Comparison between fast neutron and gamma irradiation of optical fibres

H. Henschel, 0. Kohn, W. Lennartz, S . Metzger, H.U. Schmidt Fraunhofer-INT, D-53879 Euskirchen, Germany

J. Rosenkranz, B. Glessner Alcatel Kabel, D-41048 Monchengladbach, Germany

B.R.L. Siebert Physikalisch-Technische Bundesanstalt, D-38023 Braunschweig, Germany

Abstract

A variety of undoped multimode step-index fibres and Ge- doped single mode and graded index fibres is irradiated by 14 MeV neutrons and 6oCo gamma rays up to the same total dose. Radiation-induced loss and breakmg stress are measured. The loss induced during gamma irradiation of Ge-doped fibres is about 2.5 times higher than during neutron irradiation with the same dose rate, up to a fluence of about 3x10'' cm-' (14 MeV). This loss ratio decreases to 52.0 after about 1013 cm-' (14 MeV).

Monte Carlo calculations of the dose enhancement as caused by fast recoil protons out of the H-containing acrylate coating are confirmed experimentally.

10'3cm-2 seems to decrease fibre breaking stress by 3 - 4 %, whereas 6oCo gamma irradiation up to the same total dose has no effect.

14MeV neutron irradiation up to

I. INTRODUCTION Conditioned by the good radiation hardness of most of the

present optical fibres [ l ] they are increasingly used for data transfer and sensor applications also in nuclear environments. At fission or fusion reactors fibres would also be exposed to fast neutrons. While there exist innumerable publications about fibre tests with gamma rays, only very few test results with fast neutrons are available, e.g. [2 - 121. Most of them [2 - 71 state that irradiation with fast neutrons and gamma rays would lead to the same increase of loss after exposure to the same dose. Therefore it would only be necessary to simulate the influence of neutrons by gamma irradiation up to the expected neutron dose.

In all these publications [2 - 71, however, the neutron dose was simply calculated with the (energy dependent) fluence- kerma conversion factor in SiOz (about 15.6 pGycm2 for 14 MeV neutrons). In [13] it was already pointed out that fast neutrons produce energetic recoil protons in H-containing coating materials which can, e.g., penetrate several fibres of a test spool, leading to an increase of dose in the fibre core. A crude estimate for this dose increase was given. Only in one of the older publications ([lo]) it was mentioned that the fluence- kerma conversion factor for Si02 would only "place a lower

bound on neutron dose since kerma factors are higher for most possible contributors like H, B, or C than they are for Si and 0". A correct neutron dose calculation would require "consideration of the range and trajectories of all neutron- induced reaction products from all materials within a range distance of the fibre core". They did, however, not try to make such a calculation.

The investigation published in [ 131 has initiated a Monte Carlo calculation of the contribution of recoil protons out of the coating material to the dose in the fibre core [ 141. It led to the surprising result that the dose of 14 MeV neutrons in fibres with acrylate coating can be increased by values between about 21 % (single fibre) up to about 170 % (deeper layers of a spool). So the authors of [2 - 71 might have measured comparable increase of fibre loss during gamma and fast neutron irradiation. Their neutron dose, however, could have been more than twice as high as their gamma dose.

The purpose of the present investigation is to confirm the calculations of [14] and to find out what really happens when fibres are irradiated with gammas and fast neutrons up to the same (total) dose.

II. EXPERIMENTAL Table I gives an overview of the investigated fibres. The

first fibre (IPHTgOs, Rare Earth doped) is highly radiation sensitive, so that only about 10 to 20 cm are needed for accurate radiation-induced loss measurements, i.e., we don't need voluminous fibre bundels with high recoil proton dose build-up. This fibre is predominantly used to study the influence of the H-containing fibre coating material on the dose in the fibre core. At first we placed two windings on a 3.5 cm diameter Al-spool. In this case recoil protons out of the acrylate coating would increase the dose in the fibre core only by about 21 % [14]. Then we put two windings on a 3.0cm diameter Al-spool that was covered with about 10 layers (thickness about 2.5 mm) of a dummy fibre in order to reach maximum proton dose build-up, i.e., a dose increase of about 170 % [14]. The total dose ratio for these two cases should be about 2, if the calculations of [ 141 and the determination of the gamma background dose [ 131 are correct. The irradiation time in both cases was one hour.

