J. Plasma Fusion Res. SERIES. Vol. 5 (2002) 245-249

Annealing Behavior of Defect Structures in Ni lrradiatedby High Energy H and He lons
ISHIZAKI Toshitaka, XU Qiu, YOSHIIE Toshimasa and NAGATA Shinji'
Research Reactor Institute, Kyoto University, Osaka 590-0494, Japanllnstitutefor Materials Research, Tohoku University, Sendai 980-8577, Japan(Received: 1l December 2001 | Accepted: 20 June 2002)

Abstract
Isochronal annealing experiments for H and He ion irradiated Ni were carried out to study theinteractions between gas atoms and vacancy clusters. The accelerating energy of H and He ions was 1.0
MeV and 3.3 MeV, respectively. The total dose was 1.0 x 1017 H ions/cm2 (0.2 dpa) and 9.6 x 101s Heions/cmz (0.3 dpa), and the irradiation temperature was 300"C. In order to study the recovery behavior ofthe defect structures, positron annihilation lifetime and coincidence doppler broadening (CDB)
measurements were carried out after irradiation and annealing for I hour up to 950"C. The microvoids
were detected after the H and He ion irradiation. The characteristic difference mainly appeared in theannealing behavior of the defect structures. The results of the positron lifetime and CDB measurements
after annealing indicated that the recovery of the defect structures in Ni irradiated by H ions finishedcompletely at 600'C, while that in Ni irradiated by He ions finished at 950"C. In addition, the microvoids
formed by the He ion irradiation grew after annealing at 750'C. The stability of the vacancy clusters, Hvacanc)' complexes, and He-vacancy complexes is discussed from these results.

Keywords:
nickel, hydrogen, helium, ion irradiation, positron spectroscopy, microvoid, gas-vacancy complex

1. lntroduction

release [6-8] and cavity growth behaviors [9-10] of
implanted H and He in Ni have been performed.
However, the thermal stability of vacancy clusters by
the presence of H and He atoms has not been thoroughly

The effect of H and He atoms on the developmentof defect stnlctures is an important problem formaterials used in fusion reactors, because the production
rate of H and He atoms generated by (n,p) and (n,a)

studied. Above all, experimental studies on theand, in addition the materials used in the fusion reactors

microvoid formation in Ni irradiated by H and He ionswith high energy for imitating the fusion environment

are directly damaged by the irradiation of gas atoms

has scarcely been performed.

nuclear reactions is much higher than in fission reactors,

with acceleration energy at the MeV level. It is

In the present study, the annealing behaviors ofparticularly well known that these atoms promote thedefect structures in Ni irradiated by H and He ions weregrowth of cavities [-2] and dislocation loops [3-5]. The

investigated in order to study the thermal stability ofdefect structures which were induced by the irradiationof the H and He ions. After the irradiation experiments,
isochronal annealing experiments were performed. Togas atoms induce void swelling and radiation
embrittlement even at low solubility. In order to study
the diffusion and trapping of H and He atoms to defectclusters in pure metal, several studios on tho thormal

Corre sponding author's e -mail : ishizaki@ post3.rri.lcyoto-u. ac.jp245@2002 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchIshizaki T. et al., Annealing Behavior of Defect Structures in Ni Irradiated by High Energy H and He Ions

investigate the formation of vacancy clusters, positron

was about 220 psec. The lifetime was determined fromthe analyses of the annihilation spectra by the positronannihilation lifetime and coincidence doppler

resolution program. In order to measure the CDB
spectra, two Ge-detectors with almost the same energy

broadening (CDB) measurements were performed. The

positron annihilation technique is effective forresolution were used. The overall energy resolution ofthe two Ge-detectors was 1.6 keV, which was definedby a full-width at half maximum of the 661.6 keY y-ray

investigating microvoid formation [1]-13]. The lifetime
measurement was used for detecting the size of themicrovoids in metals. The CDB measurement, which
determines the longitudinal momentum distribution ofelectrons annihilated with positrons, was used to detect

generated by the 137Cs standard radiation source.

The isochronal annealing for Ni irradiated by the H
and He ions was carried out for t hour by a 50"C

the total amount of residual defects. The CDB

increment from 300oC. The positron annihilation
lifetime and CDB measurements were performed aftermeasurement is a remarkable technique, which has thegreat advantage of reducing disturbances from thebackground spectrum on the higher energy side of theannihilation peak (51I keV) [4-18]. The stability of thevacancy clusters, H-vacancy complexes, and He-

each annealing process.

