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Nuclear Instruments and Methods in Physics Research B 245 (2006) 176–179

NIMBBeam Interactions

withMaterials &Atoms

Magnetic flux loss in Nd–Fe–B magnet irradiated with 660 MeVcarbon ion beam

Yoshifumi Ito a,*, Keisuke Yasuda a, Ryoya Ishigami a,Ken Ohashi b, Shintarou Tanaka b

a Wakasa-wan Energy Research Center, 64-52-1, Nagatani, Tsuruga, Fukui 941-0192, Japanb Shin-Etsu Chemical Corp., 2-1-5 Kitago, Takefu, Fukui 915-8515, Japan

Available online 17 February 2006

Abstract

Irradiations effect on the magnetic flux of Nd2Fe14B magnets have been investigated using 660 MeV C6+ and 200 MeV protons over adose range from 24 Gy to 105 Gy. For both cases, the flux loss as a function of the absorbed dose are nearly the same values over a widedose region from 50 Gy to 30 kGy. An absorbed dose of 1.3–1.4 kGy results in a flux loss of about 50%. Re-magnetization of the irra-diated magnets recovers the magnetic flux to the original level similar as thermal heating.� 2005 Published by Elsevier B.V.

Keywords: Radiation-induced damage; Nd–Fe–B magnet; High energy carbon irradiation

1. Introduction

The high remanence and the intrinsic coercivity of rare-earth permanent magnet materials such as Nd–Fe–B andSm–Co make them suitable in a variety of applications,including particle accelerators [1]. The maximum value(BH)max of the B * H product of Nd–Fe–B magnets arehigher than those of Sm–Co magnets, although the Curietemperature of Nd–Fe–B magnets is lower. The price ofNd–Fe–B magnets is rather low since the raw materialsare relatively abundant. Thus, the use of Nd–Fe–B magnetsis more practical even in radiation-rich environments if thedegradation of their magnetic properties due to radiation-induced damage is acceptably small.

Recently, energetic ion beams have been successfullyapplied to cancer therapy, where carbon ions as well asprotons are utilized. The use of such permanent magnetsas parts of accelerator beam lines would be fascinating

0168-583X/$ - see front matter � 2005 Published by Elsevier B.V.

doi:10.1016/j.nimb.2005.11.097

* Corresponding author. Tel.: +81 770 24 5626; fax: +81 770 24 5605.E-mail address: yito@werc.or.jp (Y. Ito).

not only for the construction of a compact irradiation sys-tem but also because of low operating costs and reducedcapital costs by eliminating power supplies. In the designstudy for such a system, the radiation resistance of theirmagnets should be known.

After the first report by Blackmore that Nd–Fe–B mag-nets are extremely sensitive to 500 MeV proton irradiationwhen compared with Sm–Co magnets [2], many groupshave found similar results of radiation-induced effects withelectron beams [3], protons (or deuterons) [4,5], and neu-trons [6]. Several investigations on ion-beam-induceddamage of Nd–Fe–B magnets have been described in theliterature [2,4,5,7]. In most cases, proton (or deuteron)beams were used, only Kahkonen et al. used 56 MeVhelium ions in some of their experiments [8]. There are onlyvery few quantitative data on the radiation effects of Nd–Fe–B magnets induced by heavy ions.

In this report, the effect of 660 MeV C6+ beam irradia-tions on the magnetic flux of Nd2Fe14B magnets isdescribed. The flux loss of the same magnet due to200 MeV proton irradiation was also examined over thesame dose region. The result of re-magnetization tests ofC6+ ion irradiated magnets is given.

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2. Experimental procedure

2.1. Sample

The samples (Grade name: N48) were made ofNd2Fe14B permanent magnets manufactured by Shin-EtsuChemical Corp. The properties of the magnet are listed inTable 1. The samples were disks with a diameter of 10 mmand a thickness of 1 mm. Here, the thickness was chosen insuch a way that 660 MeV carbon ions pass through themagnet. The samples were fully magnetized in a directionparallel to the axis.

