MEA SURE MENT OF
60CO GAMMA RA DI A TION IN DUCED
AT TEN U A TION IN MULTIMODE STEP-IN DEX POF AT 530 nm
by
Milan S. KOVA^EVI] 1,2*, Alexandar DJORDJEVICH 2, Svetislav SAVOVI] 1,2, Jovan S. BAJI] 3, Dragan Z. STUPAR 3, Miloš P. SLANKAMENAC 3,
and Milojko KOVA^EVI] 4
1
Fac ulty of Sci ence, Uni ver sity of Kragujevac, Kragujevac, Ser bia 2
City Uni ver sity of Hong Kong, Kowloon, Hong Kong, China 3
Fac ulty of Tech ni cal Sci ences, Uni ver sity of Novi Sad, Novi Sad, Ser bia 4
Vin~a In sti tute of Nu clear Sci ences, Uni ver sity of Bel grade, Bel grade, Ser bia
Sci en tific pa per DOI: 10.2298/NTRP1302158K
As op ti cal fibres are used ever more ex ten sively in space ap pli ca tions, nu clear in dus try, med i -cine and high-en ergy phys ics ex per i ments, it has be come es sen tial to in ves ti gate the in flu ence of ion iz ing ra di a tion on their char ac ter is tics. In this work, the ra di a tion-in duced at ten u a tion at 530 nm is in ves ti gated ex per i men tally in step-in dex multimode polymethyl-methacrylate plas tic op ti cal fibres ex posed to low dose-rate gamma ra di a tion. Cu mu la tive doses ranged from 50 Gy to 500 Gy. The ra di a tion in duced at ten u a tion has been em pir i cally found to obey the power law RIA= aDb, where D is the to tal ra di a tion dose and a and b are the con stants de
-ter mined by fit ting.
Key words: plas tic op ti cal fi bre, gamma ra di a tion, ra di a tion in duced at ten u a tion
INTRODUCTION
The im por tance of plas tic op ti cal fibres (POF) has grown tre men dously over past de cades. The POF are highly prom is ing trans mis sion me dia for short-range ap pli ca tions in clud ing lo cal area net work (LAN), multinode bus net works, sen sors, power de liv ery sys tems, and light guides (as in toys, en ter tain -ment and med i cal de vices). The at trac tive ness of POF is chiefly due to their low cost, flex i bil ity and ease of han dling and in ter con nect ing, while their rel a tively high at ten u a tion lim its the range of sys tems us ing them [1]. The main types of POF, their man u fac tur ing and pos si ble pres ent and ex pected fu ture ap pli ca tions have been re ported [2, 3]. The sta tus of POF de vel op -ment over the past half cen tury, fo cus ing pri mar ily on the loss re duc tion and band width en hance ment has been pre sented by Koike [4, 5]. In many cases, plas tic op ti cal fibres may suf fer from all kinds of ir ra di a tions. A drawback of POF is that their at ten u a tion is in -creased by ef fects of ion iz ing ir ra di a tion: the fibres do de grade ir re cov er ably with ex po sure [68]. This deg -ra da tion is pri mar ily by the main chain scissioning and cross-link ing in the ma te rial of the fi bre core, which give rise to ma te rial inhomogeneities and re sults in the ra di a tionin duced op ti cal at ten u a tion. Such loss de
-pends on the fi bre type, fi bre tem per a ture, wave length and in ten sity of the in jected light, and ir ra di a tion con -di tions (es pe cially dose rate and to tal dose). In spite of all fibres un der go ing some deg ra da tion of op ti cal trans mis sion when ex posed to ra di a tion, fi bre op tic sys tems are nev er the less im ple mented in stress ing ra di a tion en vi ron ments, par tic u larly as op ti cal fi bre sen -sors. In ra di a tion do sim e try, for ex am ple, op ti cal fibres of fer a unique ca pa bil ity for re mote mon i tor ing of ra di a tion in dif fi culttoac cess or haz ard ous lo ca tions be cause fi bre sen sors can be op ti cally in ter ro -gated from a safe dis tance. Thus, the use of a polymethylmethacrylate (PMMA) based plas tic op ti -cal fi bre has been re ported as an in trin sic real-time gamma do sim e ter [9]. Sim i larly, any fi bre sys tem ap -plied in the or bit ing space sta tion would be ex posed to g-rays from space, and so would be those in man made ra dio ac tive en vi ron ments such as in nu clear power plants, nu clear waste re pos i to ries [10], and do sim e try [11]. The in flu ence of ir ra di a tion should be con sid ered for all these fi bre sys tems. In re cent years, changes in op ti cal fibres ir ra di ated by g-rays have been ac tively in ves ti gated [1214]. The fun da men tals about the the -ory of ra di a tion-in duced ab sorp tion in op ti cal fibres can be found in [15-17]. Much work has been re ported by Ichikawa in ves ti gat ing the ra di a tion deg ra da tion of PMMA [18].