0-7803-4071 -X/98/$10.00 0 1998 IEEE. 430

Table I: Investigated Fibres ~

Manufacturer Fibre Fibre Type Preform Core CoreICladdingl Year of Designation Manufacturing Dopants Coating Manufacture

Process Diameters [pm]

IPHT 90s SM Pr, Al, P 4.811301220 1995 Heraeus Fluosil SSU 1.2 MM SI POD OH 10411 251250 1995 Heraeus SZU 1.2 MMSI POD 10411251250 1996 Polymicro FVA MM SI OH 100/140/250 1995 Poly micro FIA MM SI 100/140/250 1995 Alcatel Kabel E9.3R3.5 SM MCVDR Ge 9.311 251250 1997 Unnamed MM GI OVD Ge 62.511 251250 1996 POF CD 15865 E l MM GI PCVD Ge 62.511 251250 1996 Siecor SMF 1528 SM OVD Ge 9.311251250 1997

----------------- ------- ---- -----_-------__---____________

-- ..................... -- -------- - --------------_--------

In order to compare the effect of neutron and gamma irradiation on this fibre we then irradiated spools of the same diameter and approximately the same fibre length at a 6oCo source. The dose rates were chosen in that way that we reach the same (total) dose values as during the two neutron irradiations in exactly the same time (one hour).

With the residual eight fibres we only compared the effect of neutron and gamma irradiations. The procedure is outlined in Fig. 1. Our intended 14 MeV neutron fluence of about l O I 3 cm-2 can be obtained in reasonable time (6 h) only when we choose test fibre spools of very low diameter. Since the Ge- doped fibres show negligible winding loss, we could perform accurate loss measurements also during irradiation. We calculated neutron fluence and total dose (according to [14]) and choose the 6oCo gamma dose rate in that way that we obtain exactly the same dose during 6 h of gamma irradiation. I.e., comparative neutron and gamma irradiations were made with exactly the same dose rate, as we did with the IPHT 90s fibre.

18 h after each neutron or gamma irradiation we made a spectral loss measurement. In order to increase accuracy and reproducibility, the fibres were now wound on a spool with 35 cm diameter. 24 hours after the respective neutron or gamma irradiation these 35 cm spools obtained a 100 Gy 6oCo gamma dose ("standard irradiation") in order to see wether they behave different after neutron or gamma irradiation up to the same dose.

When the MM SI fibres are wound to spools of only about 4 cm diameter, they show high, fluctuating bending losses. Accurate loss increase measurements during (first) gamma and neutron irradiation were not possible. We therefore only made the spectral loss measurements 18 h after the respctive neutron or gamma irradiation, and the standard gamma irradiations where we could use spool diameters of 35 cm.

When we intend to compare the effect of neutron and gamma irradiations up to the same dose, it is very essential to know the accuracy of the respective dose in order to decide whether possible differences are significant or could also be explained by incorrect dose determination.

Neutron fluences were measured by small calibrated fission chambers and by A1 activation foils (absolutely). The neutron fluence relative to preceeding irradiations could be determined with errors d 5 % for spools of identical size and at (nearly) the same place. Determination of the absolute value of the neutron fluence in (the center of) a fibre spool was only possible with errors g 16 %. The neutron dose determinations are correct within g 19.3 %, whereas the total dose during neutron irraditions (sum of neutron dose D,, recoil proton dose D, and gamma background dose DJ might have errors C 26.8 %.

The gamma dose rate was measured with small calibrated ionization chambers. Possible errors of gamma doses during our present measurements are L 5 %.

Light power measurements during irradiation of the Ge- doped fibres could be performed with high accuracy with a HP8153A light power meter. With the SM-fibres the radiation- induced loss was measured at 1303 nm (laser source), so that we observed final loss values of only 0.14 to 0.34 dB during irradiation times of 6 h. The drifts before the neutron irradiations were < 0.0005 dB/h. We used a reference channel to compensate for light power variations. Loss of the Ge-doped GI fibres was measured at 830 nm (LED source) with even higher accuracy.

After this irradiation procedure (first irradiation of each sample with 14 MeV neutrons or 6oCo gammas up to the same dose, second irradiation with 100 Gy(Si02) standard gamma dose) breaking stress measurements were made (by Alcatel Kabel). The test method follows the "International Standard IEC 793-1-B2A: Tensile strength for short lengths of optical fibres" (1995-10). The measurements were made with a PC controlled Zwick 1445 device with rubber coated mandrels 10 cm in diameter. Gauge length was 50 cm, and pulling speed 5 %/min. In two cases (unnamed and Alcatel Kabel fibres) we also made, for comparison, a measurement with an unirradiated fibre.

