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XEROX

NOLTR 61-45

RADIATION DAMAGE THRESHOLDS FOR

PERMANENT MAGNETS

A,)

18 MAY 1961

UNITED STATES NAVAL ORDNANCE LABORATORY, WHITE OAK, MARYLAND

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NOLTR 61-45

RADIATION DAMAGE THRESHOLDSFOR PERMANENT MAGNETS

Prepared by:

R. S. SeryR. H. LundstenD. I. Gordon

ABSTRACT: SevpraIl sets of permanent magnets, representative ofcommercially important magnet materials, were irradiated atBrookhaven National Laboratory and Argonne National Laboratoryto integrated neutron flux levels from 3 x 10"' to 4 x 100epicadmium n/cm4 . In spite of this relatively high dose,Alnicos II, V and XII showed negligible change in propertieswhether irradiated at 600G, 235 0 C, or 325°C. Cunico I, thoughaffected, showed changes less than a threshold of radiationdamage of - ICF'. Cunife I and 3-1/2 Chromium Steel showedslight improvements in properties. The Barium Ferrites,Silmanal, 36 Cobalt Steel and others exceeded the 10% damagethreshold by various amounts which exteided up to severedemagnetization. Differentiation between temperature andradiation effects was accomplished by the use of controlmagnets. and by the 600C irradiation. Limitations on the useof Alnicos II, V, XII and Cunico I in combined heat and nuclearradiation environments may be imposed by the higher vulnera-bility of associated soft magnetic circuit components, eog.,pole pieces of soft iron, to radiation damage and by high gammaheating which can occur if a magnetic circuit must be used in asealed container (for protection from corrosion or otherreasons).

Of the two most widely used groups of permanentmagnets, the Alnicos exhibit the highest resistance toradiation, while the barium ferrites show the least.

PUBLISHED JUNE 1961

U. S. Naval Ordnance LaboratoryWhite Oak, Silver Spring, Maryland

NOLTR 61-45 18 May 1961

The first irradiation experiment in this survey of the effectsof nuclear radiation on permanent magnets showed that none ofthe materials tested were affected by the same amount ofradiation which caused some soft magnetic materials todeteriorate. The initial permanent magnet results werepresented in NAVORD Report 6276 and the work on soft magneticmaterials in NAVORD Report 6127. This report gives the resultsfor the behavior of the same materials at -10, 100 and 1000times the initial dose to which they were subjected. The workwas performed as part of a broad pro gam for developingmagnetic materials (Task No. RRMA-O2008/2121/R0O7_O01O01).

W. D. COLEMANCaptain, USNCommander

L. Ro MAXWELLBy direction

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CONTENTS

Page

INTRODJCT I•ON I

EXPERIMENTAL 2

RESULTS AND DISCUSSION 3

irradiation to 5 x 10 epicad nvt, Changes LessThan 10% 3

Irradiation to 5 7. 10 epicad nvt, Changes GreaterThan 10% 4

Irradiation to 5 x 1020 epicad nvt, PossibleImprovements in Properties 7

Stability of Change-: 8Effects of 3 x 10-17 and 2 x 1018 Epicad nvt on

Demagnetization Cwrves 8Effects of 3 x 101" Epicad nvt.on BOMC (900C) 10Effects of 2 x 1018 Epicad nvt on BOMC (31OOC L 5) 1OEffects of 3 x 1011 Epicad nvt on BOMC (25OOC.+20) 10

C ONCLUS IONS 11

Results up to 5 x 1020 Epicad nvt 11Results at 3 x 1017 to 3 x 10'" Epicad nvt 12General 12

AC iKWLEDGEMENTS 13

REFERENCES 15

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NOLTR 61-45

TABLES AND ILLUSTRATIONS

Page

Table I. Permanent Magnet Materials Irradiated:Composition, Nominal Characteristics,Design Factors, Heat Treatment 16

Table II. Percentage Changes in Open MagneticCircuit Induction (BOMC) of PermanentMagnets as a Result of Irradiation with

4 x 10"0 nvt (epicad) 17

Table III. Irradiated and Control Magnets for Experi-ment at Elevated Temperatures at - 4 xI10 nvt (epicad): Stability of Induction4-1/2 Months After Events 18

Table IV. Percentage Changes in Open MagneticCircuit Induction (BOMC): IrradiatedMagnets at Intermediate Levels of Inte-rated Neutron Flux - Compared withontrols 19

Figure 1. Nominal Demagnetization Cuxves of thePermanent Magnet Materials Irradiated 20

Figure 2A. High Temperature Irradiation AssemblyPartially Disassembled 21

Figure 2B. Container Used for Optimum Utilizationof Reactor Water Cooling to Hold GammaHeating Temperature to ,-/ 600C. Magnetsand Aluminum Plugs Alternately Spaced 22

Figure 3. Comparison of Percentage Changes in OpenMagnetic Circuit Induction BOMC, forMagnets Irradiated to -,- O0 epicad nvtat Normal and High Temperatures 23

Figure 4. Method of Analysis of Changes in BOMCProduced by Fictitious DemagnetizingFields, (AH 's)on Nominal DemagnetizationCurve for 36 Cobalt Steel 24

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NOLTR 61-45

Page

Figure 5. Alnico V, Alnico XII, 36 Cobalt Steel,Cunife I: Effect on DemagnetizationCurves of Irradriation with 2 - 1018epicad nvt at '-' 5OO°C ?5

Figure 6. Platinum Cobalt, Barium Ferrites Orientedand Unoriented. Silmanal: Effect onDemagnetization Curves of Irradiationwith 2 x 1018 epicad nvt at -5OO 0 C 26