low in ten sity ir ra di a tion by gamma rays. Green light emit ted di ode (LED) was used for light source to rep re sent the fre quency that is of in ter est for PMMA fi -bers. This ef fect was then in ves ti gated ex per i men tally and quan ti fied em pir i cally. The fi bre was ex posed to the field of gamma ra di a tion gen er ated by a 60Co head pro duced by Cisbio In ter na tional (Cyrus model). The over all per for mance of the plas tic op ti cal fi bre sub -jected to ra di a tion is dis cussed.
RADIATION-INDUCED ATTENUATION IN STEP-INDEX POF EXPOSED
TO GAMMA RADIATION
The ir ra di a tion de grades poly mers by ef fects such as cross-link ing, change of un sat u rated link num ber, and pro duc tion rate of fallen free rad i cals. All these changes can re flect the change of poly mer ma te rial char ac ter is tics. In many poly mers, both pro cesses – main-chain scissioning (deg ra da tion) and cross-link ing – take place in par al lel. How ever, in cer tain cases, the scission dom i nates the crosslink ing, and such poly -mers are known as de grad ing poly -mers. The PMMA is one such poly mer. The ir ra di a tion cross-link in PMMA is the phe nom ena of the poly mer mol e cule link ing to gether through bounds, the re sult is the poly mer mo lec u lar mass in creases with ab sorbed ir ra di a tion dose. Ir ra -di a tion deg ra da tion is the pro cess of the poly mer main link break ing off un der high en ergy ra di a tion, and the re sults is the mo lec u lar mass de creases with ab sorbed ir ra di a tion dose, so much as some poly mer mol e cule deg ra da tion into mono mer mol e cule. In both pro cesses, af ter ir ra di a tion, the chem i cal struc ture of poly mers changes, lead ing to ra di a tion dam age ef fects that cause the POF ra di a tion in duced at ten u a tion to in crease.
Us ing the BeerLam bert law, it is pos si ble to de -ter mine the ra di a tion-in duced at ten u a tion (RIA) in the op ti cal fi bre. As op ti cal at ten u a tion is di rectly re lated to the fi bre length, by mon i tor ing the op ti cal power in the fi bre it is pos si ble to de ter mine the at ten u a tion that was due to gamma ra di a tion [9]
RIA=
-é
ë ê
ù
û ú
10
0 L
P t
P log ( , )
( ) [ ]
T
T
dB/km l
l (1)
where L is the ir ra di ated length of the test fi bre, PT(l, t) – the mea sured op ti cal power in the ir ra di ated test fi -bre, and PT0( )l – the op ti cal power of the ref er ence fi -bre which is not ir ra di ated.
EXPERIMENTAL PROCEDURE
To con duct the ra di a tion test, a PMMA-based plas tic op ti cal fi bre was used as sup plied by Mitsubishi Rayon Eska Op ti cal Fi bre Di vi sion with the core di am e ter of 0.98 mm and clad ding di am e ter of
1 mm (core in dex 1.49, at ten u a tion 0.2 dB/ml). The fi -bre was pre pared with a length of 4 m and ar ranged into a spi ral on an 80 mm di am e ter Perspex coil and placed into an ir ra di a tion con tainer for ex po sure to gamma ra di a tion. The an LED il lu mi nated the fi bre (IF-E93; l = 530 mm). A low cost CMOS web cam era dis played and re corded the beam pro file on a note book PC com puter us ing LabVIEWtm soft ware [19]. The over all ex per i men tal set-up for in situ mon i tor ing of the fi bre at ten u a tion dur ing ir ra di a tion is shown in fig. 1. To ac count for the in sta bil ity of the LED out put, an op ti cal beam split ter and a photo-de tec tor IF-D93 (Dar ling ton type) were used. The in te grated de sign of the IF-D93 makes it a sim ple, cost-ef fec tive so lu tion in a va ri ety of ap pli ca tions. The op ti cal re sponse of the IF-D93 ex tends from 400 nm to 1100 nm, mak ing it com pat i ble with a wide range of vis i ble and nearin fra -red LED and other op ti cal sources. Light source and photo-de tec tor is housed in a “con nec tor-less” style plas tic fi bre-op tic pack age, to which the POF can be eas ily con nected.