43 1

Comparison of neutron and gamma irradiation

4 ISt spool: Neutron irradiation, loss measurement during and

4 Determination of neutron fluence, neutron dose and total dose

lo3 s after irradiation

Comparison of preirradiated and non-preirradiated fibres

v

2"d spool: Gamma irradiation up to the total dose, loss measurement during and

-b lo4 s after irradiation -------

I Shortly after end of irradiation: rewinding on 35 cm diameter spool 1 4 +

1 18 h after respective preiradiation: spectral loss measurement I Spectral loss measurement of non- preirradiated fibre (reference value)

4 24 h after respective preirradiation: Gamma standard irradiation (10 krad), loss measurement during and IO4 s after irradiation

Gamma standard irradiation (10 krad), loss measurement ri during and lo4 s after irradiation

+ + Difference between irradiated and non-irradiated fibre = radiation- induced loss increase

Fig. 1: Irradiation test procedure of the undoped MM SI and the Ge-doped fibres.

III. RESULTS

The results of the spectral loss measurements are not shown here for two reasons. Firstly, we found no distinctly deviating shape of gamma- and neutron-induced loss curves, e.g., no new absorption bands after neutron irradiation. Secondly, the accuracy of (off-line) spectral loss measurements is lower than that of the on-line loss increase measurements with a light power meter at fixed wavelength.

A. Rare Earth Doped Fibre IPHT 90s

Fig. 2 shows the results of the 14 MeV neutron irradiations with and without proton dose build-up (dotted lines), as well as of the comparative gamma irradiations up to the same respective dose (solid lines). The ratio of the total dose values during the two neutron irradiations (D, + D, + D, , see [ 131) is 1.93, whereas the ratio of the induced losses (ratio of the dotted curves of Fig. 2) is about 2.6 (Fig. 3a).

The agreement is relatively good. We can't expect total agreement, since in this situation the loss increase will not simply be proportional to the dose. In one case (two windings on Al) about 213 of the total dose is caused by the very densely ionizing Si-and 0 recoil atoms (dose contribution D,), whereas in the other case (proton dose build-up in dummy fibres) these atoms cause only 1/3 of the total dose and about 57 % are caused by not so densely ionizing protons (dose contribution

D,). According to [15], the radiation effect is lower in case of higher radiation-induced charge carrier density. The authors report that 14 MeV neutron irradiation of thermoluminescence dosimeters (TLDs) would lead to a distinctly lower response than y irradiation with the same dose and explain this by charge carrier recombination and saturation mechanisms in the dense ionization tracks of heavy ions.

0 500 1000 1500 2000 2500 3000 3500 4000

3.0 .

2.5

# I 2.0 3 ; 9 ;

1.5 -

1.0 -

0.5 - ......... .. .- i

0 500 1000 1500 2000 2500 3000 3500 4000

Irradiation Time [SI

Fig. 2: 14 MeV neutron and "CO gamma irradiation (with same respective dose rate) of the Rare Earth doped fibre IPHT 90s. Neutron fluence 1.3~10'~cm-~ in irradiations 1 and 2. Total dose = gamma dose about 31.6 Gy (n2, "(2) and 61.1 Gy (nl, K). k = 830nm, P = 10 pW, room temperature. n1/n2: high/low proton dose contribution.

432

3.5 . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

a

2'o 1.5 0 2 500 1000 1500 2000 2500 3000 3500 4000

Irradiation Time [SI

5.0 1

2.0 0 500 1000 1500 2000 2500 3000 3500 4000

Irradiation Time [SI

Figs.3a,b: Ratio of induced losses as a function of irradiation time during the two neutron irradiations of fibre IPHT 90s (above) and during the corresponding gamma and neutron irradiations (below); see Fig. 2.

This is confirmed by our comparison of neutron and gamma irradiation (Fig. 2). Gamma rays (of the same repective dose) cause about 3 times higher loss than neutrons. If we calculate the ratio of gamma- and neutron-induced loss for the two spool configurations (Fig. 3b), we even can detect the influence of the different stopping power of protons and SUO recoil atoms: With proton dose build-up (lower curve) this ratio is only about 2.8, compared with about 4.0 without proton dose build-up (= higher dose contribution of Si and 0 recoil atoms).

