ENERGY DEPENDENCE OF PROTON INDUCED L-SUBSHELL IONIZATION CROSS SECTION RATIOS FOR Ag

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ENERGY DEPENDENCE OF PROTON INDUCED L-SUBSHELL IONIZATION CROSS SECTION

RATIOS FOR Ag

A. Costa, F. Parente, M. Costa-Lima, M. Ramos

To cite this version:

A. Costa, F. Parente, M. Costa-Lima, M. Ramos. ENERGY DEPENDENCE OF PROTON IN-

DUCED L-SUBSHELL IONIZATION CROSS SECTION RATIOS FOR Ag. Journal de Physique

Colloques, 1987, 48 (C9), pp.C9-255-C9-258. �10.1051/jphyscol:1987943�. �jpa-00227361�

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JOURNAL DE PHYSIQUE

Colloque C9, suppl6ment au n012, Tome 48, dgcembre 1987

ENERGY DEPENDENCE OF PROTON INDUCED L-SUBSHELL IONIZATION CROSS SECTION RATIOS FOR Ag

A.M. COSTA, F. PARENTE, M.T. COSTA-LIMA and M.T. RAMOS

Departamento de Fisica and Centro de Fisica ~ t 6 m i c a da Universidade de Lisboa (INIC), Avenida Pr. Gama Pinto, 2 , P-1699 Lisboa Codex, Portugal

ABSTRACT

Des c i b l e s de Ag ont 6tG bombardges par des protons de 0.6

-

1.8 MeV produits par l ' a c c 6 l e r a t e u r Van de Graaff de 2 MV

5

Sacavem, Portugal. Les spectres L a d'6mmission X de Ag ont ?it6 obtenus

5

1 'a i d e d'un spectrographe

5

c r i s t a l plan e t d'un detecteur de local i s a t i o n . La resolution en 6nergie du systeme permet de bien sGparer l e s principales r a i e s des spectres. p a r t i r desmesures d 1 i n t e n s i t 6 des r a i e s X L B nous avons calculg l e s rapports o / o l e t 03/01 en fonction de 1 'Gnergie des protons. Les k s u l t a t s de nos mesures ont e t 6 comparGs avec des valeurs experimentales e t theoriques obte- nues par d ' a u t r e s auteurs.

Thick t a r g e t s of Ag were bombarded by 0.6

-

1.8 MeV protons accelerated by the 2 MV Van de Graaff generator a t Sacavem, Portugal. The LB X-ray spectra were obtained with a plane crystal spectrometer and a position-sensitive detector. The energy resolution of the system i s such t h a t a l l the diagram 1 ines of the spectra a r e well resolved. From the L B X-ray l i n e i n t e n s i t i e s and using other inner-shell q u a n t i t i e s taken from the l i t e r a t u r e , the r a t i o s o / o and o / o were obtained a s function of proton energy and $omJared

t8

oiher theoretical and experimental r e s u l t s .

1 . Introduction

The ionization of atomic inner s h e l l s by charged p a r t i c l e s has been the object of intense research in the l a s t two decades. A great deal of experimental and theoretical work can be found in the l i t e r a t u r e [ I ] . Although the K-shell can be considered very well studied and understood, f o r the L-shell problems s t i l l remain.

There e x i s t s c l e a r l y a need of experimental work t o t e s t the theoretical models which have been proposed t o describe L-shell ionization by protons, such a s the c l a s s i c a l binary encounter approximation (BEA)

,

the semiclassical approximation

(SCA) and the plane-wave Born approximation (PWBA), with d i f f e r e n t types of correc- tions.

In t h i s work we report experimental r e s u l t s on the r e l a t i v e L-subshell ionization cross sections of Ag, obtained with bombarding protons in the energy range 0.6

-

1.8 MeV and a high-resolution spectrometer.

Benka e t aZ. [2] published experimental values of r e l a t i v e ionization cross sec- tions 02/01 and 03/02, f o r Ag, Sb and Au in the proton energy range 0.2

-

^0.9^MeV.

Rosato [3] reported L-subshell ionization cross sections f o r proton bombardment of Ag, In, Sn and I in the proton energy range 0.3

-

5.0 MeV. Both these authors used

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987943

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C9-256 JOURNAL DE PHYSIQUE

S i ( L i ) detectors. Kropf [4] measured L-subshell i o n i z a t i o n cross s e c t i o n s f o r e l e - ments i n t h e range 40

<

Z 6 51, f o r p r o t o n energies 0.1

-

0.9 MeV, u s i n g a c r y s t a l spectrometer and a p r o p o r t i o n a l counter. Experimental r e s u l t s on L - s h e l l X-ray pro- d u c t i o n and i o n i z a t i o n cross sections f o r p r o t o n impact have been summarized by Sokhi et aZ. [5].

On the t h e o r e t i c a l side, Cohen et aZ.[6] p u b l i s h e d K- and L - s h e l l i o n i z a t i o n cross sections f o r protons and helium i o n s c a l c u l a t e d i n t h e PWBA approximation w i t h c o r r e c t i o n s f o r energy loss, Coulomb d e f l e c t i o n , p e r t u r b e d s t a t i o n a r y s t a t e and r e l a t i v i s t i c e f f e c t s (ECPSSR).