Figure 7. ESD Fine iron, Cunico I, 3-1/2 ChromiumSteel: Effect on Demagnetization Curvesof Irra' 1.lation with 2 x 10"' epicad nvtat - 5OOC 27

Figure 8. ESD Fine Iron-Cobalt, Alnico II: Effecton Demagnetization Curves of IrradiationWith 2 x 10" epicad nvt at - 5000 C 28

Figure 9. Alnico V, 36 Cobalt Steel, ESD Fine Iron-Cobalt: Effect of Temperature ( - 5O00 C)on Unirradiated (Control) Magnets 29

Figure 10. Alnico XII, ESD Fine Iron, 3-1/2 ChromiumSteel: Effect of Temperature (-- 5OOC)on Unirradiated (Control) Magnets 30

Figure 1i. Alnico V, Alnico XII, Alnico II: Effectsof Irradiation and Reactor-Temperature-Simulation Events on Open Magnetic CircuitInduction (BOMC) 31

Figure 12. 3-1/2 Chromium Steel, Cunife I: Effectsof Irradiation and Reactor-Temperature-Simulation Events on Open Magnetic CircuitInduction (BOMC) 32

Figure 13. Cunico I, Barium Ferrites Oriented andUnoriented, Silmanal: Effects ofIrradiation and Reactor-Temperature-Simulation Events on Open MagneticCircuit Induction (BOMC) 33

Figure 14. ESD Fine Iron-Cobalt, ESD Fine Iron:Effects of Irradiation and Reactor-Temperature-Simulation Events on OpenMagnetic Circuit Induction (BOMC) 34

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NOLTR 61..i45

Page

Figure 15. 36 Cobalt Steel, Platinumi Cobalt: Effectsof Irradiation and Reactor TemperatureSimulation Events on Open Magnetic CircuitInduction (BOMC) 35

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NOLTR 61-45

RADIATION DAMAGE THRESHOLDSFOR PERMANENT MAGNETS

INTRODUCT ION

1. What are the effects of nuclear radiation on permanentmagnet performance? To answer this question, a series ofexperiments was performed to test a number of materials topredetermined values of integrated neutron flux or until themagnetic properties of most of the magnets showed majordegradation. However, materials such as Alnicos II, V and XIIshowed little or no change in properties after high levelirradiation and, except for adverse temperature effects, CunifeI and 3-1/2 Chromium Steel showed a slight tendency towardimprovement. Eleven miaterials in all were subjected toirradiations up to 5 x 10' epicadmium nvt at maximum tempera-tures of 60, 235 an•d 3250C. Two materials showed changes ofthe order of a ± 10. threshold of radiation damage, Cunife Iand Cunico I. The two Barium Ferrites (oriented and unoriented)Platinun Cobalt, 36 Cobalt Steel and Silmanal showed severeproperty degradations.

2. In the first of these experiments the total integratedepicadmium flux (n/cm2 or nvt) reached was only about 3 x 10171.None of the thirteen materials irradiated were affected by thisrelatively low dose. This was not an unexpected result sinceprevious work 2 with soft magnetic materials revealed a roughrule of thumb measure that materials having coercive forcesless than 0.5 oersted tended to be degraded by iriadiation tothis level, whereas those having coercive forces greater than0.5 oersted were not affected. Permanent magnet coercive forceslie between 50 and 4500 oersteds (see Fig. 1). Subsequentirradiations of -i0'8, 1O0' and 1020 nvt were performed untilsome of the magnets not only crossed the threshold of damagebut were in some cases severely demagnetized.

3. The initial experiment with permanent magnets to 3 x 10"'nvt was performed at the Brookhaven National Laboratory.(NOTE: Unless otherwise specified, all values of integrated

flux are for epicadmium neutrons (neutrons whose energies>' .4 ev)). All succeeding experiments were performed in theCP-5 reactor at Argonne National Laboratory (ANL). Since thiswork was of interest to the Reactor Engineering DJ-iision atArgonne, the experiment was conducted as a joint effoct by theU. S. Naval Ordnance Laboratory and the Argonne NationalLaboratory. Projected applications by ANL included the use ofpermanent magnets as components of electromagnetic flowmeters

NOLTR 61-45

immersed in the coolant of a reactor in high nuclear radiationfields for extended periods of time. ANL provided thefacilities such as space in CP-5, the use of hot cells,temperature monitoring equipment, and dosimetry fabricationand evaluation, while NOL conducted the experiments on thepermanent magnets.

EXPER IMENTAL

4. Because reactor space available is limited, the number ofpermanent magnet materials to be tested in this survey wasrestricted to a representative group of thirteen materials.Figure 1 lists these materials and shows their nominal demag-netization curves. Table I gives additional information. Twomaterials, the ElongaLed Single Domain (ESD) magnets, wereeliminated after reaching the 10"' nvt level because one ofthem had completely disintegrated into a powder sometime beforethis value had been reached and had clung to the other testspecimens. This disintegration was probably due to the meltingof the lead alloy matrix under high temnerntiuro, The- PljatiCobalt magnet was eliminated from the normal temperatureexperiment due to lack of space.

5. In order to be able to differentiate between radiationinduced changes and changes caused by high temperatures,duplicate sets of magnets were used as controls. Temperaturelevels and temperature drops (which occurred at reactor shut-downs) encountered by the in-pile test magnets were simulatedfor the control magnets in ovens. To further insure that thetest and control assemblies had the same treatment, both setsof mra '-nets for each experiment were, wherever practicable,stabilized, tested and handled in the same manner and at thesame time. They were also packaged in identically designedcontainers. Most of the containers were approximately 12" longby 1.25" in diameter. These dimensions were dictated by thegeometry of the CP-5 holes available and by the necessity forminimizing the length of container monitored by a single set ofdosimeters.

