Radiation Physics and Chemistry 81 (2012) 57–63

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Radiation Physics and Chemistry

0969-80

doi:10.1

n Corr

Drive, C

E-m

journal homepage: www.elsevier.com/locate/radphyschem

Evaluation of potential induced radioactivity in medical productsas a function of electron energy in electron beam sterilization

Mark A. Smith a,b,n

a Sterigenics International, 10811 Withers Cove Park Drive, Charlotte, NC 28211, USAb Nuclear Engineering Teaching Laboratory, University of Texas at Austin, Austin, TX, USA

a r t i c l e i n f o

Article history:

Received 25 March 2011

Accepted 17 August 2011Available online 7 September 2011

Keywords:

Activation

Induced activity

e-beam

Sterilization

6X/$ - see front matter & 2011 Elsevier Ltd. A

016/j.radphyschem.2011.08.007

espondence address: Sterigenics Internationa

harlotte, NC 28211, USA. Tel.: þ1704 587 89

ail address: msmith@sterigenics.com

a b s t r a c t

Commercial sterilization of medical devices may be performed using electron beam irradiators at

various electron energies. The potential for activating components of the devices has been discussed,

with current standards stating that electron energy greater than 10 MeV requires assessment of

potential induced radioactivity. This paper evaluates the potential for induced activity in medical

products sterilized in electron beam as a function of the electron maximum energy. Monte Carlo

simulation of a surrogate medical device was used to calculate photon and neutron fields resulting from

electron irradiation, which were used to calculate concentrations for several radionuclides.

The experiments confirmed that 10 MeV is a conservative assumption for limiting induced radio-

activity. However, under the conditions as evaluated, which is a limited total quantity of metal in the

material being irradiated and absent a limited number of elements; the amount of induced activity at

12 MeV could also be considered insignificant. The comparison of the sum-of-fractions to the US

Nuclear Regulatory Commission exempt concentration limits is less than unity for all energies below

12.1 MeV, which suggests that there is minimal probability of significant induced activity at energies

above the 10 MeV upper energy limit.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The issue of whether radioactivity may be induced in medicaldevices irradiated in electron beams has been considered forsome time. In International Standards Organization publicationISO 11137-1, the relevant standard on radiation sterilization, theproblem is stated as (ISO, 2006)

‘‘If energy for electrons exceeds 10 MeV or energy level forelectrons used to generate x-rays exceeds 5 MeV, the potentialfor induced radioactivity in product shall be assessed.’’

However, the standard and other relevant regulations, such asthe US Food and Drug Administration regulations on food irradiationat 21 CFR 179.26, which also gives the 10 MeV electron energy limit,do not give a technical justification for imposing this energy limit. Asdiscussed in subsequent sections, most relevant publications tacitlyassume that 10 MeV is a practical limit for considering activation,which may be in part based on the most prevalent availability ofcommercial accelerators used for electron beam sterilization.

Publications from the International Atomic Energy Agency (IAEA,1995,2002) give extensive analysis of the potential activation

ll rights reserved.

l, 10811 Withers Cove Park

14.

mechanisms. From this analysis, supported by other publications,three mechanisms are considered significant for inducing radio-activity in products irradiated by electrons: photoneutron produc-tion, neutron capture, and isomeric excitation.

Other potential activation mechanisms have been studiedin other published papers, specifically photoproton production(Gregoire et al. 2003;Findlay et al., 1992). Findlay’s analysis of thephotoproton production mechanism, in particular, shows that thispathway is an insignificant contribution to the total inducedradioactivity in electron beam processing as compared to photo-neutron production and neutron absorption.

Direct activation from electron interactions, rather than frombremsstrahlung, is addressed in detail in one publication (IAEA,2002). In this analysis, the amount of radioactivity that could beexpected from the electron interactions is small in comparison tothat resulting from bremsstrahlung interactions. As a result, thefocus of this paper is on the photon interaction and excludesconsideration of direct activation by electrons.

2. Calculating induced radioactivity concentrations

As discussed in a previous publication (Smith, 2008), theinduced activity in an electron beam system is calculated forthree mechanisms: photoneutron reactions, neutron capture, andisomeric excitation.