Since we have two measurements with different dose contributions of protons (p) and recoil atoms (R), we can set up two equations in order to calculate the "signal reduction factors" fp and fR, respectively, that suffer protons and SUO recoil atoms, compared with gamma rays of the same dose [13]. From literature (e.g. [16]) it is known that the increase of attenuation A (= radiation-induced loss) with dose D (or irradiation time t) can be approximated by

A = k x D " = k x D " x t " (1)

where Dis the dose rate. k and n are constants. With doped fibres this relation is valid up to dose values of about 10' rad = lo3 Gy. At higher doses we observe saturation effects (see,

e.g., [l]). With Ge-doped fibres we have 0.6 L n L 0.7, whereas (Ge+P)-doped fibres show nearly linear increase of loss with dose, i.e. n = 1, in the same dose range [17].

From the dotted curves of Fig. 2 we can determine n = 0.985 with proton dose build-up and 0.959 without, whereas gamma irradiation of this fibre yielded 0.967. The exponent n seems to be independent of radiation type and about 1 for this RED fibre. Analogous to [ 131 we can write

or, with ktotDtot = ktotDtott = ktot t

k,(D, +D,fp +DnfR) = k'tot t .

(4)

( 5 )

ktOt can be determined from the two dotted curves and k, from the two solid curves of Fig. 2. D, is the product of the respective neutron fluence a,, and the kerma value (15.6 pGycm2 for 14 MeV neutrons in SiOz, see e.g. [14]). D, can be calculated as described in [14], i.e. D, = 1.7 D, for maximal proton dose build-up. For the calculation of the gamma background dose D, we use the relation found in [ 131, i.e. (0.49 k 0.17) Gy per 10" n/cm2(14 MeV). From the two equations obtained in this way one can calculate fR = 0 and fp = 0.43. fR = 0 seems to be unrealistic. The reason might be a too high value for D,. If we take the lower limit, i.e. 0.32 Gy per

n/cm2(14 MeV), we obtain fR = 0.1 1 and fp = 0.43. With D, = 0, an unrealistic low gamma background dose, we finally obtain fR = 0.31 and fp = 0.43. The most reasonable values of fR= 0.11 and fp= 0.43 might depend on fibre type, i.e., undoped SiOz fibres, heavily doped SiOz fibres, and fibres made of lead glass, for example, might have (slightly?) different signal reduction factors f.

According to [15] 14 MeV neutrons will only cause about 1/10 of the response of 6oCo gammas of the same dose (fR = O.l), whereas [ 181 reports that the efficiency for electrons in LiF TLDs is about 8.8 times higher than for heavy ions

One can now try fo find a relation between f and the respective charge carrier density. Mean values of these densities can roughly be calculated from the maximum energy transfer in collisions of 14 MeV neutrons with H-, Si-, and 0- atoms and of 6oCo gamma rays with electrons (Compton effect). From the range of these particles and an approximate track diameter of 1 Hm for all particles one obtains the volume of the ionization tracks. Multiplication with the density of SiOz gives the mass in these tracks. Division of the maximum particle energy by the mass yields approximate values for the dose deposited in these tracks (= microdose MD) as a measure

(fR=o.11).

433

for the charge carrier density. We obtain values of about 106 Gy (Si,O), 1.1 1 Gy (p), and 0.063 Gy (e-), respectively. If we draw the corresponding signal reduction factors (0.1 1, 0.43 and 1, respectively) as a function of the microdose values we obtain, in log-log representation, a straight line (Fig. 4). The slope of this line is -0.297, i.e. about -1/3. An a factor of thousand higher charge carrier density would thus lead to 1/10 of the loss increase.

1

L

+. 0

h

2 C -

5 - ." (I)

0.1

Microdose [Gy(SiO,)]

Fig.4: Signal reduction factor f as a function of average dose deposited in the ionization track (="microdose").

When we use the relation f 0~ (MD).'" to calculate fR and fp from the above-mentioned MD values, we obtain fR = 0.084 and fp = 0.38. These results sound quite reasonable, but they should be confirmed by measurements with proton and heavy ion beams.

If we assume for simplicity that the charge is homogeneously distributed in a sphere, the result of Fig. 4 means that f E d, the distance between the charge cariers (d (MD)"").

