Optically stimulated luminescence of ultra-high molecular weight polyethylene: A study of dosimetric features

Accepted Manuscript Optically stimulated luminescence of ultra-high molecular weight polyethylene: A study of dosimetric features M.L. Chithambo, J.M. Kalita PII:

S1350-4487(17)30834-X

DOI:

10.1016/j.radmeas.2018.06.006

Reference:

RM 5928

To appear in:

Radiation Measurements

Received Date: 6 December 2017 Revised Date:

21 May 2018

Accepted Date: 2 June 2018

Please cite this article as: Chithambo, M.L., Kalita, J.M., Optically stimulated luminescence of ultra-high molecular weight polyethylene: A study of dosimetric features, Radiation Measurements (2018), doi: 10.1016/j.radmeas.2018.06.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Optically stimulated luminescence of ultra-high molecular weight polyethylene: A study of dosimetric features M.L. Chithambo1, J.M. Kalita

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Department of Physics and Electronics, Rhodes University, P O Box 94, Grahamstown 6140, South Africa

Abstract

We report the dosimetric features of ultra-high molecular weight polyethylene (UHMWPE)

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using luminescence optically stimulated using 470 nm blue light. Samples irradiated to between 1 and 1000 Gy produces luminescence that increases with irradiation dose to

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produce a linear dose response between 1 and 1000 Gy. The sample was determined not to be affected by pre-dose in tests using a pre-dose of 4000 Gy. This characteristic precludes the need for elaborate background erasing routines typical of dosimetry experiments. The signal has good reproducibility. We used this property to test recovery of ‘unknown’ doses with encouraging results. It was observed that luminescence can also be stimulated using 870 nm infrared light. The dose response, fading, pre-dose effect and the ability to optically stimulate

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luminescence from the polymer is discussed in terms of curing involving free-radicals.

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Keywords: OSL; UHMWPE; dosimetry; free-radicals

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Corresponding author. Tel.: +27-60308450; fax: +27-6225049.

E-mail address: [email protected] (M.L. Chithambo)

ACCEPTED MANUSCRIPT 1. Introduction The use, for dosimetry, of materials intentionally or unintentionally found on one’s person is an area of growing research interest and one can list a few examples. Fielder and Woda (2011) reported the feasibility of using inductors extracted from circuit boards in a mobile phone for accident dosimetry using thermoluminescence. The components contain whose

response

to

ionising

radiation

provided

thermoluminescence to assess its dosimetry characteristics.

a

means

for

using

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alumina

Fielder and Woda (2011)

observed a linear response for doses from 100 mGy to 5 Gy. Shalom et al. (2011) studied the dosimetric characteristics of optically stimulated luminescence of various materials including

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business cards, buttons and finger nails (the latter obtained from willing subjects). Despite these materials not being necessarily crystalline, they produced OSL whose dose response was linear for irradiation up to 3 Gy pointing to a possible use in personal dosimetry.

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We report optically stimulated luminescence from ultra-high molecular weight polyethylene (UHMWPE). The physical and chemical properties of this thermoplastic make it suitable for use in prosthetics (Wang et al., 2006). The resistance of UHMWPE to high impact is due to its linear and branched molecules which make up the collinear backbones (Bower, 2002). Before any medical use, UHMWPE is sterilized by various means including

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exposure to electron beams or to high dose gamma irradiation (Jahan et al., 1998; Wang et al., 2006). However, the irradiation cleaves the C-C and C-H backbone bonds leading to creation of free radicals (chemical groups with an unpaired electron). The free radicals form both on the surface and in the core (Jahan et al., 1998). The initial free radicals produced

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following irradiation consist mainly of allyl (–CH2–CH=CH–CH●–CH2–) and alkyl (–CH2– CH●–CH2–) species although polyenyl types (–CH2–(–CH=CH–)nCH●–CH2–) may also exist

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on the surface (Jahan et al., 2001). Apart from medical use, UHMWPE, in the form of composite plates, is used in bullet-proof vests. In addition, UHMWPE is resistant to puncture and as such is embedded in protective wear in fencing. The material is therefore of potential use as a dosemeter in cases involving unintended and unexpected exposure to radiation. UHMWPE is only partially crystalline. In terms of mass, the degree of crystallinity