M.A. Smith / Radiation Physics and Chemistry 81 (2012) 57–6358

2.1. Photoneutron production

The photon interaction with a nucleus can result in theejection of a neutron from the atom, leaving a resultant radio-active species. Estimating the activity produced in target atomsfollowing ejection of the photoneutron can be estimated to withina factor 10 by a simple relationship. (IAEA 2002)

Axrays ¼ 10�13DNAUfIsUlnð2Þ

AIsUT1=2ðE�EthÞ

3ð1Þ

where Axrays is the induced activity in Bqg�1; D is the delivereddose in kGy; NA is Avogadro’s number; fIs is the fraction ofphotoneutron reactions that result in radionuclide, assumed tobe 1.0 for the purpose of this analysis; AIs is the atomic weight ofthe nuclide under consideration; T1/2 is the half-life in seconds; E

is the electron energy in MeV; and Eth the threshold energy forphotoneutron production, also in MeV.

In the derivation of Eq. (1), (IAEA, 2002) the x-ray energyspectrum was evaluated from bremsstrahlung in a single element,high atomic number target, as would be used in an industrial x-rayirradiator. This energy spectrum removes the low-energy compo-nent that corresponds to the K-shell cut-off in the target material,which was estimated to give an overestimate of activity per unitdose of approximately 20%. However, in the situation consideredhere, the x-rays are produced within the irradiated product itself,which consists of materials with lower atomic number. The equa-tion was for food irradiation applications, with the compositiongiven as oxygen, nitrogen, and carbon, similar to the assumedmedical product composition. The lower energy cutoff then isrelated to the K-shell of the product material and not the target.The actual radioactivity produced per unit x-ray dose in productirradiated with 10 MeV electrons is approximately 6% lower thanwould be produced in the irradiated product with the sameintensity x-rays produced from a tungsten target. (IAEA 2002).

2.2. Neutron capture

The photoneutron production process is the source of neutrons,which are generated within the irradiated product and subsequentlyabsorbed to create additional induced radioactivity. While manyneutron-generating reactions may occur, depending on photonenergy (IAEA, 2002), the most significant source of such neutronsin the material being considered here is the 2H(g,n)1H reaction, whichhas threshold energy of 2.23 MeV. (Wakeford and Blackburn, 1991)This reaction dominates neutron production for absorption reactions,such that the contribution from other photoneutron reactions, e.g.,13C(g,n)12C and 17O(g,n)16O, do not contribute significantly to theneutron yield in considering photoneutron production from brems-strahlung in these conditions. (Stichelbaut et al., 2006)

The estimated induced activity from neutron capture in asingle element can be calculated from the equation (Lieser, 2001)

A¼Nfsð1�e�ltÞ ð2Þ

where A is the induced activity in Bq g�1; N is the number oftarget atoms per gram; f is the neutron fluence rate; s is thethermal neutron absorption cross-section; l is the decay constantfor the radioactive species and t is the irradiation time.

2.3. Isomeric excitation

The equation for calculation of induced radioactivity fromisomeric excitation is the same as Eq. (2) above, changing thefluence rate to be photons instead of neutrons and the cross-sectionto be that for the isomeric excitation reaction. As noted previously,(Smith, 2008) these cross-sections are difficult to locate in publishedliterature. For this experiment, the same isomers used in the

previous paper were considered as the most probable to occur asinduced radioactivity.

3. Description of methodology

The experimental methodology consisted of two phases:(1) mathematical modeling of the electron beam irradiationprocess as it is expected to be encountered with medical devicesterilization and (2) confirmatory measurements at two electronenergies for a given radionuclide production reaction. Because thescope was conceived as being broad, specifically evaluating thepotential for activation for the entire range of radionuclides witha production mechanism that fell within the defined electronenergy parameters, confirmatory measurements could only beconducted on a limited scale.

3.1. Calculation of photon and neutron fluence

The modeling work described below was performed withMCNPX 2.6.0 (Los Alamos National Laboratory (LANL), 2008). InMCNPX, the simulated medical device product, which was essen-tially a phantom to be irradiated, was represented by a rightprism 20 cm by 10 cm in the horizontal dimension and 10 cm inheight, comprising twenty laminar sheets of unit density poly-ethylene with 0.5 cm thickness. Natural isotopic abundances forcarbon, hydrogen, and oxygen were assumed as the compositionof the irradiated material. A titanium layer was added at thebottom of the stack, simulating a conveyor in a commercialsystem.