3. Results
3.1 Positron lifetime measurements
Figures I and 2 show the results of the positron

vacancy complexes was discussed from the recovery
behavior of the defect structures after annealins.

annihilation lone lifetime and the mean lifetime for the2. Experimental ProceduresHigh-purity Ni of 99.997o supplied by Johnson &Matthey Chemicals Ltd. was employed for theexperiments. The Ni specimens were annealed in a highql,itr>:*i>!@vacuum below 1.0 x 10-4 Pa at 900"C. The irradiation
experiments were carried out by using an accelerator inthe Institute for Materials Research, Tohoku University.
The total dose was 1.0 x 1017 H ions/cm2 and 9.6 x l0rs
He ions/cm2, respectively. In order to align the damageq)

ciduo9and gas-atom range for both irradiations, theacceleration energy for the H and He ions was chosen tobe 1.0 MeV and 3.3 MeV, respectively. The damage andgas-atom distributions were calculated by the TRIM

*1Annealing temperature ("C)

code with a displacement energy of 24 eY for Ni. The
depth of damage and gas-atom peak was about 6 pm

Fig. 1 The isochronal annealing behavior of a long life-time component and the intensity in Ni irradiatedfrom the surface ofthe specimen, and the effective gascontaining zone was about I pm in the vicinity of thegas peak for both irradiations. The amounts of effective
damage and gas-concentration were estimated as theaverage of the damage and gas-atom distribution in theregion of a full-width at half maximum. In the case of
the H ion irradiation, the total damage was 0.2 dpa, theby H and He ions at 300'C.

Ni(H ion hadiation at 300t)----O-Ni(He ion iradiatior at 300t) ----a0)
avtc)

damage rate was 2.4 x l0-5 dpa./sec, and the amount of,q)

H concentration was about 2 at%o.In the case of the Heion irradiation, the total damage was 0.3 dpa, thedamage rate was 1.8 x 10-6 dpa./sec, and the amount ofHe concentration was about 0.3 atvo. The irradiationc!

Iotemperature was 300"C.
The system for the positron annihilation lifetime
measurement in this study was the conventional fast-fast

400

600

800

1000

Annealing temperature (T))
Fig. 2 The isochronal annealing behavior of a mean lifetime component in Ni irradiated by H and He ionsat 300"C.

circuit with two BaF2 scintillators, and a 22Na source ofpositrons was used. The time resolution of this system246

Ishizaki T. et al., Annealing Behavior of Defect Structures in Ni Irradiated by High Energy H and He Ions

Ni irradiated by H and H ions at 300oC as a function ofthe annealing temperature. The long lifetimeaNi(rFir{dialod)
Niol ion imildi--. _. _ atad. l00t)-

corresponds to the size of the vacancy clusters, its
intensity corresponds to the density of the vacancy
clusters, mainly microvoids, and the mean lifetime
corresponds to the total amount of defects. As can beseen from these results, the formation of microvoids wasobserved after the H and He ion irradiation. However,the difference between the H and He ion irradiation wasmainly marked in the annealing behavior of the long andmean lifetimes. In the case of the H ion irradiation. thelong lifetime component disappeared at 600'C, while inr)

N 0.

h0.0-15 -10-505

l015

P1 (10':l mc)

the case of the He ion irradiation the long lifetime
component diserppeared at 950'C. This suggests that therecovery of microvoids formed by the H ion irradiation
occurred completely at 600oC, whereas that by the HeFig. 3 Comparison of spectra from the CDB measure-ment between un-irradiated Ni and H ion irradiated Ni at 300"C.

ion irradiation occurred at 950"C. Another feature was aslight increase of the long lifetime after annealing above
750"C in the case of the He ion irradiation. This meansthat the vacancy clusters grew above this temperature.