It is reasonable to assume that the proportionality o f f with d or (MD>-'" is only valid within a certain range of d or corresponding MD values.

B. Ge-doped Fibres

We tried to perform continuous and as accurate loss measurements as possible at 830 nm (GI fibres) and 1300 nm (SM fibres) during neutron and gamma irradiation in order to be able to detect if there possibly exists a neutron fluence above which fibres begin to behave different. The spool frames were now made of PMMA (1.5 mm thickness) so that maximum recoil proton dose build-up was obtained for all windings

Figs.5a,b show the increase of loss during neutron and gamma irradiation of the GI and SM fibres. Again gamma irradiation leads to a more than two times higher loss than neutron irradiation up to the same dose. In order to detect whether this ratio remains constant during the whole irradiation time (6 h), we calculated this ratio for all fibres (Fig. 6a). The tendency is always the same, apart from times 5 250 s. With single mode fibres the ratio might be a little bit lower (= 2.3 compared with = 2.5 for the GI fibres), whereas

the RED fibre (Fig. 3b, lower curve with proton dose build-up) might show a little bit higher values (= 2.8). The measurements, however, were made at different wavelengths (GI and RED at 830 nm, SM at 1303 nm).

Dose [Gy(SiO,)]

Dose [Gy(SiOp)J

Figs. 5a,b: 14 MeV neutron irradiation (dotted lines) and "CO gamma irradiation (solid lines) with the same dose rate. Irradiated length: 80m with GI fibres and about 70m with SM fibres; room temperature. 1: Unnamed fibre, $,, = 9.1~lO'~cm-~; 2: POF fibre, $,, = 9.9~1O'~cm~; 3: Alcatel Kabel fibre, @n = 8.1x10'2cm-2; 4: Siecor fibre, $,, = 8x10'2cm-2.

The behaviour of the loss ratio in Fig. 6a at early times seems to be different for all fibres. Since we know that the stability of our neutron generator is quite low at the beginning, we performed two additional short irradiations (13 s and 306 s) of the unnamed fibre where we tried to have relatively stable neutron output from the very beginning. Measurements were taken every 0.5 (13 s irradiation) and every 2 s (306 s). After determination of neutron fluences and respective total doses we made 6oCo gamma irradiations of same duration and with approximately the same dose rate. The ratio of the gamma- and neutron-induced losses of these two short irradiations is compared with the ratio of the 6 h irradiations (Fig. 6b). We conclude therefrom that with all fibres the ratio would be constant from the very beginning, provided we would have constant neutron output.

After irradiation times of 5000 to 8000 s all fibres show a decrease of the ratio of gamma- and neutron-induced loss (Fig. 6a). The corresponding neutron fluence is about 3 x 10l2 14 MeV), and the corresponding dose values are

434

4.0 I L a 1

1.0 t i o 0 1000 io000

Irradiation Time [SI

0.1 10 loo 1000 10000

Irradiation Time [sj

Figs. 6a,b: Ratio of induced losses during gamma and neutron irradiation of corresponding Ge-doped fibres. Above: 6h irradiations of the four fibres. For fibre designations and irradiation conditions see Figs. 5a,b. Below: Irradiations of different duration of the unnamed GI fibre. With the two short irradiations approximately constant neutron flux from beginning.

about 140 Gy (= 14 had). The reason could be that the neutron-induced displacement damage now becomes comparable with the defect concentration already present in unirradiated fibres, i.e., that neutron-induced loss now shows faster increase with dose (or irradiation time) than gamma- induced loss.

If we simply extrapolate (in Fig. 6a) the approximately linear curve parts at irradiation times > lo4 s we can roughly estimate the neutron fluence where y and neutron irradiation will lead to the same loss (induced loss ratio = 1). As mean value from the 4fibres we obtain a fluence of about 5 x 1013 ~ m - ~ ( 1 4 MeV). This shows the results of [2-71 (neutron and gamma irradiation up to the same dose will lead to the same loss increase) in a new light. All authors, however, did not take into account the dose enhancement of recoil protons from the fibre cladding, in spite of the fact that at least in [2] and [6] it is mentioned that they use fibre bundles where the additional proton dose might be of the same size than their "neutron dose". Gamma background dose is neglected in all cases.

