for 30 min. to remove‘the major effects of the cold drawing operation. Electric resistance measurements were made by passing a measured current of approximately 50 ma. through a section of wire and measuring the potential drop between two Cu&u taps spot welded, or in some instances hard soldered, to the test wire 10 cm. z!= 1 mm. apart. The current was measured by the potential drop across a standard resistor. Potential drops were measured with Rubicon precision potentiometers, or with Leeds and Northrup recorders and microvolt amplifiers Speedomax using precision potentiometers to provide a backing potential. In this way small variations in resistancesuch as that due to temperature rise on reactor start up-could be amplified and observed. A laboratory vacuum furnace assembly enabled resistance-temperature curves to be observed for comparison with those of Sykes [2]. In the reactor a special assembly was used, illustrated schematically in Figure 1. The sample assembly could be irradiated with or without the Cd liner or the U cylinder, so that the effects of fission spectrum, reactor epithermal, and thermal neutrons respectively could be separated. Adjustment of the cooling air permitted steady controlled temperatures to be maintained between 25°C and 110°C. Temperature checks were made during the initial reactor start up, using the measured immediate rise in electric resistance and the known initial thermal coefficients to calculate the temperature rise. Further, whenever a shut down

\‘OL.

1,

1953 SPEC!MEN WIRE B LEADS .~ .~. _(_ ._~. _

j

I

CO;;"" TEFLON CERAMIC

PLUG

INSULATING

\ TUBlNG

\

’ NATURAL ” CYLINDER $ THfCK I 8” LONG

lj,? ? ‘\ ‘\/

00 136” ” WATER COOLED ON BOTH SlOES ‘\ALUMINUM

“\\0025”Cd

FIGURE 1. reactor.

Irradiation

assembly

SAMPLE

,NSERT

SHEET IWHEN USED)

as used in the NRX

of the reactor occurred, a new thermal coefficient was obtained (by assuming the temperature rise to be unchanged). Thus not only could the change in electric resistance be observed, but also the concomitant change in the thermal coefficient of electric resistance. X-ray diffraction Debye-Scherrer pictures were taken on short wire samples with a General Electric XRD unit, before neutron irradiation and again two months afterwards. It was found that very little -y-ray background was developed on the films two months after irradiation, when small samples were used. These X-ray samples were mounted in the reactor in an open holder so that the cooling air could contact the samples themselves, as in the electric resistance experiments. No other direct control of temperature was carried out. It was assumed that the temperatures during neutron irradiation would be close to those of the electric resistance specimens which had similar exposed surface air cooling. The heat treatment programs used were: (a) To prepare disordered samples, air quench from 500°C. (b) To prepare ordered samples, 6 hours at 350°C 16 hours at 340°C 4 hours at 330°C 4 hours at 320°C 16 hours at 300°C furnace cooled to room temperature.

Results I. Resistance- Temperature Curves for Cu ~Au Wire Samples of CU&W wire in the ordered and disordered states were heated at a rate of 1.75”C/min., and the electric resistance measured (Figure 2, A and B). A sample cooled slowly from 5OO”C, down curve 3, was air quenched at temperature T, and the resistance-temperature curve taken (curve

0.25

COOK

i I-

AND GUSHING:

ORDER-DISORDER

BEFORE IRRADIATION AFTER IRRADIATION HEATING CURVE AFTER IRRADIATION COOLING CURVE HEATING RATE - l.75eC/mir

I

541

Cuo.L\u

observations confirmed this: first throttling the cooling air increased the resistance; second the difference in resistance increase for ordered and disordered specimens was exactly the expected ratio of the thermal coefficients as measured from Figure 2. The ice-point resistance of each specimen was determined prior to irradiation, and hence the actual temperature of the sample could be determined

1

100

IN

I

300 200 TEMPERATURE

FIGURE 2. Electric resistance irradiated and irradiated samples.

I

400 ‘C

I

500

versus temperature:

1 I.

un-

C). It is evident that the initial rate of relaxation at a temperature of, say 25O”C, is much more rapid in a partially disordered sample (curve C) than in a fully disordered sample (curve A). This effect was found by Sykes [2] and attributed to the fact that in the partially disordered sample there are large “in phase” ordered nuclei which can grow easily, whereas in the fully disordered sample there are no nuclei initially--even when they do form they are “out of phase” and growth is hindered. As a final check the initial rate of change of resistance at fixed temperature was determined as follows : Relaxation rate Temperature (70 of resistance/day) --fully disordered 210°C
I

I

I

I

I

!,

II

I

I

I

I

FIGURE 3.