6. The buildup of temperature by gamma. heating to high valuesin the can type of container shown in Fig. 2A was prevented inthe normal temperature experiment by the use of the tube typeof assembly shown in Fig. 2B. (The gamma flux was estimated tobe appreciably greater than 1O7 roentgens per hour). Themagnets and their aluminum spacers fitted closely enough in thetubes of this assembly so that adequate cooling was effectedbut not so closely that they could not be removed withoutforcing. A non-magnetic stainless steel spring in each tubeprovided positive contact of the magnets with the assembly both

2

NOLTR 61-45

at the ends of the tube and, to some extent, along the Length.The magnets were aligned with the north poles in the samedirection.

7. In-pile temperatures were recorded continuously on a stripchart. The sensing elements were chromel alumel thermocouplesinserted in holes in dunmy samples of type 304 (non-magnetic)stainless steel. A set of three dosimeters was included ineach container and in the last three experiments (at - 1iOo nvt)two such sets were included. The three dosimeters per setincluded a cobalt aluminum foil or wire for monitoring thethermal flux, a second Co-Al wire in a cadmium jacket for theepicadmiun flux, and an aluiintun sulphate pellet for theneutrons having energies greater than 2.9 Mev. 3 ' 4 '5

8. No in-pile magnetic measurements were made because existingmeasurement techniques and presently available in-pile spaceare incompatible with such measurements. Because of the highinduced activities, in these magnets, all post-irradiationmeasurements were made in hot cells. All of the magneticproperties measured were made with specially dsigi1ed searchcoils6 which allowed data to be taken by remote handling inthe hot cell. Since pre- and post-irradiation measurementswere made with the same or identical search coils, leakageflux errors as high as ± 270 were eliminated.

9. For nyst of the magnets only one property was measured,the open magnetic circuit induction (BOMC). BOMC is theoperating induction value of the magnet under open circuitconditions, It is represented by a point on the demagnetization

curve or within it. For a given material, it depends on theshape and dimensions of the magnet, as well as on the geometryand material of thc entire magnetic circuit. Each magnet hadthat length to diameter (L/D) ratio which fixed its initialoperating point at or above the knee of the demagnetizationcurve. This insured operation of the magnet at an optimumpoint for open circuit conditions, Experimental work at the1017 and 10 ' nvt levels included an additional set of magnetswhich was used for the closed magnetic circuit tests in whichdemagnetization curves were obtained.

RESULTS AND DISCUSSION

Irradiation to 5 x 1020 nvvt s Less Than 10%

10. Three sets of magnets were irradiated to a total integratedflux of about 4 x lO nvt, one at a normal temperature of 600C+ 100C, one at an intermediate temperature of 230 0 C J 200C,and one at a high temperature of 330 0 C ± 201C. The results of

3

NOLTR 61-45

these three experiments are shown in Table II and Fig. 3.There are two main groupings of the results. The first groupincludes those magnets which were unaffected by this high valueof integrated flux or at the least showed radiation damage ofless than an arbitrarily chosen threshold change of l0%o. Thesecond group consisLs of those materials in which changesgreater than this 10% threshold occurred.

11, Alnico II, Alnico V, Alnico XII and Cunico I belong tothe first group. The first two showed negligible changes inproperties to within an experimental error of ± 2%. TheAlnico XII magnet irradiated at a normal temperature showed a-6.5% change which is still considerably below the 10% thresholdof damage. The three Cunico I samples approached the 10%threshold value. For Cunico I comparison of the results (seeTable TI or Fig. 3) for each of the three test magnets with itscontrol reveals that the change produced was due to irradiation.The average of the differences between percentage changes ofthe test and control magnets is about -7% (Note the similarityin behavior of this material and 36 Cobalt Steel, particularlywith reference to the analysis on*36 Cobalt Steel in t-he nextseCtiuLI).

Irradiation to 5 x 1 0 "0 nvt, Changes Greater Than 10%

12. The three irradiated 36 Cobalt Steel magnets showedchanges of -37, -37.5 and -34.5% in BOMC at normal, intermediateand high temperatures respectively. The changes in the threecorresponding control magnets were -1, -20.5, and -22%. Ananalysis of the demagnetizing effects of nuclear radiation andtemperature on BOMC for this material is ill-ustrated with theaid of Fig. 4.

13. The nominal operating point for each of the six magnetsafter magnetization and stabilization was point A. If weconsider the high temperature control sample alone, heating to3250C may be considered equivalent to a fictitious demag-netizing field of magnitude - 6HT. This field causes a shiftin the operating point from A down the curve to C. At the firstsimulated reactor shutdown, the drop in temperature, i.e., theremoval of the equivalent demagnetizing field, - AHT will causethe point to move along the idealized minor loop CD until itintersects the load line AO at A'. With succeeding rises anddrops in temperature the point commutes between C and A'. Itsfinal position at the end of the temperLture simulation period.is A . The net change in BOMC caused bv temperature alone isA BT. For the control sample run at 2350C the same analysisholds with the exception that ABT would be smaller.

14. For the irradiated sample at 3250C the same temperatureinduced change, ABT, o-urs initially since the first reactor

4

NOLTR 61-45

shutdown takes place within one or two days after irradiationbegins. However, the cumulative effect of sixteen weeks ofneutron bombardment causes the operating point to move fromA through C to C'. The net change in BOMC (ABI - 34.5%) canbe considered as resulting from a fictitious demagnetizingfield -,&HI which is the equivalent of the demagnetizationinfluence of irradiation. This field -AHI may be consideredas being permanently applied since the effects of irradiationare cumulative whereas those of temperature are partly rever-sible, i.e., BOMC moves from C to A once the equivalent field- AHT is removed. It does not move up a similar minor loopparallel to CA' since the fictitious field cannot be removedupon cessation of irradiation except perhaps by remagnetizationof the magnet in which case temperature effects could also beerased (provided that no irreversible metallurgical changeshad occurred in either case). For the magnet irradiated at235°C the same equivalent demagnetizing field - 4HI gives riseto approximately the sameABI (-37.5-°).