=

/(

+

) where

and

are the masses of the crystalline and non-

crystalline zone (Bower, 2002) has, for UHMWPE, been reported to be 50% (Wang et al., 2006). Free-radicals in either zone may be involved in reactions to form cross-links between polymer molecules. However, free radicals not involved in such curing processes in the crystalline regions remain stable whereas those in the amorphous parts, where oxygen can diffuse, undergo oxidation (Wang et al., 2006). The wear that is induced by the oxidation 2

ACCEPTED MANUSCRIPT may be minimised by annealing the material after irradiation. The heating, usually to below 200 oC, that is below melting temperatures, facilitates free-radical recombination reactions and prevents the said undesirable effects of oxidation e.g. (Jahan et al., 2001). Although the primary purpose of partial heating is to cure the dislodged chains, it has long been known that the process can also produce thermoluminescence (Charlesby and Partridge, 1965). The

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interpretation of the thermoluminescence (TL) in terms of standard mechanisms built for crystalline solids poses particular challenges given that UHMWPE is non-crystalline. Nevertheless, in previous studies the emission of TL in UHMWPE was ascribed to various processes such as molecular reactions (Wintle, 1974) or electron-hole recombination (Nehate

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et al., 1993).

The UHMWPE studied in this work is a particular thermoplastic obtained from Poly Hi Solidur (Germany).

This is the same material investigated previously under TL

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(Chithambo, 2006; 2012) and also by phosphorescence (Chithambo, 2008). In these studies, the aim was to exploit the efficacy of TL in sensing changes in defect concentrations which for UHMWPE was deduced to consist of reactive free radicals. These studies, particularly the phosphorescence measurements showed that luminescence could ensue from the polymer without any need for dynamic heating. The sample studied deliquesces between 150 and

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200oC (Chithambo, 2008), a feature that is thought to affect its TL properties. Therefore any optically stimulated luminescence from the material offers an attractive option worth investigating.

The aim of this paper is to explore the possible use of UHMWPE as a material for

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dosimetry using optically stimulated luminescence.

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2. Experimental details

Samples of UHMWPE prepared into discs of 1 mm thickness and 5 mm diameter

were used. The luminescence was optically stimulated in a steady-state mode whereby the light-source is maintained at a constant intensity and the luminescence is detected in the presence of scattered stimulation light, the two being discriminated by transmission filters. These CW-OSL measurements were made using the RISØ TL/OSL DA-20 Luminescence Reader (Bøtter Jensen et al. 2010) on samples irradiated using a 90Sr/90Y beta source at a dose rate of 0.10 Gy/s. Unless otherwise stated, luminescence was stimulated using a set of 470 nm blue LEDs providing a combined optical power density of 40 mW/cm at sample position.

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ACCEPTED MANUSCRIPT The luminescence was detected by an EMI 9235QB photomultiplier tube through a 7 mm Hoya U-340 filter (transmission band 250–390 nm, FWHM).

3. Results and discussion 3.1 OSL decay curves measured under 470 nm blue light stimulation

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Fig. 1 shows examples of decay curves corresponding to irradiation to 40, 400 and 1000 Gy measured at room temperature. The background signal is included for comparison. The luminescence was measured for 20 s, but to better illustrate the decay, the plot is only shown for the first 5 s.

It is observed that the sample produces optically stimulated

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luminescence whose intensity increases with irradiation dose.

The luminescence intensity decreases promptly to background level within the first 5 s. In order to assess the influence of dose on the decay curve, the data was fitted by a set of

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arbitrarily chosen exponential functions. The fit is not based on any kinetic model. The decay curves could be fitted by a sum of two exponential components as ( )=

exp(− ) +

exp(− )

(1)

where a and b are decay constants (a > b) and A and B are scaling parameters. Fig. 2 shows an example of a fit for a decay curve measured after irradiation to 1000 Gy. The transient

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nature of the luminescence (meaning not stable in time) can be further demonstrated by plotting the ratio of each component to the sum of the two components against the time of optical stimulation in the same way as done for quartz previously (Chithambo and Galloway, 2001). This is shown in the inset to Fig. 2 where the components corresponding to a and b

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are arbitrarily labelled as C1 and C2 respectively. Initially the percentage proportion of components 1 and 2 are 69% and 31% respectively. Within the first 1 s, the contribution of

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the component C1 decreases to an insignificant percentage of the total and component 2 increases in importance to 98%.