Electric resistance change during reactor start up due to temperature rise. under operating conditions immediately actor start up. With the arrangements 25-35°C was the usual operating range, controlled temperatures up to 110°C used.

after redescribed, although were also

I II. Irradiation in Uranium with Cadmium Shielding (Fission Spectrum Neutrons) The general behaviour of the electric resistance of ordered and disordered CL&U when irradiated in uranium with cadmium shielding is shown in Figure 4. The resistance of the ordered sample (curve A) rose asymptotically to that of the disordered sample; the resistance of the disordered sample (curve B) rose very little, about 0.5 per cent. X-ray diffraction pictures taken before and after irradiation are shown in Plate I. The disordered alloy remained disordered. The ordered alloy became disordered, shown by the disappearance of the superlattice lines. As a further check, an ordered sample was irradiated until its resistance (relative to the ordered and disordered values) reached point 2, Figure 2. Then the sample was removed to the

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The fast neutron component normally present in the NRX reactor neutron spectrum disorders ordered Cu:&u, but has little effect on disordered Cup~u.

Irradiation with NRX Reactor Spectrum Neutrons

V.

0

IO

-

-m‘-

IRRADIAT1ON

30 PERIOD-DA=

40

50

FIGURE 4. Fission spectrum neutron irradiation with Cd shield: temperature 35°C (identical curves obtained at 106°C).

The general behaviour of the electric resistance of ordered and disordered Cu3_lu in KRX reactor spectrum neutrons is shown in Figure 6. Since in this example the epithermal (Figure 5) plus thermal neutron components are present, one must look for differences between the curves of Figure 5 and those of Figure 6, and attribute them to the thermal neutron component. The difference is that the initial dips in resistance continue over a longer period

and an electric resistance-temperature laboratoo curve taken at *1.7”Clmin. (curve C’ and B’). It is evident that the course of these curves is very close to the course of the curves C and B, and that the sample is in essentially the same state at 2 whether prepared by quenching from a temperature T, or by irradiation of a fully ordered specimen with fast neutrons. Thus, fission spectrum neutrons disorder ordered Cu3Au but have little effect on disordered CuaAu. IV. Irradiation with NRX “Reactor Epithermal” Neutrons (“Reactor Spectrum” Neutrons with Cadmium Shielding) The general behaviour of the electric resistance of ordered and disordered CusAu in NRX reactor spectrum neutrons with cadmium shielding is shown in Figure 5, curves A and B. The general effects are qualitatively identical to those with fast neutrons, but quantitatively much less pronounced. A slight initial dip in resistance is apparent in both curves.

2

4

IRF?ADIAT&

Irradiation with “reactor Cd shield: temperature 28°C.

FIGURE 6.

without

~ERl?D

I

12

- DAYS

14

spectrum”

and become more pronounced before process takes over; then the curves behave like those of Figure 5. The thermal neutron component spectrum has a resistance-lowering ordered and disordered CURALL

16

IS

neutrons

the disordering flatten out and of the reactor effect on both

Temperatures

An irradiation in uranium with cadmium shielding (fission spectrum) was done at 106°C. The behaviour of the electric resistance was identical with the behaviour at 28°C (Figure 4) and a separate graph is therefore not shown. An irradiation with “reactor spectrum” neutrons (no cadmium shielding) at 111°C (Figure 7) gave the same general effects as at 28°C (Figure 6), but quantitatively more pronounced and quicker.

0’

0.0900

% * 0.0600 $ t: 0.0700 az 0 E0.0600 z w ii 0.0600

0

I

0

VI. Irradiation at Diferent

~0.l000 r

oDc?a-----

2

4

14 IRRA6DIATIOBN PEkqOO-J2DAY9

FIGURE 5. Irradiation with “reactor with Cd shield: temperature 44°C.

spectrum”

16

I6

neutrons

VII.

Comparison of Results in D
Irradiations have been done’ in three different positions in the NRX reactor and over a relatively

I~ISORDEREI~ BEFORE I RIlrll~IA’rIOK

I~ISOKDEIZED XF’I‘EK I Ii li.-\DI.\‘l‘lOX

COOK

.isu

GUSHING:

ORDER-DISORDER T,\BLE

____

__~_.__ _------

-------

In graphite

Nominal thermal neutron flux in that position

4

I

545

CUsATJ

___-

____

shield

__~_.

Position _________________~_-_.___-

-__-

R ____________________-__~_____--

A Position

IN

Inside caliandria

X lOI2 n/cm*/sec.

