# Energy deposition and fragments production resulting from gamma-ray or ion beam irradiation of an ion exchange resin

B. Nsouli, M. Draye, N. Allali, M. Lemaire, J.-P. Thomas

► To cite this version:

B. Nsouli, M. Draye, N. Allali, M. Lemaire, J.-P. Thomas. Energy deposition and fragments production resulting from gamma-ray or ion beam irradiation of an ion exchange resin. International journal of mass spectrometry and ion processes, 1996, 154 (3), pp.179-191. 10.1016/0168-1176(96)04381-9. hal-00006451

HAL Id: hal-00006451

https://hal.science/hal-00006451v1

Submitted on 16 Feb 2022

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
# Energy deposition and fragments production resulting from gamma-ray or ion beam irradiation of an ion exchange resin

B. Nsouliª, M. Drayeᵇ, H. Allaliª, M. Lemaireᵇ, J.-P. Thomasª,\*

The extraction efficiency of ion exchange resins aimed at the reprocessing of nuclear effluents is strongly dependent on gamma-ray induced modifications of their chemical structure. A poly(4-vinylpyridine) resin, or P4VP, has been subjected to gamma irradiation using a ¹³⁷Cs source (Eγ = 662 keV) and we have attempted to characterize its structural modifications using TOF-SIMS techniques based on low (keV) and high (MeV) primary ion beam energies. These analytical techniques are based on the formation of fragments resulting respectively from atomic and electronic collisions of heavy ions. As a result, the differences in nature and relative intensity of the emitted secondary ions, as well as the variation of their intensity as a function of the absorbed dose of gamma rays (up to 14 400 kGy), can be interpreted from different types of bond breaking. While gamma-ray absorption results in benzylic or pyridinic scissions, MeV ion bombardment may induce simultaneous (and close) pyridinic–pyridinic or pyridinic–benzylic scissions, or both. Accordingly, the technique is very sensitive to the presence of fragments preformed under gamma irradiation. This is not observed with keV bombardment which may destroy the pyridine nucleus. The SIMS analysis of the soluble fraction of the irradiated material (free radicals and fragments) and of the remaining material confirms the proposed fragmentation paths leading to positive and negative ion emission. Such results fully support the important finding that these gamma-ray induced modifications are only observed from negative emission under MeV ion bombardment.

**Keywords:** Gamma-ray irradiation; Ion beam irradiation; Ion exchange resin; Time of flight

## 1. Introduction

Because of its particular “resistance” to ionizing radiations [1], there has been in recent years great interest in the use of the poly(4-vinylpyridine) resin (P4VP) in nuclear waste reprocessing programs [2]. This property has been recently quantified in terms of variation of

specific physico-chemical characteristics such as number of protonatable sites, quantity of extracted Pd, etc. as a function of the gamma-ray dose delivered from a ¹³⁷Cs source [3]. On the other hand, various analytical techniques have been used for the characterization of the chemical structure modifications and a coherent picture of the observed evolutions has greatly benefited from the plasma desorption

<footnote>ªInstitut de Physique Nucléaire (et IN2P3), Université Claude Bernard Lyon I, 43, Bd. du 11 novembre 1918, 69622 Villeurbanne Cédex, France</footnote>

<footnote>ᵇInstitut de Recherches sur la Catalyse, Laboratoire de Catalyse et Synthèse Organique, Université Claude Bernard Lyon I, 43, Bb. du 11 novembre 1918, 69622 Villeurbanne Cédex, France</footnote>