A directional electron beam was placed 10 cm above the topsurface of the polyethylene stack and directed perpendicular to thesurface. The typical scanning function of a commercial electronbeam irradiator was not simulated in the model. Instead, the beamwas assumed to cover the entire surface of the polyethylenephantom, which will also account for the horizontal transit of theproduct along the conveyance system under the beam.

MCNPX tallies were used to calculate the neutron and photonfluence across the surfaces of the product laminar sheets and tocalculate energy deposition at the same locations. The totalelectron plus photon energy deposition was calculated at theinterface of each laminar sheet.

The same geometry was used to evaluate different initial electronenergies. All electrons generated from the source term wereassumed to be monoenergetic, with the model being repeated forinitial electron energies of 8, 9, 10, 11, 12, and 13 MeV.

In the model, it was assumed that the photon field would onlybe present as the beam spot was passing over the surface of thephantom directly above the location of the metal foil. Inducedradioactivity would only occur during direct irradiation, but apenumbral photon field would be present immediately before andafter the beam spot passes over the foil due to scattering withinthe phantom, thereby creating fringe photon fields outside ofthe principal field. This effect has not been quantified, but couldcontribute to an underestimate of the induced radioactivityconcentration.

Potential bremsstrahlung production in the window of theelectron beam accelerator also needed to be taken into account.These windows are typically thin titanium foils through whichthe electrons pass from the vacuum into the air (IAEA and iiA,2010). Production of x-rays within the window would occur, butthe photons would not be attenuated in the window itself andwould contribute to the surface dose to the polyethylene stackand to the number photons interacting with the metal foil tocreate induced radioactivity, taking into account attenuation inthe polyethylene to the depth of maximum photon fluence.

M.A. Smith / Radiation Physics and Chemistry 81 (2012) 57–63 59

Specific values on the amount of bremsstrahlung production inthe window have not been generally published for industrialprocessing, but published values are available to show the photoncontamination of an electron beam from a 10 MeV electrontherapy device (Kase and Bjarngard, 1979). In that study, thephoton dose from the window contributed between 2% and 7% ofthe electron dose at the surface of the target. In order to assessthis effect, the MCNPX model geometry was established both withand without inserting the thin window into the electron beamand the calculations run separately for each situation. At thedepth within the phantom where the maximum photon fluenceoccurs, the contribution of bremsstrahlung produced in thewindow is 2% or less of the total photon fluence at that location,depending on the initial electron energy.

3.2. Calculation of induced radioactivity

After running the model for each energy, the results werecompiled into a series of spreadsheets to evaluate distributions ofenergy, fluence, and dose within the phantom. To calculate thepotential activation for a variety of metals in the conditions of thestudy, a spreadsheet was set up to use the Section 2 equations tomake manual calculations based on the parameters for eachradionuclide. The radionuclides considered in the calculationsfor isomeric excitation and neutron capture are the same asconsidered in the author’s previous publication (Smith, 2008).The list of radionuclides considered for the photoneutron produc-tion reaction was taken from IAEA-TECDOC-1287 for the respec-tive values of Eth (IAEA, 2002).

The induced radioactivity due to photoneutron production andisomeric excitation was calculated using the maximum photonfluence and dose rate within the phantom for the given initialelectron energy. The depth at which that occurred was dependenton the electron energy and was determined by the model results.Doses at that point in product were calculated based on a 25 kGysurface dose to the phantom. Energy distribution of the photonfluence was calculated within the MCNPX model. For neutroncapture reactions, MCNPX was used to directly calculate theneutron fluence and the associated energy distribution resultingfrom the 2H(g,n)1H reaction.

3.3. Confirmatory measurements

Two data points were selected for making confirmatorymeasurements. The electron energies of 10 MeV and 12 MeVwere chosen because accelerators with that energy specificationare operated by Sterigenics International in Bridgeport, NewJersey, San Diego, and California, with those electron energies,which made them accessible for irradiations as required. The SanDiego accelerator is a linear accelerator originally manufacturedin the 1960s, while the Bridgeport accelerator is a RhodotronTM

TT300 developed commercially by Ion Beam Applications andinstalled in the Bridgeport facility in 2000.