1.5

c

near its edge

Near centre

of reactor

6 X lOI3 n/cnil/sec.

X lot3 n/cm”/sec.

Type of uranium cylinder used

4” dia. O.D. i” wall

1.4” O.D. a” wall

1.4” 0 .D . $” wall

Initial rate of increase of electric resistance of ordered CL&I with irradiation time

l.O30/o/day

2.98%/day

11.7%/day

long period of time. Consequently a precise quantitative comparison of the effects with flux is not possible. However, some quantitative comparison is possible.

IRRADIATION

FIGURE 7.

without

PEROD-

DAYS

Irradiation with “reactor Cd shield: temperature 111°C.

spectrum”

neutrons

The three positions used and the initial rates of change of electric resistance of ordered CuPAu in them are listed in Table I. The initial rate of change of electric resistance of ordered CusAu increases with the nominal slow neutron flux.

Discussion

of Results

There are two distinct and opposed effects of neutron irradiation on CusAu alloy. First, a drop in electric resistance caused mainly by thermal neutrons; second, a rise in electric resistance and a progressive disordering caused by epithermal and fission spectrum neutrons. The first effect may indicate a limited ordering process. Since the so-called “ordered” alloy possesses

in fact a degree of order appropriate to 200°C at least, it is, at room temperature, not in real equilibrium. Hence it is tempting to interpret the resistance decrease observed as an “ordering” of both the ordered and disordered states. However, since CuaAu rapidly becomes a mercury amalgam if irradiated by the slow neutron component in the NRX reactor spectrum (with 0.025” Cd shielding, the formation of Aulg8, and hence Hglg8, was found by activation analysis to be reduced to a little under 10 per cent of the unshielded value) such “ordering” must be attributed in the first instance to the thermal neutrons only through the influence of the mercury formed. Normally one expects an impurity to raise the electric resistance; the fact that introducing mercury produces a decrease in resistance in both alloy states tends to make one think that an ordering process must be taking place. For comparison, a table of mercury quantities is given (Table II). A thorough study of the effects of t-races of merTABLE

II

Per cent of gold converted to mercury Irradiation (assuming (r = 95b., F = 6 X 10’3 n/cmz/sec.) days __________________ _ _ _~_.____-__ 1

0.0050/,

3

0.037%

5

0.09%

10

0.26yn

20

0.66oj,

m’ith Cd shielding the quantities are an order of magnitude lower

546

ACTA

METALLURGICA,

cur-y on the thermal behaviour of Cu&u lay outside the scope and objective of this investigation. However, it is of interest that thermal neutron irradiation is probably the ideal method for introducing controlled amounts of mercury atomically dispersed in gold alloys for any such studies. Blewitt and Coltman [4] reported an increased relaxation rate of disordered CusAu at 200°C when irradiated in the Oak Ridge graphite reactor. Attributing this to the fast neutron component of the flux, they drew conclusions about the formation of Frenkel defects by fast neutron irradiation. It now appears, however, that relaxation of this type is caused by thermal neutrons. Unless screened out (which Blewitt and Coltman did not mention), one would expect a thermal flux to be present approximately equal to the fast flux. If we assume that a thermal flux of m10r2 n/cm2/sec. was associated with the fast flux of 1 X 1012 n/cm2/sec., the relaxation effect observed by Blewitt and Coltman could be accounted for by the thermal neutron effect alone. Adams and Dugdale [3], and Glick et al. [5] have reported initial decreases in electric resistivity, followed later by a rise, in “reactor spectrum” neutron irradiations. It is seen from the present work that the use of “reactor spectrum” neutrons produces just this mixture of thermal and epithermal neutron effects. In fact, the only work reported so far in which the thermal neutron component appears to have been negligible was that of Siegel [6]. Since Siegel did not mention thermal neutron screening, it is to be assumed that his samples were irradiated very close to uranium fuel elements, or perhaps actually in a fuel element. Siegel pointed out that the fast neutron disordering effect seemed larger than was to be expected on the basis of the mechanism proposed by Seitz [7], and suggested that a “thermal spike” mechanism (suggested earlier in another connection by Dessauer [S]) might be useful in explaining the effect. LarkHorowitz [9] has pointed out that a neutron collision which gave lo5 e.v. to a recoiling germanium atom (mass -72) would provide enough energy to instantaneously melt a region 1OP cm. in radius, and that in *lo-r2 sec. the region is quenched. On this picture one would visualise small volumes here and there throughout the ordered alloy suddenly converted to complete disorder. This picture can be quantitatively applied to the results of the present investigation. Figure 8 illustrates the formation of disordered islands in the ordered matrix, and the fact that as

VOL.