3.2 Dose response

The luminescence intensity increases with dose as is apparent in Fig. 1.

The

dependence of the luminescence on dose was studied for a larger range of dose up to 1000 Gy. Fig. 3 shows the dose response for which each data point corresponds to the area under the decay curve within the first 5 s. Each such data point is an average from three identical measurements with the margin of error being the standard deviation for each set. The data shown was corrected for background. Fig. 3 shows that the intensity of optically stimulated luminescence increases at a constant rate with dose as evident by the linear fit through data. 4

ACCEPTED MANUSCRIPT The relevant point here is that the linear dose response is a desirable characteristic for a potential dosimeter.

3.3 Effect of pre-dose It is known that when some materials are heated above an ‘activation’ temperature

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after being pre-exposed to irradiation, the sensitivity of any subsequent OSL or TL is affected. There are numerous exemplars such as OSL of porcelain e.g. (Hübner and Göksu, 1997) or TL of synthetic quartz (Chithambo and Niyonzima, 2014). For the example of quartz, this change in sensitivity is ascribed to accumulation of holes thermally transferred

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from a non-radiative to radiative recombination centres (Chen and McKeever, 1997).

The influence of pre-dose on OSL from UHMWPE may be of interest for dosimetry and was therefore studied by pre-exposing the sample to a dose of 4000 Gy. After the pre-

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dose, the sample was reset by measuring OSL for 20 s. The dose response was again built up from 1 to 1000 Gy. Fig. 4(a) shows examples of OSL decay curves corresponding to the doses between 1 and 1000 Gy. Fig. 4(b) shows the background counts before and after the pre-dose showing that the background is independent of any pre-dose. The decay curves of the sample after pre-dose are similar to as those of the sample used before any pre-dose. This

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implies that the sample does not accumulate charge to any significant extent even after preirradiation to 4000 Gy. If the sample is used for dosimetry, then its radiation history is irrelevant. This is an important feature of the material. Fig. 5 shows the dose response of the sample after a pre-dose of 4000 Gy with the

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intensities determined as before and corrected for background. The solid line drawn through the data points indicates that the intensity increases at a constant rate throughout the doses

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between 1 and 1000 Gy. The dose response of the sample after the pre-dose is similar to that measured before the pre-dose. This implies that the sample shows similar behaviour even after pre-exposure to an extensively high dose.

Any fine change due to pre-dose was

examined by plotting the ratio of the intensities measured as the area under the decay curves within the first 5 s before the pre-dose to that after the pre-dose against the irradiation dose. This is shown in the inset to Fig. 5. No effect would be indicated by a ratio equal to 1 and independent of dose. However, there is a slight shift in the ratio from 0.4 to 0.8 within margins of error when the dose is changed from 1 to 1000 Gy. Thus although there is some effect on the intensity due to pre-dose, it is negligible. The decay curve could also be described by a sum of two exponentials. Interestingly, the proportions of C1 and C2 initially were 59 and 41% each with this changing to 0.5 and 99.5% after 1 s. 5

ACCEPTED MANUSCRIPT 3.4 Fading Fading of signal with delay between irradiation and measurement has been observed in UHMWPE previously using TL where it was noted that the TL intensity decreased exponentially with time (Chithambo, 2012). The same effect was studied under OSL and the results are shown in Fig. 6. The inset figure shows that the intensity does not follow any

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power law or exponential behaviour. Here the sample was irradiated to 400 Gy and OSL was measured for 20 s after delays of up to 18000 s between irradiation and measurements. The OSL intensity fades with time decreasing by 27 % within the first 600 s and by 70% after

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18000 s.