15. &B in Fig. 4 merely indicates the difference betweenirradiation and temperature effects. That 6BI is actually themagnitude of the change produced by irradiation becomes evidentwhen the results for the two magnets subjected to temperaturesof only 600C are examined (see Fig. 3). The control magnetshowed a negligible change indicating no effects due totemperature. Its operating point remains at A. However, thechange in the test magnet of 6B = 37% was due entirely toirradiation. This corresponds to the application of aneffective, permanent fielq -AHT which caused the operatingpoint to move from A to C . The radiation induced change inBOMC occurred independently of the presence of temperature.The fact t"h' various demagnietizing influences can operateindependently of each other makes it possible to stabilize amagnet against any changes which are smaller than the changecaused by the stabilization process itself. This analysisindicates therefore that this material may be amenable tostabilization such that it would not be affected by irradiationto the nvt's achieved in spite of the large changes in BOMCwhich actually occurred.

16. An alternative method of illustrating the change in 36Cobalt Steel is to consider the operating point BOMC asremaining always on the load line. Since the load line is afunction only of the geometry of the magnet it does not change.Therefore, the demagnetization curve must change with tempera-turre or irradiation. Its change in shape is unspecified.However, its intersection with the load line marks the positionof BOMC and thus the net change A BI or & BT. As before thechanges due to temperature and irradiation are independent ofeach other; the larger of the two changes masks out the presenceof the other.

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NOLTR 61-45

17. The oriented and unoriented Barium Ferrite magnets areidentical in composition but have marked differences inmagnetic properties (Fig. 1). However, both were affected tothe same extent by irradiation. For approximately the sameintegrated fluxes the changes in the magnets irradiated atnormal temperatures were -63% and -54.5% respectively; but atintermediate and high temperatures the percentage changes werepfor both materials, within a range of -23 ± 2%. At the inter-mediate temperature (-235 0 C) the two ferrite controls showed nochanges. This is to be expected since normally they can with-stand temperatures up to 4500 C without appreciable changes. Thehigh temperature controls exhibited peculiar behavior, however,They were completely degraded by prolonged heating at the 3250Ctemperature. It should be pointed out that both the irradiatedsamples and their controls were not: only subjected to the abovetemperatures but also to periodic drops in temperature whichfollowed reactor shutdowns (for the controls quenching in airby abrupt removal from the oven simulated the reactor tempera-ture drop). These thermal shocks may have caused the deteriora-tion in the high temperature controls. No explanation hassuggested itseIf for the const;Aney of tle -23y values (seeFig. 3), for the irradiated magnets at intermediate and hightemperatures as compared to the lack of change in the controlsat the intermediate temperature and the almost completedegradation for the controls at the high temperatures.

18. Although no Platinum Cobalt magnet was irradiated at anormal temperature there is some evidence of radiation damagein this material (Fig. 3). Based on the analysis for 36 CobaltSteel the damage should be at least as great under normaltemperature irradiation as that which occurred at the elevatedtemperatures. The control magnet at 3250C showed anomalousbehavior similar to that of the Barium Ferrite controls at thistemperature. Actually all eleven of these controls were checkedafter only two weeks of the simulation of the in-pile tempera-tures. All but one (3-1/2 Chromium Steel, -13.7% change) showednegligible or slight effects, e.g., Platinum Cobalt -4.6%,Silmanal -3.2%, the ferrites -- I%. However, at the end of the16-week simulation of reactor temperatures, all but Cunico Iand the three Alnico magnets showed major changes in BOMC (seecolumns 8, 9, Table II and Fig. 3). This was not unexpectedfor 3-1/2 Chromium Steel or Silmanal which are affected bytemperatures above 1200 (Table I) and 2350 C, respectively.But materials such as the ferrites and Cunife are notappreciably affected at temperatures below 4500C. The presenceof thermal shocks was probably a factor in causing thedegradation which occurred in some of the magnets.

19. Silmanal is normally annealed by a slow bake at about2500 C. Therefore, the high temperature alone, which peaked at

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NOLTR 61-45

3250C, caused an almost complete loss of magnetization in boththe test and control magnets. However, at a mtore normaltemperature of operation (- 600C) there was a change in BOMC of-53% due almo.st wholly to radiation damage. The magnets at2350C showed opposing trends. The test magnet was adverselyaffected by radiation; the control magnet was helped by thesimulated reactor temperature. But, since none of the Silmanalmagnets had been given an optimum heat treatment initially,this control magnet had been improved only because of afortuitous heat treatment which occurred during the in-piletemperature simulation. The test magnet did not show improve-ment becadse of the large radiation damage effect.