Table 1Physical properties of the magnet sample

Grade name (Shin-Etsu Chemical) N48Materials Nd2Fe14BResidual magnetization; Br [kG] 13.68Intrinsic coercive force; iHc [kOe] 13.38Coercive force; bHc [kOe] 12.83Maximum energy products; (BH)max [MGOe] 45.43Curie temperature; TC [K] 628Permeance Coefficient; Pc 0.247

Fig. 1. Schematic drawing of the experimental setup (a)

2.2. Irradiation

The irradiations were performed with 660 MeV carbonions and 200 MeV protons at the Wakasa-wan Multi-purpose Accelerator with Synchrotron and Tandem(W-MAST). The carbon and the proton beam, acceleratedby the synchrotron, had a repetition rate of 0.5 Hz and apulse width of 0.25–0.3 s. The fully magnetized sampleswere irradiated with the different doses of carbons (or pro-tons) in a vacuum chamber at room temperature. Fig. 1shows a schematic drawing of the experimental irradiationsetup. Ten magnets were mounted on a disk-shape sampleholder which can rotate around the axis. The samples wereplaced on a circle with radius of 8.5 cm and each intervalof the magnets was about 3 cm as shown in Fig. 1(b).After passing through a collimator (Cu block 50 mmthick with a hole of 11 mm in diameter), the ion beamirradiated uniformly the samples. The energy loss of660 MeV carbon ions in the sample was estimated to beabout 264 MeV and the loss of 200 MeV protons wasabout 2.5 MeV. The stopping power was calculated withthe Bethe formula using a sample thickness of 1 mm

and sample holder (b) for the ion beam irradiations.

Table 2Results of re-magnetization test for the N48 (Nd2Fe14B magnet) irradiatedwith 660 MeV C6+ beam

Sample 1 Sample 2

Before irradiation 1.000 1.000After irradiation 0.394 0.314After re-magnetization 1.018 1.015

The intensity is normalized by the fully magnetized one before the carbonirradiation.

178 Y. Ito et al. / Nucl. Instr. and Meth. in Phys. Res. B 245 (2006) 176–179

and a density of 7.56 g/cm3. The beam current was mea-sured by a Faraday cup placed about 20 cm downstreamof the sample. The time averaged beam currents were0.1–0.3 nA for carbon ions and 1–3 nA for protons. Thedose absorbed by the magnet averaged over the cross-section was calculated from the time-integrated beamcurrent and the beam energy loss. Conversion factors were1 lC/cm2 = 61.25 kGy for 660 MeV carbons and 3.48 kGyfor 200 MeV protons.

After the irradiation of the allotted dose for a given sam-ple, the holder was rotated to the next sample position.After irradiation, the total magnetic flux of each samplewas measured at room temperature by using a flux meterwith a Helmholtz type open coil. The coil radius and thedistance between the coils were about 6 cm. The samplewas inserted between two open coils of 750 turns each,the induced voltage being integrated. The error in the mea-surements was estimated to be below ±0.1%. The value ofthe observed flux U was compared to the initial one U0 andthe relative flux loss DU = (U � U0)/U0 was determined.

3. Results and discussion

Fig. 2 shows the dose dependence of the magnetic fluxloss DU relative to the initial magnetization of the samplesin the dose range between 24 Gy and 105 Gy. Here, dataare plotted as DU versus logD. In the dose region up to104 Gy, DU is proportional to logD. Above 104 Gy, the fluxloss remains constant at �75%. The radiation-induced fluxlosses for 660 MeV C6+ ions are nearly the same as for200 MeV protons over the dose range from 50 Gy to30 kGy. The result indicates that the flux loss is scaled onlyby the absorption dose and seems to be independent of theprojectiles.