3.5 Reproducibility

Fig. 7 shows the results of measurements made to investigate the reproducibility of

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the OSL in ten identical measurements carried out on a sample after irradiation to randomly chosen doses of 30, 100 and 600 Gy. In each case, the OSL was measured for 20 s and the intensity calculated as the area under each decay curves for the first 5 s. For irradiation to 30, 100 and 600 Gy, the statistical coefficient of variation (CV) for the 10 identical runs was each found to be 8.4, 4.0 and 2.0 % respectively. The results show that the material properly

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replicates its response under identical experimental conditions. The decrease in CV with dose indicates that the reproducibility improves with dose.

3.6 Dose recovery test

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The preceding discussions have shown that the OSL dose response of UHMWPE is linear between 1 and 1000 Gy, that any effect due to pre-dose is negligible and that the The sample was thus used in a blind test to identify an

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reproducibility is acceptable.

‘unknown’ dose in a dose recovery test. In the measurements, a sample was irradiated, in turn, to three different ‘unknown’ doses denoted d1, d2 and d3 and the OSL measured immediately after the irradiation under identical experimental conditions. The intensity corresponding to each of the ‘unknown’ doses was calculated as the area under the decay curve for the first 5 s and corrected for background. The corresponding intensities were then interpolated onto the dose response curve in Fig. 5 and used to extrapolate to the corresponding dose on the ‘x’ axis. Fig. 8 shows the dose response and the solid horizontal lines are each drawn for intensities corresponding to the doses d1, d2 and d3. The dotted lines enveloping each solid line indicate the margin of error for each data point. The margins (dotted lines) were estimated from the experimentally measured intensity using the square6

ACCEPTED MANUSCRIPT root rule for counting experiments. The lines corresponding to the unknown doses d1, d2 and d3 intersect the dose response curve giving x-coordinates of 28.5, 152.0 and 496.5. Taking into account the margin of error, the ‘unknown’ doses d1, d2 and d3 were estimated as (25 ± 5) Gy, (152 ± 11) Gy and (498 ± 14) Gy respectively. The test doses used to generate these unknown values were 25, 150 and 500 Gy respectively. The dose recovery

UHMWPE in dosimetry is thus encouraging.

3.7 OSL measured under 870 nm infrared light stimulation

The potential for use of

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test produces results meaningfully close to the actual dose.

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It was found that OSL could also be stimulated using infrared stimulation in measurements made at room temperature using a set of 870 nm infrared LEDs set at 130 mW/cm2 optical power density. The luminescence was detected from the same sample used

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earlier within 330–470 nm using a combination of BG39 and BG3 filters. The dose response between 1 and 1000 Gy is shown in Fig. 9 and an example of a decay curve is included as an inset. The luminescence intensity was found in this case to be slightly less than that for blue light stimulation. This may be due to change of either detection filter or stimulation wavelength. The OSL was measured for 20 s but as for blue light stimulation, the signals

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decreased to background level in 5 s. The decay curve could also be resolved into a sum of two exponential components.

4. Discussion

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The aim of this work is to examine the dosimeric features of luminescence optically stimulated from UHMWPE. Although the mechanisms involved in the emission are not

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immediately obvious, the emission of luminescence may be explained along the same lines used to account for TL in the same sample (Chithambo, 2012). We now discuss possible causes for the dose response as observed, fading, pre-dose effect and ability to stimulate luminescence from the polymer. As with TL (Chithambo, 2012), luminescence was only observed from irradiated

samples.

The emission of luminescence in polyethylene was ascribed to electron-hole

recombination (Nehate et al., 1993) and in some cases to recombination of free radicals (Jahan et al., 1998). Indeed, using electron spin resonance, Jahan et al. (1998) were able to associate alkyl and allyl free radicals to particular glow peaks in UHMWPE. It seems plausible to assume that the dose response of Figs. 3, 5 and 9 are caused by an increase in OSL intensity brought about by a corresponding increase in the concentration 7

ACCEPTED MANUSCRIPT of free radicals due to irradiation. We assume that the free radicals responsible for the emission are produced in the same way as explained before, namely, when bonds get severed by beta irradiation as seen in polyethylene (Nehate et al. 1993) or in particular when C–C and C–H backbone bonds rapture as in gamma irradiated UHMWPE (Jahan et al., 2001). Identifying the actual free radicals might need use of ESR but this was not our concern.