<footnote>\* Corresponding author.</footnote>
mass spectrometry (PDMS) technique [4,5]. In fact, the effects of the gamma irradiation are explained in terms of competition between scissions, leading to free radicals or modified chains (in which double-bond formation is observed) and recombinations of such species as a function of the integrated dose. Such interpretations are based on secondary ion emission, which by its nature, also results from the production of fragments in the vicinity of the primary particle trajectory. In the present work we have not only considered the results of high energy ion interactions leading to PDMS spectra but have also compared them to those resulting from low energy ion interactions using static SIMS and the so-called “spontaneous” desorption [6,7]. Moreover, in addition to a comparison of analytical performances (as reported by the Munster group [8]), we have attempted some rationalization of the mechanisms of fragment formation under these two different regimes of energy deposition, and we propose a tentative interpretation for the gamma irradiation effects. Since both analysis techniques are based on time-of-flight (TOF) measurements, the terminology proposed by Sundqvist [9] will be adopted. Accordingly, low velocity impact solid sample fast extraction secondary ion mass spectrometry (LSF-SIMS) with KeV ion irradiation will be used in place of static SIMS and high velocity impact solid sample fast extraction secondary ion mass spectrometry (HSF-SIMS) with MeV ion irradiation instead of PDMS.

## 2. Experimental

### 2.1. Sample preparation

The P4VP resin, cross-linked with 2% divinylbenzene, was supplied by Reilly Industries Inc. Indianapolis, Indiana. This material appears as a thin (specific area of about $0.5 \text{ m}^2\text{g}^{-1}$) white powder, insoluble and infusible. It has the

<figure><img src="image_2.png" /><figcaption>Fig. 1. Sketch of the molecular structure of the P4VP resin cross-linked with 2% divinylbenzene.</figcaption></figure>

chemical structure sketched in Fig. 1, with, on average, one cross-link every 57 pyridine units. Plugged glass containers filled with 1.8 g of this powder were exposed to various irradiation doses (here expressed on the current SI unit of Gray, equivalent to $6.24 \text{ eV g}^{-1}$) from a $^{137}$Cs gamma source. The doses absorbed were determined after calibration to be 148, 480, 3360, 7200 and 14400 kGy at a dose rate of $20 \text{ kGy h}^{-1}$. Then pellets 1 mm thick were made by compression at $2 \text{ tonne cm}^{-2}$ of 0.1 g of the powder in a stainless steel cylinder of 8 mm internal diameter. This cylinder and the sliding block in contact with the sample surface were systematically cleaned to avoid any contamination from the lubricant.

The soluble portion of the irradiated samples was extracted into ethanol (0.15 g of material into 2 ml of solvent) and the corresponding soluble fraction (%) was determined from the weight of dried material found on the filter and/or, for the most soluble samples (high irradiation doses), from the weight of material left after evaporation. Thin films were prepared from these solutions at various concentrations, after the required number of dilutions (see below), by spin-coating deposition on aluminized mylar backings. Since a typical volume of solution was $10 \mu l$, and the typical area of deposit was around $2.5 \text{ cm}^2$, the largest thickness value never exceeded $0.5 \mu g \text{ cm}^{-2}$.

### 2.2. Instrumentation

The different TOF detection procedures used in this work are sketched in Fig. 2. The
<figure><img src="image_3.png" /><figcaption>Fig. 2. Sketch of the experimental TOF spectrometer setup, showing the different ways of obtaining start signals: TFA, time-filter-amplifier: SBD, surface barrier detector: CFD, constant fraction discriminator.</figcaption></figure>

most important requirement for the analysis of
such targets with the present spectrometer is to
establish a reproducible electric field between
the sample surface and the extraction electrode
2 mm away [4,10]. Once correctly applied to
the target and controlled, the 5 kV biasing
allows the secondary ions to be accelerated
over the flight distance of 125 mm toward the
microchannel plate (MCP) assembly. With
such compactness the efficiency is high (85%
transmission) but conversely the mass resolu-
tion is poor, typically 190 FWHM at mass 133.
Based on “ion by ion” detection conditions the
spectrometry implies that each primary ion
must be detected. This is easily done for the
thin films, from which fast projectiles can be
transmitted and detected by a surface barrier
detector giving the start signal. For the thick
samples, but for negative ions only, the
emitted electrons can be used to generate
the start signal [10]. Also, for thick targets,
the incident projectiles can be detected before

hitting the target from the electron emission of
a thin carbon foil placed close enough to the
target site (to avoid too large a scattering
dispersion). Such a “start foil” signal is obtained
from a time-pick-up device described previously
[11]. Then, either positive or negative ions can
be detected, depending on the target biasing.