The fi bre was ir ra di ated at the Vin~a In sti tute of Nu clear Sci ences, Bel grade, Ser bia, in the ir ra di a tion cham ber shown in fig. 2.
Fig ure 1. Ex per i men tal set-up
RESULTS AND DISCUSSION
The fi bre sam ple was ir ra di ated by ten dif fer ent ra di a tion doses es ti mated to range from 50 Gy to 500 Gy. With pol ished fi bre end-sur faces for re li able op ti cal cou pling, the power trans mis sion test was con -ducted be fore and af ter ir ra di a tion us ing the set-up shown in fig. 1 that also pro vided vi su al iza tion as in di cated in fig. 3. The sys tem per forms realtime im -age-stream ing and fa cil i tates mea sure ment, cap ture, dis play, and re cord ing of pa ram e ters be ing mea sured. The ex per i men tal re sults are sum ma rized in tab. 1.
As the func tion of the ab sorbed dose, fig. 4 shows nor mal ized light out put at the fi bre end. This in -ten sity was ex tracted from the mea sured op ti cal beam pro file as the av er age in ci dent light out put re corded by the cam era. With the LED per for mance ver i fied ev ery 10 sec onds drift-free fi bre in put was noted through out the mea sure ment with sta ble av er age value. The fi bre out put, on the other hand, was found to de crease with the ra di a tion dose that the fi bre re ceived. The func -tional re la tion ship be tween the nor mal ized op ti cal
out put and ab sorbed dose that best fits the curve of fig. 4 is
NO=(NO)0 +ae-bD (2) where NO and D are the nor mal ized out put and ab -sorbed dose, re spec tively, with the other con stants de ter mined by curve fit ting: (NO)0 = 0.8 a = 0.2, and b = 0.005.
Nor mal ized LED sta bil ity mea sured through the ref er ence fi bre that was not ir ra di ated is shown in fig. 5.
For the mea sured data, BeerLam bert law cal cu -la tions (1) of the RIA as a func tion of the ab sorbed dose are shown in fig. 6. It fol lows that as the gamma ra di a tion dose in creases, so does the ra di a tion in duced ab sorp tion in plas tic op ti cal fi bre. The in crease in the in duced fi bre at ten u a tion is di rectly re lated to the dose of in ci dent ra di a tion. The re sults in fig. 5 in di cate that the ra di a tion in duced ab sorp tion RIA var ies with the ra di a tion dose D as RIA = aDb, where a and b are con stants de ter mined by fit ting of the ex per i men tal re -sults (a = 0.01 and b = 0.5).
This is at trib uted to the ra di a tion-in duced mainchain scissioning and to crosslink ing of the poly Fig ure 3. Front panel for op ti cal beam pro fil ing
Table 1. Normalized output and radiation-induced POF attenuation
Dose [Gy] Normalized output RIA [dB/m]
0 1 0
50 0.94340 0.063260
100 0.89598 0.119254 150 0.86806 0.153626 200 0.83876 0.190905 250 0.82525 0.208536 300 0.80967 0.229229 350 0.80230 0.239158 400 0.78960 0.256482 450 0.78287 0.265775 500 0.77639 0.274800
mer mol e cules re sult ing in the mo lec u lar mass in crease with ab sorbed dose. As PMMA is ir ra di ated with ion iz -ing ra di a tion, such as gamma ra di a tion, a free rad i cal is gen er ated on the es ter side-chain-COOCH2 (see fig. 7). This side chain rad i cal may be gen er ated in a num ber of ways [18]. This pro cess leads to ra di a tion in duced at -ten u a tion, i. e. ir ra di a tion dam age ef fects. The ir ra di a tion sta bil ity is de pend ent on the en ergy trans mit ted in -side poly mer and the mol e cule spe cial ef fect.