Nevertheless the authors of [2] obtained identical results for neutron and gamma irradiation "up to the same dose" dose for 14 MeV neutron fluences of 8.9 x 10" cm-2 as well as of 7.5 x 1013 and 3.6 x 1014 cm-2. They show, however, only

relatively inaccurate spectral loss curves measured at certain times after irradiation. Fig. 2 of [6] shows the increase of loss of Ge-doped fibres during 14MeV neutron and gamma irradiation with the "same dose rate". The maximal neutron fluence of about 3 x 1014cm-2 was reached after about 13000 s. The authors did not point out that below 1000 s (i.e.i 2.3 x 1013 cme2) yirradiation showed higher loss than neutron

irradiation, whereas after about 3 x 1014 cm-2 neutron irradiation showed about 1.5 times higher loss.

In order to detect faster loss increase during neutron irradiation also in the loss growth curves (Figs. 5a,b), we normalized the loss at the end of the gamma and neutron irradiations of the unnamed GI fibre (at D, = Dmt = 424.1 Gy; D = 0.02 Gy(SiO@) to "1" and compared the resultant curves also with that of a gamma irradiation with D = 0.18 Gy(SiOz)/s (Fig. 7). Whereas both gamma curves show unchangd behaviour in the whole dose range, the neutron curve seems to change to nearly linear increase of loss with dose above about 100 Gy (> 2.1 x 10l2 cm-2), just where we expect to see the beginning of domination of the fast neutron- induced displacement damage. This linear increase of loss during neutron irradiation is even more pronounced with the POF fibre (dotted curve 2 of Fig. 5a).

Dose [Gy(Si02)]

Fig.7: Comparison of the loss growth curves during gamma and neutron irradiation of the unnamed GI fibre. Curves ly and In are normalized versions of curves 1 of FigSa. For comparison (curve l'y) we show a gamma irradiation of the same fibre with D= 0.18Gy/s (instead of = 0.02Gy/s with curves ly and In). For irradiation parameters see Fig.5.

It also could be possible that annealing of neutron- and gamma-induced loss occurs with different time constant, at least after longer irradiations times, i.e. higher neutron fluences. To find such behaviour, we normalized the loss at the end of the respective irradiations to "1". Fig. 8 shows the results for the three above-mentioned neutron and gamma irradiations of the unnamed GI fibre. As already shown in [ 191 we observe an increase of annealing time constant with irradiation time, but no difference between neutron and gamma irradiation. The annealing time constants after the 6 h neutron and gamma irradiation of the residual three Ge-doped fibres were also identical.

435

I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . '"ml

3.0 - .

1.001 TTF Irradiation Time 1

...... 1 . . . . . . . . . . . . ...... ...... ._. . , ._ . . ' - .....

0.oot . """" . """" . """" ' " " " ' ( . """" . . . . . . . 0.01 0.1 1 10 100 1000 10000

Time after Irradiation [SI

............................................... Heraeus SSU 1.2

Fig. 8: Annealing of the loss induced in the unnamed GI fibre during gamma and neutron irradiation. Comparative neutron and gamma irradiations where performed with the same dose rate. The loss at the end of all irradiations was set to J". Solid curves: gamma irradiation, dotted curves: neutron irradiation. For irradiation conditions see Fig. 5.

"Standard" gamma irradiation (with 0.04 Gy(Si02)/s up to 100 Gy) of the Ge-doped fibres 24 h after the comparative neutron and gamma irradiations brought no result. All fibres showed exactly the same order of the induced loss as shown in Fig. 9 for the unnamed GI fibre: highest loss of the unirradiated fibre, lowest loss after gamma irradiation. This can (only qualitatively) be explained by the curve shape: at the beginning we see sharp loss increase. With increasing dose (or loss) we observe decreasing slope. A new fibre (no residual loss) therefore shows highest loss increase, whereas the fibre preirradiated with gammas has a higher residual loss and therefore a lower increase of loss during a new irradiation than the fibre preirradiated with neutrons.

35 . . . . . . . . . . . . . . . . . . . . . . . .

30 -

0 20 40 60 80 100

Dose [Gy(SiOp)]

Fig.9: Increase of loss during standard "CO gamma irradiation of the unnamed GI fibre 24h after neutron irradiation ("""') and gamma irradiation(-) up to the same dose. The dashed line is obtained with a non-preirradiated fibre sample. D y = 0.04 Gy/s, k 830nm, P=lOpW, L =70m, room temperature.