Argon ions accelerated up to 9 MeV by
the 4 MV van de Graaff machine of the Institut
de Physique Nucléaire were used as primary
particles. Compared to the use of a conven-
tional $^{252}$Cf source (where primary ions are
fission fragments) such a device is somewhat
sophisticated but has several decisive advan-
tages, although not fully exploited in these
experiments. Let us simply mention that a
sharp and well-aligned spot can be obtained
which offers the possibility of lateral homoge-
neity probing, even at a few micrometers level
of resolution [11]. In addition, the optics of the
TOF spectrometer allow the determination of
the angular distribution of the emitted ions
[10] which can be very important in the future understanding of the spatial distribution of the energy deposited by the incident high energy ion [12]. Finally, these experimental conditions lead to the highest yield values if one compares our results on reference materials such as alkali halides [10] with those published in the literature.

The experiments are performed under conventional vacuum conditions ($10^{-7}$ mbar). With a continuous beam of around 1000 ion $s^{-1}$, the analysis is essentially non-destructive and no charge compensation is required. This does not mean that the detection is always free from secondary effects (background or peak broadening) but with typically $5 \times 10^9$ primary ions received per cm$^2$, no surface evolution can be detected.

# 3. Results and discussion

## 3.1. HSF-SIMS: positive ion emission

The positive PDMS spectrum of an untreated P4VP sample is shown in Fig. 3. The most intense peak corresponds to the protonated monomer while the peaks corresponding to the $[2M+H]^+$ and $[3M+H]^+$ ions are much less important. Although

<figure><img src="image_4.png" /><figcaption>Fig. 3. Positive ion mass spectrum of an untreated P4VP pellet sample.</figcaption></figure>

many other fragments (for example from masses 27 to 73 Da) result from polyfragmentation of the polymer chain and cannot be considered as characteristic, some others are clearly associated with the monomer (m/z 118 and 120) and the dimer (m/z 197). The tropylium cation described in Ref. [13] should be responsible for the peak corresponding to m/z 93. As sketched on Fig. 4, all the fragments result from benzylic scissions often associated with a loss of H or $CH_3$ radicals, ionization being a simultaneous process. Such a process of bond scission with localization of the charge at the functional side-group has already been described from static SIMS experiments which lead to similar spectra [13].

All the irradiated samples exhibit a very similar spectrum, without any significant changes in the ion yield whatever the integrated dose. Thus it can be provisionally concluded that the probability for the analyzing beam to induce the previously described process is the same from the intact network or from $\gamma$-irradiation-induced fragments, if any are formed.

## 3.2. HSF-SIMS: negative ion emission

From the negative ion spectrum of Fig. 5(a) only four peaks will be considered as characteristic of the material. Apart from the usual contaminants (m/z 49, 73, 97, 121), many $C_nH^n$ fragments (bearing the * label on the spectra) come from multifragmentation of the polymer backbone and do not reflect its chemical structure. As discussed in the following, the remaining fragments can be considered as characteristic of the polymer structure since their emission yield is sensitive to the $\gamma$-induced modifications. As sketched on Fig. 6, these fragments result from pyridinic scissions (A-type in the following) which can be single or in coincidence with a vinylic or benzylic scission (B-type) on a neighboring site. The m/z 78 peak is a typical illustration of scission A.
<figure><img src="image_5.png" /><figcaption>Fig. 4. Proposed mechanisms of positive fragment formation under ion beam irradiation.</figcaption></figure>

Simultaneous scissions A + B involve the pro-
duction of double bonds, leading to *m/z* 41,
when occurring near a chain end or to *m/z*
146 when occurring within a chain. Even con-
jugated double bonds could be produced
through A + A + B processes (*m/z* 158). The
poor mass resolution of our system cannot
be exclusively responsible for the width of
these last two peaks. A doublet can be sus-
pected at *m/z* 146–148 or even resolved at *m/z*
158–162 (see Fig. 5(c)): these doublets include

peaks that are interpreted are the capture of an
additional H instead of the release of an Hα
atom and double bond formation.