CONCLUSIONS
The ef fect of a low-dose gamma-ra di a tion on light trans mis sion in step-in dex plas tic op ti cal fibres
was in ves ti gated in this work. The re sults showed that the trans mit ted power is re duced by ir ra di a tion. This is be cause poly mer mol e cule con sists of mil lions of mono mer nits and only a small chem i cal change caused by ir ra di a tion may change the phys i cal per for -mance. The ir ra di a tion cross-link is the phe nom ena of the poly mer mol e cule link ing to gether bonds; the re sults are poly mer mo lec u lar mass in creases with ab -sorbed ir ra di a tion dose. This pro cess leads to ra di a tion in duced at ten u a tion, i. e. ir ra di a tion dam age ef fects. The ra di a tion in duced at ten u a tion de te ri o rates the per -for mance of the plas tic op ti cal fi bre caus ing the POF trans mis sion loss to in crease. The re sults in di cate that the ra di a tion in duced ab sorp tion fol lows the power law RIA = aDb. A good agree ment of our ex per i men tal data and pre dic tion of the ory pro posed by Duncan et al, [15] was ob tained.
ACKNOWLEDGEMENTS
The work de scribed in this pa per was partly sup ported by the stra te gic re search grant from the City Uni -ver sity (Pro ject no: 7002775) and partly by Ser bian Min is try of Ed u ca tion, Sci ence and Tech no log i cal De -vel op ment (Pro jects 171011, 171021, and III43008).
AUTHOR CONTRIBUTIONS
The o ret i cal anal y sis was car ried out by M. S. Kova~evi}, A. Djordjevich, and S. Savovi}. Lit er a ture re search was car ried out by M. P. Slankamenac and M. S. Kova~evi}. Ex per i ments were car ried out by J. S. Baji}, D. Z. Stupar, M. S. Kova~evi}, and M. Kova~evi}. Cal -cu la tions were per formed by S. Savovi}, A. Djordjevich and M. S. Kova~evi}. Figures were pre pared by J. S. Baji}, D. Z. Stupar, and M. P. Slankamenac. All au thors dis cussed the re sults and par tic i pated in the writ ing of the manu script.
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Fig ure 6. Real-time ra di a tion in duced at ten u a tion at l = 530 nm
Fig ure 7. The PMMA poly mer mol e cule that ir ra di ated with ion iz ing radiation
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[19] Baji}, J. S., et al., Im ple men ta tion of the Op ti cal beam Profiler Sys tem Us ing Lab VIEW Soft ware Pack age and Low-Cost Web Cam era, Pro ceed ings, 35th Internationl Conventrion MIPRO 2012, May 21-25, 2012, Opatija, Croatia
Re ceived on March 19, 2013 Ac cepted on June 10, 2013
Mi lan S. KOVA^EVI], Aleksandar \OR\EVI], Svetislav SAVOVI], Jovan S. BAJI], Dragan Z. STUPAR, Milo{ P. SLANKAMENAC,
Milojko KOVA^EVI]
MEREWE SLABQEWA U MULTIMODNOM OPTI^KOM VLAKNU
NA 530 nm USLED OZRA^IVAWA GAMA ZRACIMA IZ 60CO
S obzirom na sve ~e{}e primenu opti~kih vlakana u svemirskim stanicama, nuklearnoj industriji, medicini i eksperimentima u fizici visokih energija, postalo je veoma va`no ispitati uticaj jonizuju}eg zra~ewa na wihove karakteristike. U ovom radu je eksperimentalno ispitivano slabqewe u multimodnom plasti~nom opti~kom vlaknu na 530 nm kao posledica ozra~ivawa gama zracima niske doze. Kumulativne doze gama zraka bile su u opsegu od 50 Gy do 500 Gy. Empirijski je utvr|eno da se slabqewe izazvano gama zracima niske doze – RIA mewa kao RIA = aDb, gde je D ukupna doza ozra~ivawa, a a i b su konstante ~ije su vrednosti dobijene fitovawem eksperimentalnih podataka.