The experimental protocol was established to reproduce theconditions of the MCNPX geometry to the extent possible. Unitdensity polyethylene laminar sheets were assembled at eachlocation with Far West Technology FWT-60 radiochromic filmdosimeters interspersed at depths in the product. This simulatedproduct was processed so as to deliver a 25 kGy surface dose,after which the dosimeters were retrieved and read to evaluatethe depth dose distribution.

The same simulated product was processed a second time,with a platinum foil inserted into the stack at the depth ofmaximum photon fluence as calculated through the MCNPXmodel. Platinum was selected because it offered the best balanceof a relatively high potential activation concentration, isotopic

half-lives that were long enough for overnight shipment to acommercial radiochemistry laboratory, and adequate detectorefficiency in gamma spectroscopy to present a high probabilityof being detected and quantified.

Following irradiation of the activation foil, a preliminarymeasurement of the gross beta-gamma activity was made usinga pancake GM detector coupled to a scaler. This evaluation alsoserved as a check that activation had occurred in the foil. Uponarrival at the commercial laboratory, the foils were analyzed withgamma spectroscopy using an HPGe detector.

4. Results and interpretation of data

4.1. Depth dose distribution

MCNPX was used to calculate the total energy deposition fromboth electrons and photons within the laminar polyethylene sheetsfor comparison against dosimetry measurements in the irradiators.The depth dose curves as calculated followed the general patternexpected for electron beam facilities, with the maximum doseappearing within the product stack and the difference between thesurface and maximum doses being a function of the electronenergy. Fig. 1 depicts the depth dose curve as calculated usingMCNPX under the conditions defined for this research. Thecalculated absorbed dose for all energies has been normalized toa 25 kGy surface dose.

The titanium conveyor simulation included in the model wasirrelevant for the depth dose distribution. As intended in definingthe geometry, all of the electrons were absorbed in the phantommaterial prior to reaching the depth of the titanium layer,resulting in no contribution to the photon fluence from interac-tion with that material.

4.2. Photon fluence distribution

In order to calculate the maximum potential induced activityconcentration for the photoneutron production reaction, themetal being irradiated should be located at the location wherethe photon dose is greatest. MCNPX was used to calculate thephoton fluence averaged over the surface of each laminar sheet ofpolyethylene. The photon fluence increased with depth in thepolyethylene to the point of maximum fluence corresponding tothe increased bremsstrahlung production tracking the depth dosecurve. Photon fluence reduces from the maximum according to anexponential absorption function typical for photon shielding. Thisis shown in Fig. 2.

Note that the maximum photon fluence, which was used forthe induced activity calculation, is determined from the brems-strahlung production in the forward direction. Backscatter fromthe titantium plate that simulated the conveyor did not appear tohave any contribution to the maximum photon fluence.

4.3. Neutron fluence

In previous publications (Smith, 2008), thermal neutrons result-ing from the photoneutron reaction with deuterium in polymersthat comprise the majority of a medical device composition wereconsidered as potential contributors to induced radioactivity. Inthese modeling results, however, the neutron fluence from thisreaction was negligible for all electron energies evaluated. Minimalneutron fields, with fluence values six orders of magnitude lowerthan photon fluence at the same depth in the phantom, were foundwith 13 MeV initial electron energy. For all other electron energies,no deuterium photoneutrons were generated above the cutoffthreshold as set in MCNPX. In considering deuterium photoneutron

Fig. 2. Calculated depth distribution of photon fluence within a unit density polyethylene phantom.

Fig. 1. Calculated depth dose distribution within a unit density polyethylene phantom.

M.A. Smith / Radiation Physics and Chemistry 81 (2012) 57–6360

production in x-ray irradiators, this reaction does appear to besignificant, (Stichelbaut et al., 2006), but according to the conditionsestablished for this model, the bremsstrahlung conversion efficiencyfrom electron irradiation of polyethylene does not generate photonfluence of sufficient quantity for deuterium photoneutrons to havean impact on induced radioactivity in the simulated product.