1,

1953

RATE OF FORMATION

OF DISORDERED

OVERLAP

FIGURE8.

= a/UNIT

TIME

?
--;f$ig .‘$6LUME

Vt

VOLUME

IS PROPORTIONAL

Illustration

TO Vt

of temperature

spike theory.

disordering proceeds by this mechanism, the islands will overlap; in fact, the overlap will be, on the average, proportional to the fraction of total volume already disordered. %

(I)

= (1 -

or VZ=

V,)r

1-ee7L

where Vt is the fractional volume disordered at time t, and y is the total fractional volume (in islands) produced per unit time. This is clearly the right shape of curve to reproduce Figure 4. One must make some assumption connecting V, and C, (the conductivity of the alloy). Landauer [lo] has considered the problem of the conductivity of a body with spherical islands of another homogeneous material imbedded in it. Using Landauer’s notation: if xi is the fraction of material of conductivity ur and resistance pi and x2 is the fraction of material of conductivity g2 and resistance p2 then, (a) pm = xlpl +

x2p2

(b) c, = x1fl1 +

x2u2

(c) urn = ; { (3x2 +

[(@X2

-

1)

gives an upper limit for pm, gives an upper limit for (TV,

1) c2 + (3x1 ~2

(3x1 -

+

1)

Ul

1) ~1)~ + 8u1u21a}

is the Landauer equation. If one uses equation (b) to connect the fractional volume disordered with electrical conductivity, one has dx2

-mda

=

u2

-

at x2 = 0 u1

whereas with (c),

The factor u2 + _____

2Ul

30 is easily calculated from Figure 9 to be 0.85. The procedure adopted in this paper has been to use the simple relation (b) throughout, since it

COOK

enables an experimental

4ND

GUSHING:

excellent fit to be obtained results and enables

with

ORDER-DISORDER

CdV,

conductivity of sample at time t, conductivity in the ordered state, conductivity in the disordered state, fractional volume of ordered material at time t, V,, = fractional volume of disordered material at time t. Combining equations (1) and (2) :

where

(3)

C, Co C, Ir,,

= = = =

C, = CO-!- AC{1

where AC = C, are fixed; a further

-

exp[

- &($$)t,_,l)

Co. In this equation measurement of

Co and

Cd

dC, ( at ) I==0 or else a normalising at some point C’ 2 on the experimental curve fixes the equation completely. To test this equation, curve A, Figure 4, was replotted to conductivity-time co-ordinates (Figure 9). Clearly AC = C, - Co = 11.46 - 20.82 = - 9.36 mhos. The equation was normalised at the 40-day point requiring

IRRADIATION PERIOD-DAYS

FIGURI;. 9. Quantitative test of theoretical ductivity curve (Fig. 4 replotted).

547

= 0.655 mhos,,‘da>-,

to be obtained accurately and easily. It also provides an accurate relative comparison of the fractional volumes disordered in different experiments. One could, if one wished, reduce all the final results by the Landauer factor (0.85) to obtain the exact fractional volumes disordered. However, this has not been done because it is very doubtful if the interpretation is really valid to this absolute accuracy. Using our notation,

ct = cove+

Cu,Xu

the

and other

(2)

IN

equation:

con-

points t days 0 1 10 20 30 40 50

calculated

as follows:

C mhos 20.8 20.2 16.1 13.77 12.6 12 04 (normalisation 11.75

point)

These points are plotted as circles in Figure 9, along with the calculated initial slope. The equation reproduces the experimental curve within experimental error over the whole course of the experiment.* Implications of this equation are: 1. The rate of “spiking” was constant throughout the run and equal to the initial rate of disordering which equals = 5.930J,/day. 2. Conductivity (at fixed temperature) is linearly proportional to the voIume disordered. 3. The “spiking” rate is linearly proportional to the flux. These implications will be discussed in detail one by one. Using implication 1, the initial disordering rates can be calculated from the initial rates of change of resistance (Table I) and are listed in Table III. Thus one concludes that the fast neutron damage in Position C amounts to the breaking of 28.9 per cent of the nearest neighbour “bonds” per day. The implication 2 deserves further comment. In view of the fact that irradiation disordering produces a structure behaving identically with one produced by a quench from an appropriate temperature (see Figure 2), one must conclude, with Sykes, that this material, too, contains “in phase” nuclei which can readily grow at temperatures of 220”-250°C. This is, of course, true. The “nuclei” in the present example consist of the ordered walls of the “spiked” islands. The ordered structure can grow inward from the walls easily, being in phase at all points, since the original ordered mass was prepared by slow cooling and was “in phase” throughout. *The slight increase in electric resistance in curve A, Figure 4, must be interpreted as indicating increased disorder. It. is unlikely that the thermal quench used in preparing the disordered specimens was perfect.

