RELATION BETWEEN CRYSTALLOGRAPHIC TRANSFORMATION AND LIFETIME OF ION-IRRADIATED CARBON FOILS

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RELATION BETWEEN CRYSTALLOGRAPHIC TRANSFORMATION AND LIFETIME OF

ION-IRRADIATED CARBON FOILS

U. Sander, H. Bukow, H. V. Buttlar

To cite this version:

U. Sander, H. Bukow, H. V. Buttlar. RELATION BETWEEN CRYSTALLOGRAPHIC TRANS-

FORMATION AND LIFETIME OF ION-IRRADIATED CARBON FOILS. Journal de Physique

Colloques, 1979, 40 (C1), pp.C1-301-C1-303. �10.1051/jphyscol:1979163�. �jpa-00218443�

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JOURNAL DE PHYSIQUE Colloque Cl , suppldrnent au n " 2, Tome 40, fe'vrier 1979, page C1-301

RELATION BETWEEN CRYSTALLOGRAPHIC TRANSFOIiMATION AND LIFETIFB OF ION-IRRADIATED CARBON FOILS

U. Sander, H.H. Bukow, and H. v. Buttlar

Institut fur Experimentalphysik 111, Ruhr-Universitiit, Postfach 102148, D 4630 Bochum 1 RBsumB: Les f e u i l l e s de carbone auto-porteuses (7-30) pg/cmL s u b i s s e n t d e s t r a n s f o r m a t i o n s s t r u c t u - r e l l e s pendant l ' i r r a d i a t i o n p a r l e s i o n s de He (E<400 keV; d e n s i t e de c o u r a n t (0,l-4,O) p ~ / m m ~ ) c e q u i cause une augmentation de l a tempgrature j u s q u ' d (600-1300) K . Apr6s une expansion i n i t i a l e , l a f e u i l l e montre une c o n t r a c t i o n l e n t e , accompagnee d'une t r a n s f o r m a t i o n v e r s un m e i l l e u r o r d r e c r i s t a l - lographique. La r a p i d i t s de l a g r a p h i t i s a t i o n depend (1) de l a p a r t i e n u c l e a i r e d e ' l a p e r t e d ' g n e r g i e s p 6 c i f i q u e c e qui v e u t d i r e : de l ' g n e r g i e d e l ' i o n etdu num6ro atomique e t ( 2 ) de 1 ' B p a i s s e u r de l a f e u i l l e . La dose de f r a c t u r e d e s f e u i l l e s de (7-10) pg/cm2 e s t (2,3f0,4) keV/pg.

A b s t r a c t : S e l f - s u p p o r t i n g carbon f o i l s (7-30) pg/cm2 undergo s t r u c t u r a l changes d u r i n g i r r a d i a t i o n wiih H e i o n s (E<400 kev; c u r r e n t d e n s i t y (0.1-4.0) y ~ / m m ~ ) with causes t h e temperature t o r i s e (600-1300)K.

A f t e r an i n i t i a l expansion t h e f o i l undergoes a slow c o n t r a c t i o n which i s accompanied by a transforma- t i o n t o a h i g h e r degree o f c r y s t a l o r d e r . The r a t e of g r a p h i t i z a t i o n depends on ( 1 ) t h e n u c l e a r p a r t o f t h e s p e c i f i c energy l o s s , i . e . i o n energy and atomic number, and ( 2 ) t h e t h i c k n e s s of t h e f o i l . The breakage dose f o r f o i l s o f (7-10) pg/cm2 i s (2.320.4) keV/pg.

I. INTRODUCTION

Self-supporting carbon foils which are used rature 161 and determines the rate of gra- in beam-foil spectroscopy have a finite phitization and the lifetime of the foil.

lifetime before destruction in the beam. e) The temperature of the foil rises up to The correlations between the macroscopic 1300 K under irradiation. A continuous phenomena (appearance of the foil, foil n'oncharacteristic electromagnetic radiation thickness, breakage) and the irradiation is emitted from the surface [6].

parameters (current density, beam energy, In this paper we propose a practical defi- atomic number of the projectile) have been nition of the foil lifetime and discuss the investigated by many authors [ I - 4 1 . Recent- of beam and foil ly we have performed a detailed study of ness. Most of the measurements are per- the microscopic transformations occurring formed using He+ ions as projectiles. The during irradiation of a carbon foil [ 5 ] . energy range is (60-400) keV.