In sharp contrast to what is observed with
positive emission, the effects of gamma irradia-
tion are well detected. It can be qualitatively
observed that even at the lowest integrated
dose of 148 kGy, there is a noticeable increase
in the intensity of the characteristic peaks of
the original material (especially *m/z* 41 and 78;
Fig. 5(b)), being more pronounced for the
<figure><img src="image_6.png" /></figure>

Fig. 5. Negative ion mass spectra: (a) from an untreated P4VP pellet sample; (b) from an irradiated sample with an integrated dose of 148 kGy; (c) from an irradiated sample with an integrated dose of 14400 kGy. Characteristic ions appear in black squares, new ions following gamma-ray irradiation are circled, contaminants or $C_nH_m^-$ fragments are marked with an asterisk.

largest dose of 14400 kGy (Fig. 5(c)). In addition, new fragments are detected at low doses of irradiation, corresponding to *m/z* 183–184, 197 and 211 (circled on the spectrum of Fig. 5(b)). Since the gamma-ray interactions are randomly distributed within the material, simultaneous scissions are not expected from this process and only single A or B scissions have to be considered. Then, if the proposed mechanisms of ion formation sketched in Fig. 6 are relevant, it will be easier for the primary beam to produce such secondaries from

<figure><img src="image_7.png" /></figure>

Fig. 6. Proposed mechanisms of the characteristic negative ion formation under ion beam irradiation.

fragments resulting from A scissions induced by the gamma irradiation. As the simplest example, if the pyridine radical is preformed, the corresponding negative ion will only require an electron capture to be emitted. Also, as sketched in Fig. 7, the final emission of the ions at *m/z* 146 and 158 will be easier from fragments resulting from either A or B scissions; in the first case the analyzing beam
<figure><img src="image_8.png" /><figcaption>Fig. 7. Proposed mechanisms for the emission of the negative ions of m/z 146 and 158, from A and B scissions induced by gamma irradiation.</figcaption></figure>

must interact with modified chains, in the
second with dinner or trimer . free-radical
fragments.

As far as new fragments are observed, the
gamma irradiation must produce new entities

such as those represented in Fig. 8. The
production of such ions from a modified
chain only requires a B scission instead of
simultaneous B+B scissions never observed
from the unirradiated material. On the other
<figure><img src="image_9.png" /><figcaption>Fig. 8. Proposed mechanisms for the emission of new negative ions resulting from B scissions induced by gamma irradiation.</figcaption></figure>

hand, it has been previously shown that the fragment of mass 211, when formed under ion bombardment, is preferentially ionized and leads to $[M+H]^{+}$ ion emission. If preformed under gamma irradiation it could be responsible for the production of the related ions. Of course, when a benzylic scission occurs, the resulting modified chain is the counterpart of the production of the dimer and trimer radicals sketched in Fig. 7.

The yield variation of all these fragments as a function of the integrated dose is given in Fig. 9. It can be observed that the smallest dose is responsible for the steepest increase. However, for increasing doses, the yields of

the fragments already observed in the unirradiated sample tend to saturate while the emission of the new fragments exhibits a fast decrease and vanishes. Such behavior has been interpreted in terms of radical recombinations [5] above a given concentration of fragments or modified chains, also leading to a decrease in the mobility of the charged fragments. The somewhat different behavior of the m/z 41 fragment can be related not only to the fact that, being the smallest, it has the greatest mobility but also to the fact it results from scissions occurring at chain ends. As an extreme consequence, the ions corresponding to the largest fragments resulting from gamma
<figure><img src="image_10.png" /></figure>

Fig. 9. Evolution, as a function of the integrated dose, of the yield of the fragments: (a) observed on the unirradiated sample (m/z 41, 78, 146 and 158); (b) resulting from gamma-irradiation induced modifications (m/z 183, 184, 197, 211).

irradiation have a lower mobility and their emission is then strongly affected by the recombination processes.