ACT:\

5-18 T:ZBLE

Position

Nominal slow neutron flux (n/cm’,/sec.) 4

x 10’2

1.5 x 10’3 C Position C without U cylinder, with Cd shielding

6

x 10’3

6 x 10’3

III

=A. 2.06

Rate (70)

~[g$,1

2.00

1 different

reactor

posi-

1 spectrum)

neutrons

Reactor flux, epithermal component

*If one uses, instead of the graphically estimated initial rate of change of resistance, the calculated initial rate of change of resistance using formula (3) fitted to the curve (Figure 9), 7.00 per cent is obtained. In general the rates calculated from the fitted equations are a little higher than one would estimate graphically.

The implication 3 that the initial disordering rates are linearly proportional to the nominal slow neutron flux, while consistent and reasonable, can not be considered strictly proved by the results as the neutron fluxes themselves are not well enough known. Moreover, according to the present picture, the initial disordering rate should, in fact, not be strictly proportional to the neutron flux, but rather to the energy transmitted by neutron collisions to the recoiling atoms. Rather than try to relate the effects observed too closely to other equally involved flux measurements, it seems best to concentrate experimentally on whether neutron flux as measured by the disordering effect on CusAu bears a direct relation to irradiation damage in other systems; it seems likely that the disordering effect of neutron irradiation on CusAu will be a useful measure of potential damage to other materials. It is of interest to estimate the average volume of a disordered island. If we put, approximately, c = 5 barns (u = average total scattering section for fast neutrons in CuaAu),

cross-

F = 1.5 X lOI n/cm2/sec., the probability

1,

1953

(5.4 x 10-6, ‘day. Since the experimental probability of an atom being disordered at this flux is 0.059,‘day (implication I), a single temperature spike must contain ~10~ atoms.

Conclusions

63* i_tions with uranium cy1linders- fast (fission

28, 9

\.C)L. matel~

Initial disordering

6,

METALLURGICA,

of an atom being struck is approxi-

The following conclusions may be drawn: 1. Ordered CusAu is rapidly disordered in a t” wall uranium cylinder in the iYRX reactor. The disordered and partially disordered states are indistinguishable, to a first approximation, from states produced thermally. 2. Disordered CusAu is little affected when irradiated in a uranium cylinder, but the electric resistance drops in a thermal neutron flux. The latter effect is probably due to the formation of mercury as a third component, and consequent changing of the ordering kinetics of the alloy. 3. The neutron irradiation disordering characteristics of Cu&u enable it to be used as a type of reactor flux monitor-particularly for epithermal and fast neutrons-or for thermal neutrons if used with a uranium cylinder neutron converter. Its value as a flux monitor probably will be greatest in neutron irradiation damage studies.

Acknowledgment The continued assistance and co-operation of the Engineering and Operating Branches of Atomic Energy of Canada Limited were essential in building and using the irradiation facilities needed for this work.

References 1. SYKES, C. and EVANS, H. J. Inst. Metals, 58 (1936) 255. 2. SYKES, C. and JONES, F. W. J. Inst. Metals, 59 (1936) 257. 3. ADAMS, J. and DUGDALE, R. *q. Nature, 168 (1951) 582. 4. BLEXITT, T. H. and COLTMAN, R. R. Phys. Rev., 85 (1952) 582. 5. GLICK, H. L., BROOKS, F. C., WITZIG, W. F., andJoHNsoN, W. E. AECD-3332 (U.S. report), 1952. 6. SIEGEL, S. Phys. Rev., 75 (1949) 1823. 7. SEITZ, F. Discussions of the Faraday Sot., 5 (1949) 271. 8. DESSAUER, F. Z. Phys., 12 (1923) 38. 9. LARK-HOROWITZ, K. Semi-Conducting lo.

Materials

don, Butterworth, 1951), p. 55. LANDAUER, R. J. AppI. Phys., 23 (1952) 779.

(Lon-