4.4. Calculated activation product concentrations

The results of the MCNPX model were used to calculate theestimate of induced activity in Bq/g for a variety of elements. Thelist of elements that was used for the initial evaluation was thesame as shown in a previous publication (Smith, 2008), which iscomposed of naturally occurring nuclides that would becomeradioactive through neutron capture, loss of a neutron in aphotoneutron production reaction, or isomeric excitation.

For the purposes of the evaluation and to ensure that confirma-tory measurements represented the same configuration as themathematical model, each element was assumed to be present inthe form of a square foil 25 mm on a side and 0.025 mm thick. The

density of each nuclide was incorporated into the calculation of doseto the element for the purpose of calculating induced radioactivityusing Eq. (1) and for the calculation of the number of atoms presentin the sample for the purpose of calculating induced radioactivityfrom neutron capture and isomeric excitation.

Based on the lack of neutron generation from the deuteriumphotoneutron reaction at electron energies of 13 MeV or less andthe even fewer number of photoneutrons generated from theamount of material present in the foil, induced activity from theneutron capture reaction was inconsequential. In comparison toinduced activity from the photoneutron production reaction ineach foil, the concentration of induced activity from the neutroncapture reaction can be considered zero.

To normalize the calculated concentrations to a quantity thatis related to a relative risk value, the concentrations were dividedby the US Nuclear Regulatory Commission (NRC) exempt con-centrations as given in 10 CFR 30.70, Schedule B (US NRC, 2008).These concentrations are the allowable amounts of radioactivitythat may be present in objects distributed to members of thegeneral public, who are not radiation workers. While the exempt

Table 2Radionuclides with high ratios of calculated specific activity to exempt

concentration.

Nuclide T1/2 Ratio of calculated specific activity to exempt concentration

M.A. Smith / Radiation Physics and Chemistry 81 (2012) 57–63 61

concentrations do not directly relate to risk to the individual, theywere originally based on a pathways analysis dose reconstructionthat does have a connection to relative risk to individuals.

The NRC exempt concentrations are defined to be the concen-tration of the given radionuclide in the object under considera-tion. For example, the exempt concentration would apply to theamount of induced radioactivity in a gemstone divided by theweight of the gemstone itself. Calculations as done for this studyresult in a value that is the amount of radioactivity induced in themass of the metal irradiated and not of the mass of the entiredevice. For the purposes of comparison, a mass concentration of0.1% of the given metal in the total mass of simulated medicaldevice was assumed in order to make a direct comparison to theNRC exempt concentrations.

For isomeric excitation, the calculated concentration of eachradionuclide is shown in Table 1. Because there are no exemptconcentrations given by the NRC for these specific radionuclides, thegeneral concentration limit of 1�10�6 mCi g�1 (3.7�10�2 Bq g�1)would apply. All of the calculated concentrations are less than 4% ofthe relevant NRC exempt concentration.

In photoneutron production, some induced radioactivity wasfound for all electron energies between 8 and 13 MeV, inclusive.At the lower energy, only a few radionuclides were produced indiscernable quantities, specifically 180Ta, 189mOs, and 185W. As theelectron energy increased, additional radionuclides were pro-duced as a result of exceeding more photoneutron thresholdenergies. However, the concentrations of 180Ta and 189mOs,relative to the exempt concentrations, remained the most sig-nificant contributors to the amount of induced radioactivity. Fig. 3

Table 1Ratio of calculated activity concentration (Bq g�1) by isomeric excitation.

Nuclide 115mIn 176mLu 87mSr 135mBa 195mPt 117mSn 123mTe

Half-life 4.486 h 3.635 h 2.803 h 28.7 h 4.02 d 13.6 d 119.7 d

8 MeV 9.4E-04 1.4E-04 2.9E-05 2.1E-05 1.3E-05 7.5E-08 4.9E-09

9 MeV 1.0E-03 1.6E-04 3.2E-05 2.3E-05 1.4E-05 8.3E-08 5.5E-09

10 MeV 1.2E-03 1.8E-04 3.6E-05 2.5E-05 1.6E-05 9.2E-08 6.1E-09

11 MeV 1.3E-03 2.0E-04 3.9E-05 2.8E-05 1.7E-05 1.0E-07 6.7E-09

12 MeV 1.4E-03 2.1E-04 4.3E-05 3.0E-05 1.9E-05 1.1E-07 7.2E-09

13 MeV 1.4E-03 2.2E-04 4.4E-05 3.2E-05 2.0E-05 1.2E-07 7.6E-09

Fig. 3. Ratio of induced radioactivity concentration calculated for photoneutron produc

depicts the range of radionuclides included in these calculations,graphically showing those with the highest ratio to the respectiveNRC exempt concentration.