The following facts have been observed:

a) Under irradiation amorphous carbon transformes from a disordered to an ordered structure approaching that of graphite

(graphitization).

b) The graphitization during irradiation is the result of heat deposited in the foil caused by the traversing ions. Graphitiza- tion can be produced by irr8diation.with ions or electrons as well as by thermal heating.

C) Graphitization is accompanied by an increase of mass density, causing mechanical stress between the irradiated and non-irradiated parts of the foil. Mechanical stress is the reason for foil breakage.

d) Only the nuclear part of the total energy loss'causes the rise of the foil tempe-

11. EXPERIMENTAL ARRANGEPENT

The ion beam traverses the foil tilted at 45O with respect to the beam axis (fig. 1) and is measured in a shielded Faraday cup.

The surface radiation from the foil is observed at 90° to the beam axis. The obser- vation system consists of an achromatic

lens, a monochromator (Jobin Yvon HL f/2) and a photomultiplier (EM1 9558 QB) in the single-photon counting mode. The monochromator is set at the wavelength X =695 nm

0

because no emission lines are observable within Xo?l0 nm. Ion current and radiation intensity are measured simultaneously. The vacuum is about 4.10 -7 mbar, thus largely reducing carbon deposition. The irradiated foil area is 0.071 cm2.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979163

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JOURNAL DE PHYSIQUE

of the first hole or fissure. The time r:tl

,

~ ~ o c h r o m a t o r

slit

1

Foil

F i g . 1 Experimental arrangement

111. DEFINITION OF THE LIFETIHE

When a foil of about 10 pg/cm2 is exposed to the beam, the surface radiation intensity S soon reaches a steady state value

(fig. 2) showing that the equilibrium temperature of the foil is established. (A slight continuous increase of S during the lifetime of the foil was observed with foils of (20-40) ug/cm2.) The current I (the charged fraction of the beam behind the foil) increases about 10 % during a time ranging between some seconds and a minute under irradiation (0.1-4.0) uA/mm 2 current density). The time to at which a steady value is established we call the

"running-in time T". After a time tl a sudden simultaneous variation of surface radiation and current marks the appearance

we call the lifetime of the foil. We note that the beam current does not change in the special case that the average charge of the beam after the foil is identical with the incoming beam.

IV

.

^RESULTS

Typical measurements are collected in table I.

(1) The running-in time T is of the order of a tenth of the total lifetime T. The mean energy dose deposited during T is (3.221.3) x10I4 keV/pg, = (0.052+0.021) J/vg, if only the nuclear part of the energy loss is con- sidered.

(2) The lifetime T depends on the surface density (i.e. the thickness) of the foil, in contrast to the result of Livingston et al. [ 4 ] : In fig. 3 the lifetime is plotted versus the specific nuclear energy Loss [ 7 1 for measurements with the same current density but with different foil thickness. Ob- viously, for a given incident energy the lifetime of a thicker foil is shorter. As the energy transfer per target atom and therefore the internal heating of the foil is independent of the foil thickness, this means that the energy dissipation from the foil (its cooling) is determined not by its volume but by its surface [6,81. This ex- plains why cooling 05 the foil frame has no influence on the lifetime [ 9 1 .

F i g . 2 S u r f a c e r a d i a t i o n i n t e n s i t y S and i o n c u r r e n t I a s f u n c t i o n o f i r r a d i a t i o n time a ) f o i l f r a c t u r i n g s t e p w i s e b) f o i l b r e a k i n g suddenly

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F i g . 3 L i f e t i m e T a s f u n c t i o n o f i o n energy (upper s c a l e ) and s p e c i f i c n u c l e a r energy l o s s (lower s c a l e ) . A l l d a t a f o r same c u r r e n t d e n s i t y .