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The emission of thermoluminescence in UHMWPE e.g. (Chithambo, 2012; Nehate et al., 1993) is a fortuitous side-issue to the fact that heating allows free radical recombination to occur (Jahan et al., 2001). However, phosphorescence, the emission of TL at a constant temperature, has also been observed in UHMWPE (Chithambo, 2006; 2012). Using the

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presence of phosphorescence in UHMWPE, Chithambo (2012) deduced that free radical recombination in UHMWPE does not require only a dynamic temperature change since the emission in this case is observed at a constant temperature. Indeed, the activation energy was

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determined as (0.76 ± 0.05) eV for the same material studied here (Chithambo, 2012) and as 0.70 eV for a low linear-density polyethylene (Nehate et al., 1993). Given this low activation energy, we adduce that heating and optical stimulation, as in this study, only catalyse the recombination process.

Optical stimulation releases unpaired electrons and the species

created, being unstable, immediately recombine or form other free radicals which are then

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involved in recombination This explains the ability to optically stimulate luminescence in UHMWPE by 470 nm blue or even 880 nm infrared light. As was explained for thermoluminescence (Chithambo, 2012), the luminescence emitted under optical stimulation can be related to the energy of recombination at the

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termination step of the free radical reaction leading to recombination. It is possible that the emission of luminescence can also be accounted for by other means including conversion of

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free radicals from one form to another or molecular reactions but these require techniques from radiation chemistry which are outside the scope of this work. As regards the pre-dose effect, the lack of any significant change of sensitivity after a

pre-dose means that charge produced after irradiation does not accumulate in the material. In terms of our explanation, the pre-dose does not leave any residual free radicals after the predose. Previous studies on UHMWPE (Chithambo, 2012) and on X-irradiated low-density polyethylene (Markiewicz and Fleming, 1988) showed that there was no TL from unirradiated samples meaning that there were no residual free radicals after irradiation. Absence of any OSL after pre-dose or from unirradiated samples can be explained in the same way.

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ACCEPTED MANUSCRIPT The OSL was observed to fade with delay between irradiation and stimulation at room temperature. Since the evidence discussed suggests that free-radical recombination does not require any heating or optical stimulation and is only aided by it, recombination of freeradicals prior to any stimulation will reduce their concentration and thus account for the fading observed. For discussion of polymerisation and other processes involving free radical

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reactions, the reader is directed to standard texts e.g Bower (2002).

5. Conclusion

We have reported the optically stimulated luminescence of ultra-high molecular

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weight polyethylene (UHMWPE) and its possible utility for OSL dosimetry. Irradiated samples produce transient luminescence under 470 nm blue or 870 nm infrared stimulation. The dose response in both cases is linear between 1 and 1000 Gy. The material is unaffected

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by pre-dose as determined in tests using pre-irradiation to an extensively large dose of 4000 Gy. A study carried out to estimate ‘unknown’ test dose of 25, 150 and 500 Gy retained encouragingly meaningful values of (25±5) Gy, (152±11) Gy and (498±14) Gy. This study shows that UHMWPE has potential for use in dosimetry.

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Acknowledgement

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We acknowledge with gratitude funding from Rhodes University and the National Research Foundation of South Africa.

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ultra-high molecular weight polyethylene. J. Phys. D: Appl. Phys. 45, 345301 Chithambo, M.L., 2008. Phosphorescence of orthopaedic-grade ultra high molecular weight polyethylene. Phys. Stat. Sol. (c)-Current topics in solid state physics. 5, 871–874. Chithambo, M.L., 2006. Orthopaedic-grade ultra high molecular weight polyethylene: Some features of the main thermoluminescence glow curve. Radiat. Prot. Dosim. 119, 157– 160.

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Chithambo, M.L., Galloway, R.B., 2001. On the slow component of luminescence stimulated from quartz by pulsed blue light-emitting diodes. Nucl. Instrum. Methods. B. 183, 358– 368.