### 3.3. *Study of the soluble and insoluble fractions of the irradiated material*

From the free radicals formed under gamma irradiations as well as their recombination products, those leading to linear fragments will be extracted with ethanol. Indeed, there is an increasing quantity of soluble material as a function of the dose of irradiation and such a variation is in good agreement with the corresponding variation of the number of spins per gram measured by electron paramagnetic resonance spectroscopy (EPR) [5]. As the simplest way to determine such a variation for a particular species in the solution, one can

<figure><img src="image_11.png" /></figure>

still use the HSF-SIMS technique applied to deposits taken from solutions containing a known amount of extracted material. However, the thickness of such deposits must be smaller than the emission depth of the emitted ions, i.e. within a range where the yield varies linearly with the layer coverage, as illustrated in Fig. 10. There the $[M+H]⁺$ emission yield has been measured from spin-coated targets corresponding to increasing soluble fraction concentrations of (a) a non-cross-linked resin or (b) a 2% cross-linked material irradiated with the largest dose. The first case corresponds to the presence of the longest chains in the solution, while the second corresponds to the highest soluble fraction with much smaller fragments. Indeed, the steeper slope is found when large chains are present, presumably leading to a better coverage. For the curve corresponding to the largest integrated dose, the concentration required for being below
<figure><img src="image_12.png" /><figcaption>Fig. 11. Variation, as a function of the integrated dose, of the yield of the positive ions [M+H]⁺ and those of masses 93 and 120, as obtained from thin films corresponding to the resulting concentration of soluble material. Also plotted are the corresponding values of the soluble fraction (%). All data are normalized to the value at 14400 kGy.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 12. Variation, as a function of the integrated dose, of the yield of the negative ions of mass 41 and 78 as obtained from thin films corresponding to the resulting concentration of soluble material.</figcaption></figure>

the saturation value (around 50 µg cm⁻² in the present example) sets the number of dilutions one has to perform on each solution corresponding to a given dose of irradiation. From the spin-coated targets so realized, the variation in yield of a given ion will reflect the extracted proportion of the related species.

Then, as shown in Fig. 11, the analysis of such targets leads to a yield variation of the main positive ions as a function of the irradiation dose very similar to the variation of the soluble fraction. Since the positive ion emission has been found insensitive to fragment formation when performed from the bulk sample, the present variation must be interpreted as a simple proportionality to the amount of extracted material whether it is a fragment or an unmodified chain. This indicates that a positive ion is formed with almost the same probability from a B scission and simultaneous ionization of the fragment formed, as from a simple ionization on the same fragment preformed by γ irradiation (see Fig. 4).

For the emission of negative ions, the yield variation will reflect the amount of free radicals from which they can be produced (as proposed in Fig. 7). Then, as observed in Fig. 12, the yields of the ions of m/z 41 and 78 remain similar whatever the integrated gamma dose, in contrast to what was observed for the bulk samples (Fig. 9). According to the proposed mechanisms sketched in Fig. 7, both A and B gamma-induced scissions will favor production of free radicals leading to the emission of the m/z 78 ion (pyridine fragment directly from A scissions or larger fragments resulting from B scissions). On the other hand, compared to the bulk situation, A scissions leading to modified but insoluble chains will not contribute to the emission of the m/z 41 ion, thus explaining the reduction of this emission relative to those of the pyridine ion. The emission of the other ions previously observed from the bulk is strongly
<figure><img src="image_14.png" /><figcaption>Fig. 13. LSF-SIMS negative ion mass spectra from (a) untreated P4VP pellet sample, (b) irradiated (and contaminated) sample with an integrated dose of 148kGy, and (c) irradiated sample with an integrated dose of 14400kGy.</figcaption></figure>

reduced ($m/z$ 158 is hardly detected). However, the fragments corresponding to the dimer are clearly identified in the solutions from the peaks at $m/z$ 197 and 211, and, in contrast to the bulk situation, the emission does not vanish when increasing the ion dose. From the mechanism proposed in Fig. 8, one may infer that the free radicals responsible for the emission can be extracted (i.e. they do not recombine). Consequently, it can be concluded that the extinction of the emission from the bulk results from some trapping of the ions instead of a recombination of the radicals produced.