(3) The lifetime depends on the ion energy:

Selecting measurements with same current density and identical thickness we see that the lifetime increases with increasing beam energy. Or, as shown in fig. 3, the lifetime decreases with increasing nuclear energy loss. This explains the trend of fig. 3 in ref. [ I ] .

(4) Further measurements with hydrogen and lithium as projectiles in the energy range

(80-400) keV show that the lifetime is re- duced by increasing the atomic number. This is because the specific nuclear energy loss increases.

V THE DOSE FOR FOIL BREAKAGE

The energy dose DT per ug leading to foil destruction is related to the impinging

Table I

current I, the unit of charge e, the surface density u, the irradiated areaA, the part of the particle's energy loss leading to foil destruction AE and the lifetime r : Dobberstein et al. [ 2 ] proposed to identify AE with the nuclear part of the total energy loss. With the specific nuclear energy loss A~,(keV/ug cm-2) we have

DT = rIAan/eA.

From our experiments (table I ) we deduce D =(2.3co.4)-10'~ keV/ug=(0.37?0.07) ~ / u g for foils of (7-10) ug/cm 2

.

This value, h o w ever, decreases with increasing foil thickness. Using this result it is possible to predict the lifetime of a foil under given experimental conditions. For example, in a 0.93 PA current of 3 1 ~ 4 + ions at 21 MeV the lifetime of a 10 ug/cm2 foil should be r=

(43+13) min. This is in agreement with the observed value T = 60 min [ l o ] .

A heat treatment of the foil [1,21 preforms the graphitization [ 5 ] . In order to 2ro- longe the lifetime, the foil should be gra- phitized

-

if possible

-

at (2000-3000) K without experiencing mechanical stress before exposure to the beam.

BIBLIOGRAPHY

[ I ] J . L . Yntema, Nucl. x n s t . and Meth.

113,

605 (1973) ;

122,

45(1974) ; IEEE Trans. Nucl. S c i . NS 2, 1133(1976)

1-21 P. Dobberstein, L. Henke. Nucl. I n s t . and Meth.

. -

119, 611(1974)

[3]

-

P.D. Dumont, A.E. L i v i n g s t o n , Y . Baudinet- Robinet, G. Weber, L. Q u a g l i a , Physica S c r i p t a

13, 122 (1976)

[4] KE. L i v i n g s t o n , H.G. Berry, G.E. Thomas, Nucl.

I n s t . and Meth.

148,

125(1978)

[5] U. Sander, H.H. Bukow ( s u b m i t t e d t o R a d i a t i o n E f f e c t s )

[6] H.H. Bukow, G. Heine, U. Sander, H.

v. B u t t l a r , Procceedings V I I . I n t . Conf. on Atomic C o l l i s i o n s i n S o l i d s September 19-23, 1977, Moscow ( i n p r i n t )

[7] J.B. B i e r s a c k , E. E r n s t , A. Monge, S. Roth, Hahn-Meitner-Institut Ber- l i n , Report HMI-B 175 (1975) [8] R.D. Hight, R.M. Schectman, H.G.

Berry, T. G a b r i e l s e , T. Gay, Phys.

Rev. A E , 1805(1977)

[9] R.B. Gardiner, R a d i a t i o n E f f e c t s 35,

151 (1978)

[ 101 P

.

^H. ^Heckmann^,p r i v a t e communicaticm Surface

d e n s i t

Y

u/ugcm

7 7 10 7 7 7 7 10

I n c i d e n t beam energy E/keV

80 80 80 100 100 100 100 100

Running - i n dose DT/keV/

119

x10 14 Y:75

-

2.01 1.84 3.53 4.66 4.49 4.42 S p e c i f i c

n u c l e a r energy l o s s Asn/keV/

pgcm-2

0.0165 0,0165 0.0165 0.0139 0.0139 0.0139 0.0139 0.0139

L i f e t i m e T / S

114 6 0 150 348 210 84 66 156 Beam

cur- r e n t I/uA

10.0 25.0 11.5 5.0 12.0 19.0 30.5 12.0

L e t h a l - dose DT/keV/

P9

x 1 ~ 1 5 1.66 2.18 2.51 2.13 3.09 1.96 2.47 2.30 Running-

i n time T/s

12 -

12 30 24 20 12 30