Re-magnetization of the samples demagnetized due tothe 660 MeV carbon irradiation was tested after theradioactivity of the samples was sufficiently low. Theresults are listed in Table 2, where the values are normal-ized to the magnetic flux before the beam irradiation. All

Fig. 2. Dose dependence of the magnetic flux loss DU for samples of N48magnets due to 660 MeV carbon beam and 200 MeV protons.

magnets recover the flux intensity before the irradiationwithin the experimental accuracy, which was the sameresults as for N48 magnets irradiated with 200 MeV pro-tons, reported before [9]. Thus, it is concluded that the irra-diation with 660 MeV carbon ions as well as with 200 MeVprotons does not affect the crystallographic structure of themagnet.

The result of the re-magnetization test indicates that themechanism of beam-induced demagnetization is similar tothe effects of thermal heating, as suggested by Talvitie et al.[7]. When the high energy ion beams are injected into mag-netic materials such as Nd2Fe14B, atoms can be excited orionized by inelastic interactions with the ions and, high-energy electrons are generated. Some part of the electronenergy is finally transferred to the lattice, which may leadto local heating. The high-temperature region may act asa nucleation center for a reverse domain [7]. For a givenabsorbed dose, the number of incident ions per unit areais for the 660 MeV C6+ ions less than 10�2 of that for the200 MeV protons. Fig. 2 shows that the absorbed dose ofthe N48 magnet in both cases of carbons and protons leadsto the same flux loss of the magnet. This result might beexplained by the assumption that the flux loss due to theion beam irradiation depends on the local temperatureTL times the volume V of the high-temperature region,TL · V, which may be proportional to the absorbed doseof the magnet.

Kahkonen et al. proposed a theoretical model for theparticle induced flux loss in permanent magnets [10]: Pri-mary knock-on atoms caused by elastic collisions withincoming particles lead to a temperature increase in a localarea of the lattice above the Curie point. Each such colli-sion causes domain nucleation in a direction opposite tothe rest of the magnet and the domain immediately growsto the size of the grain. They explained the results on thetemperature dependence of the magnetic flux loss due to20 MeV proton irradiation, using their model. This model,however, dose not apply to our experimental results,because it was questionable that the effects of the elasticcollision on the temperature increase and on the magnetproperty due to 660 MeV carbon irradiation was the sameas the effect due to the 200 MeV proton irradiation whichgives rise to the same absorbed dose as for the 660 MeVcarbons. The mechanism of the flux loss due to the ionbeam irradiation has not been fully understood yet, somore detailed studies are required on the mechanism ofthe radiation-induced loss of the Nd–Fe–B magnets.

Y. Ito et al. / Nucl. Instr. and Meth. in Phys. Res. B 245 (2006) 176–179 179

4. Concluding remarks

The effects of 660 MeV C6+ irradiation as well as200 MeV protons on the magnetic flux of Nd2Fe14B mag-net with an intrinsic coercive force iHc of 13.4 kOe, a per-meance coefficient Pc of 0.25, and a Curie temperatureTC of 628 K have been investigated over the dose rangeform 24 Gy to 105 Gy.

(1) The radiation-induced flux losses for C6+ beam arenearly the same as for protons over the dose regionfrom 50 Gy to 30 kGy. The absorbed dose of 1.3–1.4 kGy leads to a flux loss of about 50%.

(2) Re-magnetization of magnets irradiated with a C6+

beam or with protons recovers the magnetic flux tothe original level. Thus, 660 MeV carbon irradiationas well as 200 MeV protons does not affect the crys-tallographic structure.

(3) Re-magnetization tests indicate that the mechanismof demagnetization under the ion beam irradiationis similar to the effects of thermal heating.

(4) Ion-beam-induced flux loss might be attributable tolocal heating in the magnet, resulting from inelasticcollision with the incoming ion beam.

Acknowledgement

The authors would like to thank the staff of the acceler-ator group at the Wakasa-wan Energy Research Center fortheir operation of the accelerator through this experiment.

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