Irradiation to 5 x 1020 nvt. Possible Improvement in Properties

20. For high level irradiation at a normal temperature, 3-1/2Chromium Steel showed a slight improvement in BOMC of 2,4%.Ordinarily a percentage change of this magnitude would not besignificant. However, this material has the lowest coerciveforce (Fig. i) of all the materials tested and proved to bethe mrst easily affected by demagnetizing influences. Thepositive percentage change, which ran counter to the expected.tendency to demagnetize, may have been even larger than thatshown in Fig. 3. However, this magnet had become jammed in itsclose-fitting aluminum jacket and had to be subjected to thedemagnetizing effect of being forcibly punched out before itsBOMC value could be measured. The effect of intermediatetemperature irradiation on chromium steel was about the sameas that of temperature alone on the control magnet, and thiswas expected since this material is permanently affected bytemperatures above 120 0 C. It is significant though, that theradiation induced change was smaller than the change producedby temperature alone (-687. as compared with -79%). Theadditional fact of a sizeable increase in BOMC at the hightemperature irradiation corroborates this tendency for 3-1/2Chromium Steel to improve with radiation (see Fig. 3). But themagnitude of the percentage increase is somewhat misleading.Because a preliminary dry run had indicated an in-piletemperature of about 325oC, the two sets of test and controlmagnets of this experiment were temperature cycled (stabilized)at this temperature. The magnetic induction, BOMC, for boththe test and control specimens therefore decreased frominitially optimum values by -89% and -79% respectively. Thesubsequent irradiation induced increase of 67% in the testmagnet yielded a value of BOMC which was still 82% below itsinitial optimum value.

21. At high temperatures (•- 3250C) the Cunife I magndt wasalmost completely degraded by temperature alone since the testand control magnets showed the same decrease in BOMC. At theintermediate temperature there was some evidence of a radiation

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NOLTR 61-45

induced change greater than that caused by temperature alone.At a normal temperature the control sample was essentiallyunaffected but the test magnet showed a 13% increase, Itsfinal BbMC value was, in fact, 3% higher than the initialoptimum value for this sample, This magnet also had to beforcibly ejected from its jaqket, which treatment would tendto demagnetize it,

Stability of Changes

22. In order to determine whether or not the changes caused byirradiation were stable, the two sets of magnets irradiated to-102 nvt at elevated temperatures were retested 4-1/2 mionthsafter irradiation. As a check, the control sets were alsoretested four months after their temperature run. The data areshown in Table III. Most of the magnets maintained, within theexperimental error, their immediate post-irradiation values.The 36 Cobalt Steel of one set (see column I of Fig. 3) was anotable exception. However, because of the intense gammaradiation resulting from the induced activity in the irradiatedmagnets the wooden containers it which they were stored brokedown in places allowing some of the magnets to move closertogether. The 36 Cobalt Steel was in tandem with the Alnico Vand its drop in BoC... may have been due to the demagnetizingincluence of the latter. Moreover, the control magnet of thesecond set (see 36 Cobalt Steel, column 4, Table ITI) showedconsiderably deterioration after four months of storage. Thus,the post-irradiation changes in both cases may be due to aninstability inherent in magnets which have undergone majorchanges in BOMC 9 .

23, Th w•,c 3.I/2?/ Chromi-. Steel test magnets changed byappreciable amounts, 4 13, -18%. The controls changed also,although to a lesser extent. These changes are probably notsignificant since the BOMGvalues from which the changesoccurred are only about 2M. of what they would be for normallyoptimum values9•

Effects of 3 x 10" and 2 x 10"8 nvt on Demagnetization Curves

24. Initially, two sets of thirteen magnets were irradiated inthe Brookhaven National Laboratoiv Reactor to 3 x 10'' nvtepicadmium. Both sets were then shipped to Argonne NationalLaboratory where they were irradiated a second time but inseparate holes of the CP-5 reactor. The set used for demag-netization curve determinations was ýrradiated in hole VT--9at a temperature of 500°C ± 20; the second set for the measure-ment oZ open magnetic circu.u% induction, BOMC, was irradiatedsimultaneously iv hole VT-5 at 3000C 4 20. The integrated fluxfor each set was about 2. 1015 ivt.

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NOLTR 61-45

25. No changes due to neutron irradiation were detectable inthe demagnetization curves for the experiment at 3 x 1017 nvtl.At 2 x 1018 nvt the differences in the before and after demag-curves for eight of the irradiated magnets were the same as thebefore and after curves for the corresponding controls (e.g.,see the curves for Alnico V and Alnico XII in Figs. 5, 9 and OThe magnets which were not affected included the Alnicos II, V,XII, Cunife I, Cunico I, Platinum Cobalt and the Barium Ferrites(oriented and unoriented).

26. Of the remaining five of this set of thirteen the ESD,Fine Iron and Fine Iron-Cobalt irradiated magnets MFigs. 7 and8) showed changes smaller than those that occurred in theircontrols (Figs. 9 and 10); the 36 Cobalt Steel (Fig. 5), 3-1/2Chromium Steel (Fig. 7) and Silmanal irradiated magnets showedchanges which were larger than those of their controls (Figs. 9and 1O). The disparity in behavior bctec=n test and controlmagnets is probably due, primarily, to temperature differencesin the test magnets which are not reflected in the value of5000 C ± 20 quoted above. This quantity represents the tempera-ture monitored at one point in the assembly (± 20 shows therange of variatioai in temperature caused by variation in-pilepower, etc., and not the experimental error). Thc amount ofgamma heating which can occur in a material is a function ofitb geometry, thermal conductivity, area of contact with theassembly in which it is mounted, etc. Therefore, the tempera-tures of some of the irradiated magnets may have exceeded 520 0 Cor fallen below 4800C by substantial amounts. In contrast, thecontrols were uniformly heated in an oven and experienced thesame temperatures to within a fraction of a degree.

27. One other consideration confirms the conclusion that theinfluence of temperature was largely responsible for thechanges in the demagnetization curves. The chan§es in BOMC ofthe magnets irradiated simultaneously to 2 x 101 nvt (but atthe lower temperature of 310°C) together with the above set weredue to temperature. Of the five materials which showed suchpronounced differences between the test and control magnetdemagnetization curves the two ESD materials showed negligiblechanges in BOMC for both test and control magnets; changes inBOMC for the Silmanal, 3-1/2 Chromium Steel and 36 Cobalt Steelirradiated magnets were the same as or less than those whichoccurred in the control magnets.