The analysis of the remaining material once the extraction has been performed must confirm the previous findings. For the most favourable case of the highest dose (14400 kGy), the residue

was dried and the resulting pellet analyzed. The yield reduction for the most significant peaks at $m/z$ 78, 41, 146 and 158 is respectively 5.0, 2.6, 1.6 and 1.2. These numbers are only indicative since they result from a single experiment, but the qualitative trend is spectacular. As expected, the $m/z$ 78 ion is more easily formed from the extracted fragments while the $m/z$ 41 ion can be formed either from fragments or from modified end chains. As far as $m/z$ 146 and 158 are concerned, there is a clear indication that the related ions are formed from insoluble modified chains (Fig. 7). Accordingly, the dimer and trimer fragments formed by B scissions will more easily give rise to positive (Fig. 4) than negative ions.
<figure><img src="image_15.png" /><figcaption>Fig. 14. "Spontaneous" desorption negative ion mass spectra from (a) untreated P4VP pellet sample, and (b) irradiated (and contaminated) sample with an integrated dose of 148 kGy (same samples as for LSF-SIMS analysis shown in Fig. 13).</figcaption></figure>

### 3.4. LSF-SIMS

One of the major difficulties of a comparative study with LSF-SIMS is, of course, to perform the analysis under comparable experimental conditions. As an example, when using a Perkin-Elmer PHI 7200 TOF-SIMS instrument with 8 keV $Cs^+$ ions as primary particles one expects typical flux rates of about $4 \times 10^{10}$ ion cm$^{-2}$ s$^{-1}$, so that about $5 \times 10^{12}$ ion cm$^{-2}$ are received by the analyzed sample (three orders of magnitude greater than in HSF-SIMS). Then charges have to be neutralized using an electron flood gun with energies limited to 70 eV. However, the analysis conditions are close to ultrahigh vacuum ($10^{-9}$ mbar). Rather unexpectedly the major contribution to the spectra of Fig. 13 is contamination from persulfates. Although the presence of the peaks already observed with HSF-SIMS for the 148 kGy irradiation in the mass range 183–211 is unambiguous, some samples also exhibit an unexpected group in the $m/z$ 278–

339 range (Fig. 13 (b)). Finally, one has to be cautious about assigning the pyridine pattern only from its fragments ($CN^-$, $OCN^-$), although the variation of the yield of these fragments with the ion dose is very similar to what was previously observed with HSF-SIMS for the pyridine fragment.

The only way to perform LSF-SIMS analyses under precisely the same experimental conditions is to use the so-called "spontaneous desorption" phenomenon which has been well explained as a bombardment of polyatomic ions [6,7] with a broad mass distributions but at an energy corresponding to the target biasing (keV). Thus when comparing these results (spectra of Fig. 14) to the previous ones, the following observations can be made: (1) the pyridine fragmentation is higher at low energy and the process appears to be sensitive to the analyzing beam flux rate; (2) the mass distribution 183–211 appearing at the 148 kGy gamma dose is also detected at low energy; (3) there is confirmation that a large mass distribution from 278 to 339 can be observed on some samples where accidental contamination has been found from the lubricant of the press used for pellet production. These peaks are sequentially separated by 14 Da and disappear after some irradiation by the $Cs^+$ beam. The fact that they are never detected by HSF-SIMS confirms that the sensitivity to surface contamination is different for the two velocity regimes. However, points (1) and (2) provide more specific information about the scissions induced in the low velocity regime. Neither pyridinic nor benzylic scissions, nor their simultaneous occurrence, are observed as they are in high energy bombardment, but more "extended" damage seems responsible for the pyridine fragmentation. Conversely, the emission of ions resulting from gamma-induced dimer or trimer fragments may indicate that the corresponding ions result from softer processes (electron capture, $CH_2$ rupture). Also interesting in the
"spontaneous" desorption process is the unexpected presence of the *m/z* 211 ion in the unirradiated sample (Fig. 14(a)). One has to note that such a spectrum results from a fairly long acquisition time and consequently the primary ion dose is somewhat high (around 2 × 10¹⁰ ion cm⁻²). Then, an effect similar to gamma irradiation could be evoked, while under LSF-SIMS conditions, where the dose rate is typically five orders of magnitude higher, the sputtering effect is dominant.