Ni(H ion iradiation at 300'C) ----€Ni(He ion inadiation et 30011) ----a-€r.
ktra)

3.2 Goincidence doppler broadening (CDB)measurements

F r. 01
gcd

Figure 3 shows the CDB spectra of un-irradiated Ni

aand H ion irradiated Ni at 300"C (0.2 dpa). P1 is thelongitudinal m()mentum of an electron annihilated witha positron. The mc of the x-axis unit means themomentum, where m is the electron rest mass and c isthe light speed. The unit of the y-axis is the count ratio
for each momentum, which is normalized by totalcounts. A remarkable feature in this result is that thecount ratio at the low momentum region for the H ion
irradiated Ni was larger than that before irradiation.

0.

400

600

800

Annealing temperature (t)1000

Fig. 4 The isochronal annealing behavior of the Sparameter ratio in Ni irradiated by H and He ionsat 300"C.

specimen is plotted as a function of the annealing
temperature. Fig. 4 implies the same temperaturedependence as the positron lifetime measurement. TheS-parameter ratio for the H ion irradiation was less than
1.0 after annealing at 600"C, while that for the He ion
irradiation was less that 1.0 at 950oC. It was also clear
from the result of the CDB measurement that themicrovoids formed by the H ion inadiation decomposed
below 600"C, whereas those formed by the He ion
irradiation did not decompose until 950"C.

Positrons trapped at open-volume defects such asvacancies are rnainly annihilate with low-momentum
electrons, i.e., conduction and valence electrons F9l.
The increase of the count ratio at the low momentum
region in the CDB spectra signifies that open-volume
defects exist in the specimen. This low momentum
region is called the S-region, and the S-region of Ni at
our CDB measurement system was less than 5.0 x 10-3
mc. The ratio o1'the S-region to total counts is called theS-parameter, which is used for measuring a total amountof vacancy-type defects. For example, if the S-parameter
ratio of high energy particle irradiated specimens to unirradiated specimens is larger than 1.0, it is consideredthat radiation induced defects will be formed in the4. Discussion
The results of the positron annihilation lifetime andCDB measurements indicate the difference in thermal
stability between the H and He-vacancy clusters, i.e.,
He-vacancy clusters were much more stable than Hvacancy clusters. According to Niwase et al. [9], thecavities in Ni formed by the 20 keV D ion irradiarion atirradiated specimen.
In Fig. 4, the S-parameter ratio of the Ni irradiatedby H and He ions at 300'C to the un-irradiated247Ishizaki T. et al., Annealing Behavior of Defect Structures in Ni Inadiated by High Energy H and He Ions

coalescence between free vacancies and vacancyclusters. After annealing at 950'C, the desorption of He360"C disappeared after annealing at 625"C. Though
there is a difference between the D and H ion, it can beconsidered that the same recovery of defect clusters
occurs below 600"C in H ion irradiated Ni. In the caseof the H atom in Ni, the migration activation energy of
the H atom is 0.41 eV [20], and the binding energy of
the H-vacancy complex is 0.57 eV [21]. The de-trapping
energy, a sum of the migration energy and binding
energy, of the H-vacancy complex in Ni is about I eV.
Therefore, H atoms can easily escape from vacancies orsmall vacancy clusters by annealing at 350oC. It canalso be explained from the thermal desorption behavior

came to end, and at the same time, the decomposition of
the vacancy clusters fully occurred.

5. Conclusions
Based on the positron annihilation measurements

for Ni irradiated by H and He ions at 300oC afterisochronal annealing, the following conclusions weredrawn. In the case of the H ion irradiation, the vacancy
clusters disappeared after annealing at 600'C, whichindicated that the desorption of H atoms might occur
after annealing at 350"C, and that the vacancy clusters
decomposed between 350'C and 600"C. The recoveryof the vacancy clusters was not influenced by H atoms
above 350oC. In the case of the He ion irradiation, thevacancy clusters grew by annealing above 750"C, andof H atoms in Ni. Tanabe et al. 16l have reported thatthe thermal desorption of D atoms from Ni finished over350"C in the case of D ion implantation into Ni by theacceleration energy of under 30 keV with fluences of
4.0 x 10la up to 9.0 x 1017 ions/cm2 at room temperature(RT). Therefore, the recovery behavior of vacancy

disappeared after annealing at 950"C. This wasexplained by the unstableness of the He-vacancy

clusters was not influenced by H atoms above 350"C. It
was concluded that in the case of H ion irradiation, thethermal decomposition of vacancy clusters occurredcomplex above 750oC, and the thermal decompositionof the He-vacancy clusters ended below 950'C.

between 350oC and 600"C.

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249