28. The set of magnets used for determining changes in theproperties of the demagnetization curves was not irradiatedfurther because temperature alone had caused large irreversiblechanges in five of the materials..

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NOLTR 61-45

Effects on BONC, 3 x 10"7 nvt (900C)

29. None of the magnets tested gave any indication of thepresence of radiation damage greater than an estimatedexperimental error of ± 2%. This together with the results forthe demagnetization curves determined at this level sets alimit of about 3 x 10"7 nvt below which one should not expectto find any radiation damage.

Effects on BOMC, 2 x 1018 nvt, (3100C 1 5)

30. Alnicos 'I and XII, Cunico T, 36 Cobalt Steel, Platinum.Cobalt, Silmanal and the ESD magnets, Fine Iron and Fine Iron-Cobalt were not affected by irradiation to this level.

31. Two Barium Ferrite magnets showed changes of -3% in BOMC.This is close to the : 2% experimental error but is consideredsignificant because Lhis material has exceptional stabilityeven at elevated temperatures (compare percentage changes withthose of the corresponding controls in Tables II and IV andFig. 13).

32. BOmC for Alnico V decreased by 9%. This change is notconsidered typical in the light of all the data obtained for,this material. At higher doses of integrated flux no radiationdamage effects were observed.

33. The large changes in properties in Cunife I and 3-1/2Chromium Steel were caused by the high temperatures. In fact,the changes in the controls were larger than those in the testmagnets ýFig. 12). This is consistent with the tendencytowarJ improvement in BOMC which was mentioned in the sectiondescribing the irradiation experiments at ,-,O2o nvt (Fig. 3).

Effects of 3 x lO• nvt on BOMC, (2500C ± 20)

34. The following magnets were not affected by this irradia-tion: Alnicos II, V, XII, Cunico I, Platinum Cobalt, Cunife I,3-1/2 Chromium Steel, ESD Fine Iron and Silmanal.

35. Materials such as Cun.fe I, 3-1/2 Chromitn Steel andPlatinum Cobalt - both test magnets and controls - w01ereaffected by the temperature. of 3100 in the previous irradiationand yet were not affected by the 2500 temperature of thisirradiation. The explanation for this is that the temperattrre-induced decreases in BOMC stabilized these magnets to anysucceeding temperature excursions whose peaks were less than3100C. By contrast, the previously unaffected 36 Cobalt Steelmagnet changed by -- 9%, Its controls did not change, thusindicating that the test magnet approached the threshold ofradiation damage.

IO'-

NOLTR 61-45

36. The ESD Fine Iron-Cobalt test sample disintegratedcompletely, probably due to the melting of the lead alloymatrix. Its controls also showed signs of deterioration, i.e.,corrosion and swelling (difficulty was experienced in placingthe search coil oi- rhe controls).

37. The two Barium Ferrites both showed near-threshold changesof _8,. At the highest intregrated fluxes of these experiments,5 x 102 nvt, the high temperature magnets showed changes of

22-.22 -The progressive damage with integrated flux occursover three orders of magnitude in nvt - 1017 to lO-. This isin s harp contrast to the effects of radiation on a typical softmagnetic material such as Supermalloy for which order of magni-tude changes occurred in some properties for a change in nvt- ofonly a factor of 2.

CONCLUS IONS

Results up to 5 x 1020 Epicad nvt

38. Alnico II, Alnico V and Alnico XII are not affected bynuclear radiation up to ý-5 x 102 epicad nvt (and -• 2 x 10'9nvt for neutrons with energies :2,9 Mev) at temperatures rangingfrom 950c to 3250C.

39. Cunife I, Cunico I and 3-1/2 Chromium Steel should beoperable at this level since they showed less than threshold ofdamage changes. Proper stabilization techniques could reduceexpected radiation induced changes to negligible amounts.These materials would have to operate at temperatures of about70 0 C or less to minimize changes due to gamma heating. Tenzer'has shown that Alnico V can withstand temperatures of 5500C forone thousand hours with little or no change in remanence (BOMC),.The Alnico m.gnets in this experiment showed no changes in thedemagnetization curves for irradiation to 2 x 1018 nvt at 5000C(although no BOMC measurements were made at this temperature).This suggests the possibility of stabilizing Alnico V, II andother materials to withstand not only high temperatures butstill higher integrated fluxes than were achieved or anenvironment combining both.

40. The Barium Ferrite magnets withstand irradiation to theselevels better if the temperatures are about 235 to 3250C thanthey' do at a normal temperature of operation. Proper stabili-zati.on could reduce irradiation induced changes or, perhaps,eliminate them.

41. It is questionable whether stabilization techniques wouldcompensate for the larse changes which occurred in PlatinumCobalt, 36 Cobalt SteeL, and Silmanal since large knockdownsof the order of -35% by stabilization may result in subsequenterratic behavior'.

Ii

NOLTR 61-45

Results at 3 x 1017 to 3 x iOCY nvt

42. None of the ma-onets were affected by nuclear radiation of10'• nvt (at 906C). Most were unaffected by che 2 r 1018

nvt irradiation; only three showed changes not attributable totemperature. The change in Alnico V. -9%, is probably not:representative. The changes in the Barium Ferrites,-.-3%, aretolerable or can be eliminated by stabilization. The resultsfor the demagnetization curves at 3 x l0'" nvt were not clearcut for five of the materials because of the high temperatureswhich were present. The differences between the curves of thetest and control magnets were attributable to temperatureeffects.