Table 2 specifically identifies the nuclides that have the highestratio of calculated specific activity to the exempt concentrations. Forthe purpose of this comparison, the ratio used a specific activitycalculation rather than a concentration, which would be directlycomparable to the NRC regulatory limits. That is, the calculatedvalue compared to the exempt concentration was the inducedactivity in the foil being irradiated divided by the mass of the foil,as opposed to being divided by the total mass of the device. Theregulations are designed to address the latter, which is taken intoaccount in the activation index evaluation discussed later.

Note that the calculations used a half-life cutoff of two hours.That is, any induced radionuclide with a half-life of two hours orless was not included in the calculated induced radioactivityconcentrations. While there is no minimum half-life specified inthe NRC exempt concentration schedule, two hours was consid-ered to be the shortest half-life that might result in some radio-activity still being present at the earliest time a medical devicemight reasonably be expected to be available for patient use,based on defined exposure pathways (Smith, 2008).

tion in electron beam irradiation to the respective US NRC exempt concentration.

8 MeV 9 MeV 10 MeV 11 MeV 12 MeV 13 MeV

180Ta 8.15 h 4.45 39.8 186 611 1600189mOs 5.8 h 9.40 48.4 170 474165Er 10.36 h 1.68 13.1 58.2 191141Nd 2.49 h 2.71 27.7 149195mPt 4.02 d 2.18 7.93 22.7193mPt 4.33 d 1.33 5.67 18.0132Cs 6.479 d 1.14 6.89 26.344Sc 3.927 h 3.37 70.0121Sn 27.06 h 2.63 9.48168Tm 93.1 d 1.50 4.41185W 75.1 d 1.14 2.85107Cd 6.50 h 1.13184Re 169 d 1.08

Fig. 5. Comparison of calculated versus measured depth dose distribution for

10 MeV initial electron energy.

M.A. Smith / Radiation Physics and Chemistry 81 (2012) 57–6362

4.5. Approximation of potential induced radioactivity

For any single medical device, there is almost no likelihood thatall the elements used in these calculations would be present.However, to approximate an estimate of the probability of inducingradioactivity in an irradiated medical device, the sum of the ratios ofthe calculated concentration to the NRC exempt concentration wasused. The term activation index was coined to indicate that, whilethis value represents the total possible induced radioactivity to beconsidered under the exempt concentration regulations and is anindicator of a probability of induced radioactivity occurring, theactual value does not precisely determine either.

The activation index was calculated as a function of the initialelectron energy for each of the conditions considered above.Graphically shown in Fig. 4, the activation index follows anexponential function. Under the regulations, for the final objectto be considered to be less than the exempt concentration, thesum of ratios as defined above must be less than one. Use a least-square curve-fit function for the graph in Fig. 4, the sum of ratioswould be equal to one at an initial electron energy of 12.1 MeV.

Fig. 6. Comparison of calculated versus measured depth dose distribution for

12 MeV initial electron energy.

4.6. Confirmatory measurements

At 10 and 12 MeV, measurements were made to determinewhether the model as presented is a relatively accurate depictionof the process for inducing radioactivity in an irradiated metal.Platinum foils were used in irradiations at two different facilities,with two measurements made for each irradiation: an initialevaluation of total induced radioactivity by gross beta-gammameasurement and a quantitative analysis of the amount of aspecific isotope induced during the irradiation.

Figs. 5 and 6 show the measured depth dose distribution in thepolyethylene stack as compared to the calculated depth dose. Themeasured total energy deposition, which included the contribu-tions from both electrons and photons, agreed with the calculatedvalue to within 4% at all depths except at 5 cm for the 10 MeVcalculation. At that depth for that energy, the total dose wascalculated at 0.8 kGy, which is below the lower end of thecalibration range for the dosimeters used. The measured dose of1.3 kGy represented a 66.7% difference from the calculated value,apparently primarily due to the shape of the dosimeter calibrationcurve at that point.