The effect of the acrylate coating on dose (chapter I11 A.) as well as the difference between gamma- and neutron-induced loss are so big that they are neither caused nor considerably affected by the possible errors discussed in chapter 11.

C. Undoped MM SI Fibres

MM SI fibres with high OH content usually are the most radiation resistant ones [ 11, but there also exist low OH MM SI fibres with quite good radiation hardness. MM SI fibres are therefore important for data transfer and sensor applications in nuclear environments where transmission lengths of 10 - 100m usually are sufficient, so that their low bandwidth of 2: 20 MHzxkm (100 pn core diameter) is no limiting factor.

Unfortunately we could not make as extensive, accurate and systematic measurements as with the Ge-doped and RED fibres. The main reason was already mentioned in chapter 11: high fluctuating bending losses limited the accuracy of on-line loss measurements with small spool diameters.

The spectral loss measurements that were made 18 h after the end of the comparative neutron and gamma irradiations showed between two and four times higher loss after gamma irradiation in the region of highest radiation-induced loss ( s 700 nm) where the results are quite reliable.

4.0 . . . . . . . . . . . . . . . . . . . . . . . .

3.5 a

High OH Fibres I

" ' ' ~ ~ ~ ~ ' ~ ' ' ' ' ~ ' " ' ~ ' ' ' ' ~

0 20 40 60 80 100

Dose [Gy(Si02)]

_ . ' g 25 1 ........... 0 20

20 40 60 EO 100

Dose [Gy(Si02)]

.. .iolymicro FIA

Low OH Fibres .

Heraeus SZU 1.2

20 40 60 EO 100

Dose [Gy(Si02)]

Figs. 10a,b: Increase of loss during standard "CO gamma irradii m of the MM SI fibres 24h after neutron irradiation (--) and gamma irradiation (-) up to the same dose. D y = O.OSGy/s, h = 829nm, P = 10 pW, room temperature.

436

The standard gamma irradiations (100 Gy) one day after the comparative neutron and gamma irradiations (Figs. 1 Oa,b) are now difficult to interpret. With Ge-doped fibres we observe steady increase of loss with dose. Saturation begins not before about lOOOGy and is less distinct than with undoped

fibres, especially of high OH content, where pronounced saturation already sets in after dose values of 5 to lOGy (Fig. loa). Some fibres even show a decrease of loss after an intermediate maximum around 10Gy. Only after dose values Z 1OOOGy they show again distinct increase of loss with dose (e.g. [ 11). Therefore on-line loss measurements during gamma and neutron irradiation of undoped fibres up to dose values between about 10 and lOOOGy should lead to nearly the same loss increase, so that the interpretation of the results obtained with the Ge-doped fibres (Fig. 9) should not be applicable

With both high OH fibres (Fig.lOa) we observe a little bit higher loss increase after preirradiation with neutrons, whereas both low OH fibres (Fig.lOb) show a little bit higher loss after gamma preirradiation. The situation might become unambiguous again after dose values > lOOOGy or higher neutron fluences, when neutron-induced displacement damage becomes the dominating source of fibre loss.

D. Breaking Stress Measurements

Gamma irradiated, neutron irradiated and unirradiated fibre pieces were always taken from the same delivery. The measured breaking tensions were drawn as Weibull plots. From a fit to the resultant curves we determined as usual the breaking stress (BS) that corresponds to 50 % failure probability. The results are summarized in Table 11. ,,No. of samples" means number of pull tests.

We only found very small BS changes. Since, however, five of the six fibres of Table I1 as well as the fibre described in [ l ] (BS decrease from 4.75 to 4.58 GPa 14 weeks after 14 MeV neutron irradiation with 2.1 x 10l2 cm-') showed the same tendency, one can be sure that 14MeV neutron irradiation up to l O I 3 cm-' leads to an average BS decrease of about 3 - 4%, whereas 6oCo gamma irradiation up to corresponding total dose values of about 500 Gy does not affect BS. It would be interesting to see whether 14 MeV neutron irradiation up to 1014 - lOI5 cm-' would lead to a stronger decrease of BS or even to an increase as it was observed after high gamma dose values of lo6 Gy [l].