43. BOMC for ten of the materials remained unaffected byirradiation to 3 x 10'" nvt. The 36 Cobalt Steel and theoriented and unoriented Barium Ferrites approached but did notexceed the threshold of radiation damage. They would thereforebe amenable to stabilization treatment to minimize changescaused by irradiation.

General

44. Various limitations precluded the performance of experi-ments in which adequate statistical results could be achieved.Nevertheless, a small measure of statistical success wasattained. The three Alnico materials, for example, althoughsomewhat different in composition, gave essentially the sameresults under six different irradiations (two for demagneti-zation curve determinations, four for BOMC measurements). Thetwo Bariunm Ferrite materials - identical in composition -likewise showed the same chan~oes after several irradiations.These experiments moreover, provide boundaries for further workin this field. Irradiation to integrated fluxes lower than3 x 1017 epicadmium would not be rewarding (excluding perhapsexperiments which would reach this value by means of very highpulses of flux of very short duration). Future work on thosematerials which were unaffected could be started at the 1020nvt level and pursued until the damage threshold was attainedand exceeded. Another experiment of interest would be toirradiate permanent magnets in the presence of intense magneticfields to see whether their nominal magnetic properties could beimproved. Prerequisites of this work would include ample in-pile space for the equipment which produces the magnetic fieldand the necessity that such equipment would itself be resistantto radiation damage.

45. Two other factors must be considered with reference to thebehavior of permanent magnets in radiation fields. First,adequate cooling must be taken into account even for materials

12

NOLTR 61-45

like the Alnicos which operate successfully in combined environ-ments. The temperature equilibrium in a sealed container, whichis inadequately cooled could exceed the Curie temperature of amagnet, this would result in its comnlete demagnetization.Second, the soft magnetic materials (high permeability materials)used in conjunction with permanent magnets in magnetic circuitsmay be more susceptible to radiation damage than the magnetsthemselves. Although previous work 6 has shown that Bmax .seeFig. 5) the saturation induction, of soft magnetic materialsis not affected at 3 x 10i7 epicad n/cm2 , not enough is knownabout how this property may be affected at the integratedfluxes of -5 x 10 epicad n/cm2 achieved in these experiments.

46. The following relations between changes due to tempera-ture and radiation have been observed in these experiments(assuming thermal shocks are not of primary importance)..

a. Radiation effects are independent of temperatureeffects, e.g., in 36 Cobalt Steel and Cunico I.

b. The presence of high temperatures during irradiationcounteracts to some extent the effects of radiation, e.g.,Alnico XII, both Barium Ferrites.

c. Irradiation effects counteract temperature inducedchanges: 3-1/2 Chromium Steel. Also, possibly, Cunife I andBarium Ferrites.

47. Alnicos II and V were not affected at the integratedfluxes achieved. Other materials for various reasons showedno conclusive trend.

48. Calculations of the formation, by transmutation, ofisotopes which would act as impurities in the magnets show thatthis is a negligible factor contributing to radiation damage.The primary factor is that of physical damage to the latticestructure by the more conventional damage mechanisms.

AC KNOWTLEDGEý1ENT S

49. The authors wish to thahk the following individuals andorganizations for their help and cooperation in performingthese experiments:

a. P. Brown, V. Hall, A. D. Rossin, R. J. Armani andothers of the Reactor Engineering Department of ANL. Thecooperative effort between ANL and NOL was initiated bMr. Brown. When Messrs. Hall and Brown left ANL, Dr. Kossinand R. J. Armani continued to provide for the time, facilities,and personnel in the Reactor Engineering Department, the Hot

13

NOLTR 61-45

Cell Laboratory and at the CP-5 reactor to continue theirradiation studies.

b. Dr. W. H. McCorkle, John Condelos and other personnelof CP-5 for carrying on the irradiations, handling the hotcontainers, etc.

c. W. Doe for providing the hot cell facilities in theHot Cell laboratory, R. Enders for assisting in the measure-ments of the irradiated magnets in the hot cell, and otherpersonnel for their assistance.

d. L. Steele of the Naval Research Laboratory for adviceon methods of cooling containers placed in a reactor.

e. F. E. Luborsky and T. 0. Paine of the General ElectricCompany for furnishing samples of ESD Fine Iron and Fine Iron-Cobalt Magnets.

f. E. A. Bye of Simonds Saw and Steel Company forfurnishing .amples of 3-1/2 Chroml4i_ Steel and 36 Cobalt Steel.

14

NOLTR 61-45

Ra'ERF(NCES

i. R. S. Sery, D. i. Gordon, and R. H. Lundcsten •N,"NuclearRadiation Effects in Permanent Magnets", NAVORD Report6276 (U. S. Naval Ordnance Laboratory, White Oak, SilverSpring, Maryland) 13 April 1959.

2. D. I. Gordon, Environmental Evaluation of MagneticMaterials, Electro.Tc ..... lo-y, 67, No. 1, 11-, January 1961.

3. R. J. Armani, Neutron Flux Measurements in CP-5 inConjunction with Radiation Damage to Materials, ANLInternal Memorandum, 16 August 1960, Private Communication.

4. R. J. Armani, Neutron Flux Measurements Durin- MaQnetIrradiation, ANL Internal Memorandn, 29 August 1961,Private Communication.

SR J. Armani, Neutron Flu,- Measurements Durinux >iet

Irradiation in a Water Cooled Thimble, ANL InternalMemorandum, II January 1961, Private Co-mmunication.

6. R. S. Sery, Cast-in-Epoxv Search Coils for PermanentManet Testina, Electronic Design, 7, No. 17, 53,19 August 1.959.

7. R. K. Tenzer, Influence of Various Heat Exposures on

Alnico V Magnets J. Appl. Phys., Suppl. to 30, No. 4p. I15S, April 1659.