Fig. 4. Sum of ratios of induced radioactivity concentration as a function of the

initial electron energy.

Fig. 7. Comparison of gross beta–gamma measurement of activation immediately

following irradiation to the calculated concentration of platinum isotopes.

Fig. 7 shows the comparison between the gross beta–gammameasurement and the calculated concentration of all platinumisotopes. The dominant isotope in terms of activity immediately

Fig. 8. Comparison of gamma spectroscopic measurement of activation to the

calculated concentration of the dominant platinum isotope.

M.A. Smith / Radiation Physics and Chemistry 81 (2012) 57–63 63

following irradiation was 195mPt, but that isotope has a relativelyshort half-life and was consequently decayed to undetectablelevels by the time the foil was transported to a gamma spectro-scopy laboratory.

With the transportation time to the laboratory, the dominantisotope still present in the foil at the time of analysis was 197Pt.Fig. 8 shows the measured concentrations of that isotope in thefoils compared to the calculated value that was expected.

For both measurements, the model predicts a somewhat lowerconcentration that was found in the measurements. For the grossmeasurement immediately following irradiation, the differencebetween the measurement value and the calculated concentra-tion was approximately 20% for both electron energies. Withspectroscopic analysis, the predicted activity of 197Pt was 50% to60% greater than that measured. This discrepancy appears to berelated to the penumbral effect of photons immediately beforeand after the time required for the electron beam spot to passover the irradiated metal foil.

5. Conclusions

The surrogate for a medical device being sterilized in electronbeam was assumed to be a block of polyethylene, with smallamounts of metal imbedded. MCNPX was used to calculate thebremsstrahlung production within the phantom and subse-quently the radiation interactions that would result in inducingradioactivity in metals. Based on the model, the overwhelminglydominant activation mechanism at initial electron energies below13 MeV is the photoneutron production reaction within metalcomponents. Between 12 and 13 MeV, the photoneutron produc-tion reaction in deuterium begins to create a field of neutrons thatcan also induce radioactivity in the metal components. However,the amount of activation by this reaction at incident electronenergies of 13 MeV or lower is insignificant in comparison to thephotoneutron production reaction in the metals themselves.

When normalized to a surface dose of 25 kGy and compared asa ratio to the US NRC exempt concentrations, the activation limitof 10 MeV as given in the ISO 11137 standard was shown to be aconservative assumption. Within the parameters of this experi-ment, there is only a small probability that 10 MeV electron

irradiation would produce significant quantities of induced radio-activity during sterilization of medical devices.

Under the conditions of this experiment, the amount ofinduced radioactivity actually remains below the exempt con-centration limitations at initial electron energies up to 12.1 MeV.This implies that, under conditions where the total amount ofmetal in a device is limited and certain susceptible metals areabsent, the electron energy may be higher than 10 MeV withoutcreating significant induced radioactivity.

Many of the elements represented in the induced activity inthese calculations are not commonly encountered in medical devicemanufacturing, although there is some limited probability that theymay be. As such, this evaluation only serves as an estimator of thegeneral probability that activation may occur during electron beamsterilization, and not as the estimate of induced radioactivity that ispresent in any given product processed at a specific irradiator. Eachelectron beam irradiator would need to evaluate the types ofproducts being processed to verify that the composition assump-tions as stated above are valid. If not, the conclusion that there is nosignificant induced radioactivity in sterilization with electron energyup to 12 MeV may not be valid.

Acknowledgments

The Sterigenics International electron beam facilities in SanDiego, California, and Bridgeport, New Jersey, irradiated the materi-als used in confirmatory measurements. Vital assistance was pro-vided by Don Braid, Randy Smith, Vicki Udvarheyli, and Carl Zinn ofthese two facilities. This work was undertaken in cooperation withthe Nuclear Engineering Teaching Laboratory of the University ofTexas at Austin. Faculty and staff members that provided valuableassistance in reviewing drafts and providing suggestions for compil-ing the final version were Sheldon Landsberger, Steven Biegalski,Erich Schneider, Mark Raizen, and David Hearnsberger.

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