Table 11: Results of breaking stress measurements

Fluencd Time between No. of Breaking Dose irrad. and meas. [w] samples stress [GPa] Fibre

Heraeus an = 1 . 1 ~ 1 0 ~ ~ cm-' ssu 1.2 10 30 4.96 9 30 4.97 D, = 346 Gy

Heraeus @,, = 1 . 1 ~ 1 0 ~ ~ cm-' 27 11 4.64 szu 1.2 D, = 337 Gy 25 12 4.77

Polymicro a,, = 1 . 8 ~ 1 0 ' ~ cm-* 10 30 4.95 FVA D., = 575 Gy 9 30 5.27

Polymicro @,, = 1 . 8 ~ 1 0 ' ~ cm-' 27 12 5.25 HA DT = 562 Gy 25 11 5.27

Alcatel Kabel unirradiated 10 4.89 E9.3fF3.3 @,, = 8 . 1 ~ 1 0 ' ~ cm-' 6 10 4.76

DY = 380 Gy 6 10 4.88

Unnamed unirradiated 10 4.83 MM GI @,, = 9 . 1 ~ 1 0 ' ~ cm-' 6 10 4.85

Dy = 424 Gy 6 10 4.90

--------------------____I_______________-------------.

IV. SUMMARY

We could confirm by 14 MeV neutron irradiation of two different fibre samples (single fibre on AI or on thick dummy fibre pile) that energetic recoil protons out of the acrylate coating will cause a significant increase of dose in the fibre core, as calculated in [14]. This enabled us to perform comparative fast neutron and gamma irradiations up to really the same (total) dose.

When irradiating doped fibre bundles that show high proton dose build-up with 14 MeV neutrons and 6oCo gammas in the same time up to the same total dose, gamma rays cause an about 2.5 times higher loss. A single fibre,

with low proton dose contribution, i.e. higher dose contribution of very densely ionizing Si and 0 recoil atoms, even showed about four times higher loss during gamma irradiation. After a fluence 2 3x 10'2cm-2 (14 MeV) this ratio begins to decrease and might become 1 after fluences around 5 x 1013 cm-2 (determined by extrapolation of curves measured with Ge-doped fibres).

We can conclude that the ratio of the induced loss during gamma and 14 MeV neutron irradiation (with same dose rate) changes with neutron fluence: At fluences

3 x lo1' cm-' where the fibre behaviour is determined by already present defects, gamma irradiation causes 2.5 to 4 times higher loss. At higher fluences, when the displacement damage caused by fast neutrons becomes

437

comparable with the initial defect concentration, the increase of loss with dose will be faster during neutron irradiation. After about 5 x lOI3 cm-2 (14 MeV) the neutron- induced loss should be of about the same size as that induced by gammas of the same total dose. At still higher fluences 14MeV neutrons will cause higher loss than gammas of the same dose, as shown with a Ge-doped fibre in [6].

Therefore it is not correct to simulate the effect of neutron irradiation by (cheaper) gamma irradiation up to the same dose. Nevertheless one can use gamma irradiations to identify the radiation hardest sample out of a set of candidates since fibres that show low loss increase during gamma irradiation will do the same during neutron irradiation (see Figs. 5a,b), at least up to fluences

It would be interesting to perform on-line loss increase measurements of comparable accuracy with highly radiation insensitive undoped fibres, especially of high OH content.

1013 cm-2 seems to reduce fibre breaking stress by about 3 - 4 % (average values), whereas gamma irradiation with the corresponding total dose has no effect.

Our present measurements were made with 14MeV neutrons under "normal" conditions, i.e., room temperature and "usual" humidity. With thermal or fission neutrons, respectively, one would observe no or nearly negligible dose increase in fibres with H-containing coating materials (see [ 141). Nevertheless gamma irradiation would lead to distinctly higher loss increase than neutron irraidation with same (total) dose rate since also with lower energetic neutrons the dose is caused by densely ionizing heavy ions, in contrast to gamma irradiation. The results would be influenced by changes of temperature as well as humidity (breaking stress).

1 0 ' ~ cm-'.

14 MeV neutron irradiation with

V. ACKNOWLEDGEMENTS

We gratefully acknowledge the gratuitous supply of test fibres by Heraeus Quarzglas GmbH (Dr. U. Grzesik, Dr. Schotz), Plasma Optical Fibre B.V. (Mr. G. Kuyt), Siecor GmbH (Dr. E. Baumanfi) and a company that wishes to remain anonymous.

VI. REFERENCES

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[15] M. Oberhofer, A. Scharmann, "Applied Thermo- luminescence Dosimetry", Adam Hilger Ltd., Bristol 1981.

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