Chithambo, M.L., Niyonzima, P., 2014. On isothermal heating as a method of separating

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closely collocated thermoluminescence peaks for kinetic analysis. J. Lumin. 155, 70–

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Fiedler, I., Woda, C., 2011. Thermoluminescence of chip inductors from mobile phones for retrospective and accident dosimetry. Radiat. Meas. 46, 1862–1865. Hashimoto, T., Ogita, K., Umemoto, S., Sakai, T., 1983. J. Polym. Sci.: Polym. Phys. 21, 1347–1356.

Hübner, S., Göksu, H.Y., 1997. Retrospective Dosimetry using the OSL-pre-dose Effect in Porcelain. Appl. Radiat. Isot. 48, 1231–1235. Jahan, M.S., King, M.C., Haggard, W.O., Sevo, K.L., Parr, J.E., 2001. A study of long-lived free radical in gamma-irradiated medical grade polyethylene. Radiat. Phys. Chem. 62, 141–144.

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ACCEPTED MANUSCRIPT Jahan, M.S., Tessema, G.X., Campbell, B.W., 1998. Effect of post-irradiation storage condition on thermoluminescence from ultra-high molecular weight polyethylene. J. Lumin. 40–41, 242–243. Jahan, M.S., Thomas, D.E., Banerjee, K., Trieu, H.H., Haggard, W.O., Parr, J.E., 1998. Effects of radiation-sterilization on medical implants. Radiat. Phys. Chem. 51, 593–

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594. Kurtz, S.M., 2004. The UHMWPE Handbook, San Diego, CA, Elsevier.

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thermoluminescence study of polyethylene polymer (LLDPE, LDPE, HDPE). J. Photochem. Photobiol. A: Chem. 74, 279–282.

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UHMWPE in total joint replacement. J. Phys D: Appl. Phys. 39, 3213–3219. Wintle, H.J., 1974. Dose kinetic of U.V. excited thermoluminescence in polyethylene.

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ACCEPTED MANUSCRIPT Figure captions

Fig. 1: Decay curves measured after irradiation to 40, 400 and 1000 Gy shown to demonstrate presence of OSL. The background is included for comparison.

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Fig. 2: The decay curve corresponding to 1000 Gy (circles) fitted with Eq. (1), a sum of two components. The inset shows the proportion of each component as a function of the duration of stimulation.

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Fig. 3: The dose response of OSL from UHMWPE stimulated using 470 nm blue light.

Fig. 4: (a) The decay curves of UHMWPE for doses as shown following a pre-dose of 4000

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Gy. The inset shows the fractional contribution of each of the two components identified. (b) Background counts before (open circles) and after (solid circles) pre-dose.

Fig. 5: The dose response of UHMWPE after a pre-dose of 4000 Gy. The inset shows a plot,

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against dose, of the ratio of OSL intensities before and after pre-dose.

Fig. 6: The change of OSL intensity with delay between irradiation and measurement. The

guides.

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inset shows the same feature but on semi-logarithmic scale. The dotted lines are visual

Fig. 7: The OSL intensity corresponding to 30, 100 and 600 Gy for ten consecutive

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measurements.

Fig. 8: The results of a dose recovery test run on UHMHPE. The horizontal lines (a), (b) and (c) correspond to intensities of three ‘unknown’ doses d1, d2 and d3 respectively. Each dose is determined by extrapolating each intercept on the dose response curve to the corresponding value on the dose axis. The doted lines indicate the envelope for the margin of error.

Fig. 9: The dose response of UHMWPE under 870 nm infrared stimulation. The inset shows a decay curve corresponding to 600 Gy.

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ACCEPTED MANUSCRIPT ●Dosimetric features of ultra-high molecular weight polyethylene (UHMWPE) is reported ●Samples were stimulated using 470 nm or 870 nm light ●The dose response is linear in the range 1-1000 Gy studied ●The sample is not affected by pre-dose as tests using a pre-dose of 4000 Gy show

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SC

RI PT

●The luminescence process involves free-radicals