8. K. J. Kronenberg and M. A. Bohlmann, Long, Term MagneticStablity of Alnico V and Other Permanent Magnet Materials,

WADC Tehnical Report 58-535, ASTIA Document No. AD 203387,December 1958.

9. Ibid, p. 32, VI, 5.

15

NOLTh 61.-4

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NLOLTR 61-45

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NOLTR 61-45

TABLE II1. IRRADIATED (-1O20 nvt) AND CONTROL MAGNETSAT ELEVATED TEMPERATNURES: T'YVJ OF OPEN

MAGNrETIC CIRCUIT INDUCTION 4-1/2 MONTHS AFTER EVENTS

PFIRcENT CHANCR IN B^.C

1_ 2350C -- '325 0 C

MateriaL Sample Control Samplel Control

ALNICO TI A <+.5 -. 5 4. 5 <+.5

ALNICO V C -4 -1.5 -- 5ALNICO XII -. 5 -i -. 5 -6.5

UCOI <+C5 1-.5 +.5 <+.5

CUNIFEI +.5 -. 5 +13* +3

3-1/2 CHROMIUM -8* -83EEL -3

,6 COBT•I,STEEL -46 -1.5 -3 -28

PLATINUMCOBALT -. 5 +2.5 -9.5

SILMANAL -1.5 +25.5t -50* -. 7

BARIUM FERRITE(ORIENTED) -. 5 -. 5 -. 5 -1.7

B.ARI!UM. FEPRT? Tr -M'' . -(UNORIENTED)I +2

nvt, epicadmium 2 x 102- 5 x 1020

Temperature 0 C 23-235 -325 -325

* BOMC so reduced by previous events (irradiation or heating)that these large changes are the result of inherent in-stability of open magnetic circuit induction at low levels.

t Improvement due to heating at temperatu27e 6f annealing.

18

NOL-R 61-45

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19

NOLTR 6 -45

16

1. 3j CHROMIUM STEEL /2. 36 COBALT STEEL

123. ALNICO V1

4. ALNICO \S. ALNICO XII J4•'

6. C UNICO I / 07. CUNIFE I

8. SILMANAL

9. ES0 FINE IRON 2!10. ESD FINE IRON COBALT

II. PLATINUM COBALT C

2. BARIUM FERRITE (UNORIENTED) ;8UM13. BARIUM FERRITE (ORIENTED) (Ino

':10

6m

/0

5 2122

2400 2000 1600 1200 800 400 0

H (OERSTEDS)

Figure 1. Nominal Demagnetization Curves of the

Permanent Magnet Materials Irradiated

20

t'OLTR 61 -45

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Cd

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NOLTR 61 -45

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"* CONTROL, 3250CA

-0 EPCADMIUM ALNICO XliA

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0 BARIUM FERRITE

, (ORIENTED)

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*0 _ _ _ _SILMANAL

*0 -CUNIFE I

-100 -80 -60 -40 -20 0 20 4(0 60

PERCENT CHANGE IN OPEN CIRCUIT INDUCTION (BOMc)

Figure 3. Comparison of Percentage Changes in Open

Magnetic Circuit Induction, BOMC, for

Magnets irradiated to •-' 102O epicad nvt

at Nor-mal and High Temperatures

23

NOLTR 61 45

A {DABT

w

AA B

C',

Hc- AHT H (OERSTEDS)

a--- I -A ------

Figure 4. Method of Analysis of Changes in BOMC

Produced by Fictitious Demagnetizing Fields,IH'son Nominal. Demagnetization Cuve for

36 Cobalt Steel

24

NOLTR 61 - 45

------- BEFORE IRRADIATION 16

-OAFTER !RRADIATION

14-

- 1 36 COBALT STEEL

ALNICO V <7W

00 -40 200 400 60010 / 7H (OERSTEDS)

I I I

Figure 5. Alnico V, Alnico XII, 36 Cobalt Steel, CunifeI: Effect on Demagnetization Curves of

Irradiation with 2 x 10"8 epicad nvt at

S50OC

25

NOLTR 61 -45

0

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z 0~ 0 C C

z 1

CIQ) 0)i

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22

NOLTR 61 -45

*----. BEFORE IRRADIATION 14

O-.... OAFTER IRRADIATION /

w TU) Id

0 10-j

100

/-6

-8.

Figure 7. ESD Fine Tron, Cunj.co I, 3-112 ChromiumSteel: Efl%2ct on Deiiaagnetization Curves

! f - rradiat - on wth 2 x -0oo Eioad o0vt

at00

27

NOLTR 61 -45-9 "-

------ BEFORE IRRADIATION

o o- AFTER IRRADIATION 8/A

-4

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/

/1

1/ "5

Io j 4

/ -w

Ci)

()

Iy/0

-j

t !IiALNICO 11

-100 800 -400 -200 200 400 600

Figure 8. ESD Fine Iron-Cobalt, Alnico IT: Effect on

Demawnetization Cnurves of Irradiation with

x 101" Epicad znvt at . •rw

28

NOLTR 61 -45

---- BEFORE is -

o OAFTERqF--4--o--FOR'1

U •o 36 COBALT STEEL

ALNICO V '0U(

0

8-

ESD FINE IRON-%COALT

0I

./i / _ _ _ __,_

-- 4o 00 -6oo -2 0 200 400 600H (OERSTEDS)

/t/I ,

2-6

Figure 9. Alniwo V, 36 Cobalt Steel., ESI) Fine Iron-Cobalt: Effect of Temperature ( .5000oc)on t1 nirradiated Control Magnets

29

NOLTR 61 -45

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NOLTR 61-45

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