## 4. Conclusion

From the SIMS analysis of the bulk samples and the soluble and insoluble fractions resulting from gamma irradiation, a coherent picture of the formation of fragments can be obtained for different energy deposition processes. One particular aspect of fast (MeV) ion interactions with the P4VP network is the possibility of inducing simultaneous pyridinic-pyridinic, or benzylic-pyridinic scissions or both. Subsequently, any single scission initiated by the gamma irradiation leads to preformed fragments from which the ion emission is easier under MeV bombardment. However, this is specific to negative ion formation. Since positive ion emission only requires benzylic scissions the phenomenon takes place either on intact or on prefragmented material. The occurrence of such simultaneous scissions is much less with low energy (keV) ions, thus explaining the lack of sensitivity of LSF-SIMS to gamma-induced preformed fragments. As a noticeable exception, dimers or larger fragments can be detected at low doses (as for MeV primary ions), but the related ion emission is strongly reduced when increasing the dose.

## Acknowledgements

The dedicated assistance of the accelerator crew of the Institut de Physique Nucléaire, M. Ferrari and A. Gardon and of M. Espagnan from COGEMA-Marcoule who performed the gamma irradiations, is gratefully acknowledged. Two of us (M.D. and M.L.) are grateful to R. Chomel from COGEMA-Marcoule for his support of the work.

## References

[1] T.E. Gangwer, M. Godstein and K.K.K.S. Pillay, Radiation effects on ion exchange materials, Report BNL-50781, Brookhaven National Laboratory, Upton, NY, 1977, p. 88.

[2] M. Lemaire, R. Chevillote, R. Chomel, P. Doutreluingne, A. Guy, J. Foos and R. Chomel, French Patent, 9, 202, 516, 1992.

[3] M. Draye, R. Chevillote, R. Chomel, P. Doutreluingne, A. Guy, J. Foos and M. Lemaire, Sep. Sci. Technol. 30(7-9) (1995) 1245.

[4] B. Nsouli, Ph.D. Dissertation (unpublished), Université Claude Bernard Lyon I, 1995.

[5] M. Draye, B. Nsouli, H. Allali, M. Lemaire and J.-P. Thomas, Polym. Degradation and Stability, submitted for publication.

[6] H. Allali, O. Debré, B. Lagrange, B. Nsouli, A.A. Oladipo and J.-P. Thomas, Nucl. Instrum. Methods Phys. Res. B, 108 (1996) 163.

[7] M. Benguerba, S. Della Negra and Y. Le Beyec, Proc. 39th ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, TN, 1991, p. 500.

[8] H. Feld, A. Leute, R. Zurmühlen and A. Benninghoven, Anal. Chem., 63 (1991) 903.

[9] B.U.R. Sundqvist, in R. Behrisch and K. Wittmaack (Eds.), Sputtering by Particle Bombardment III, Springer-Verlag, Berlin, 1991, p. 257.

[10] H. Allali, B. Nsouli and J.-P. Thomas, Int. J. Mass Spectron. Ion Processes, 127 (1993) 111.

[11] J.-P. Thomas, A.A. Oladipo and M. Fallavier, Nucl. Instrum. Methods Phys. Res. B, 42 (1989) 127.

[12] R.M. Papaléo, G. Brinkmalm, D. Fenyö, J. Eriksson, H.-F. Kammer, P. Demirev, P. Hakansson and B.U.R. Sundqvist, Nucl. Instrum. Methods Phys. Res. B, 91 (1994) 667.

[13] K.J. Hook, T.J. Hook, J.H. Wandas and J.S. Gardella, Jr., Appl. Surface Sci., 44 (1990) 29.