2. Experimental Method

Proton beams i n t h e energy range 0.6

-

1.8 MeV were obtained from t h e 2 MV Van de Craaff a c c e l e r a t o r a t Sacavem and analysed w i t h i n

+

1 keV. An aluminum t a r g e t chamber o f 70 cm diameter was used. I n t h i s chamber a plane c r y s t a l spectrometer was placed, f i t t e d w i t h a q u a r t z c r y s t a l . The 1010 planes ( d = 4 . 2 5 0 ~ 1 0 - ~ pm) were used t o Bragg s c a t t e r t h e X r a d i a t i o n e m i t t e d by a Ag f o i l 0.026 g/cm 2 t h i c k placed i n

FIG. 1. ( a ) F i t o f

L$

Ag X-ray spectrum, f o r 1.64 MeV protons, w i t h the f u n c t i o n described i n t h e

4.0

2.0

g 3 ,

1

-20

-4.0

160 310 4GO 610

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t e x t . (b) Residuals given as m u l t i p l e s o f one stan- dard d e v i a t i o n .

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t h e p r o t o n beam. A t h i c k t a r g e t had t o be used due t o t h e poor e f i c i e n c y o f t h e ana- l y s i n g system.

The t a r g e t f o i l was k e p t a t an angle o f 45' r e l a t i v e t o the beam d i r e c t i o n . We used a commercial backgammon-type p o s i t i o n s e n s i t i v e d e t e c t o r manufactured by INEL.

The d e t e c t o r has a 50 mm x 12 mm Be window 250 vm t h i c k . 3. Results

The LB l i n e s o f Ag, f o r 7 values o f p r o t o n energy, were recorded i n a Camberra multichannel analyser and computer analysed by a l e a s t squares method. As shown i n f i g . 1, t h e spectrometer was a b l e t o completely separate t h e LB1, LB3, LB4, LB6 and LB2 l i n e s o f Ag. The f i t t i n g f u n c t i o n was a q u a d r a t i c background and several peak f u n c t i o n s , each one being an admixture o f a L o r e n t z i a n and a Gaussian f u n c t i o n .

The r e l a t i v e L-subshell i o n i z a t i o n c r o s s sections were computed from t h e expres- sions:

OLB, = (fI2o1 + u2) u2FgI2

where the oLa. ( f o r j=1,4) and the a. ( f o r i=1,3) a r e the X-ray p r o d u c t i o n and i o -

1

n i z a t i o n crossJsections, r e s p e c t i v e l y . The Coster-Kronig y i e l d s fij (i=1,2; j=2,3), and the subshell fluorescence y i e l d s wi (i=1,3) a r e taken from Krause [7]. The F

B i j (i=1,4; j=1,3) a r e the f r a c t i o n o f t h e r a d i a t i v e t r a n s i t i o n t o the j subshell asso- c i a t e d t o the L B i peak. Values o f FBij were obtained from S c o f i e l d [8].

One advantage o f t h e method i s t h a t we can o b t a i n the r e l a t i v e i o n i z a t i o n cross

w..

? i

FIG. 2. Experimental and t h e o r e t i c a l r a t i o s 02/01

o f Ag L-subshell i o n i z a - t i o n cross sections.

r- Cohen C61; A- Kropf [4]; 0 - Benka [21;

e-

present work.

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JOURNAL DE PHYSIQUE

FIG. 3. Experimental and t h e o r e t i c a l r a t i o s u3/al o f Ag L-subshell i o n i z a - t i o n cross sections.

m- Cohen [6]; A- Kropf [4];

o-

Benka [2];

a- present work.

s e c t i o n s u s i n g d i f f e r e n t combinations o f l i n e i n t e n s i t i e s from the same spectrum.

Thus, t h e 02/a1 r a t i o was computed s e p a r a t e l y from I / I and ILg /ILg

,

and the a3/a, r a t i o from I /I and I / I

,

where

LB2 LB3 LB2 LB4

kB1

^Lai

~PzI,~]

i s b e i t t e n s i t y o f l i n e LBi.

I n f j g u r e s 2 and 3 a r e p l o t t e d t h e r a t i o s a / a and a3/al, r e s p e c t i v e l y , obtained 2 1

i n t h i s work, as f u n c t i o n o f p r o t o n energy, together w i t h t h e o r e t i c a l and experimen- t a l values o f o t h e r authors.

The u n c e r t a i n t i e s i n t h e atomic parameters used i n t h e c a l c u l a t i o n (fluorescence y i e l d s and Coster-Kronig r a t e s ) a r e t h e main source o f e r r o r and l e a d t o 30-35% e r - r o r bars i n t h e i o n i z a t i o n cross sections. The c o r r e c t i o n f a c t o r s due t o d e t e c t o r e f f i c i e n c y and c r y s t a l r e f l e c t i v i t y on the i n t e n s i t y r a t i o s a r e negligeable. The t a r g e t thickness c o r r e c t i o n s a r e included i n the e r r o r bars.

The experimental i o n i z a t i o n cross s e c t i o n r a t i o s obtained i n t h i s work a r e i n b e t t e r agreement w i t h t h e o r e t i c a l values than those o f Benka e t aZ. [Z]. However, t h e o r e t i c a l p r e d i c t i o n s (ECPSSR) appear overestimated. The experimental values o f Kropf [4] a r e h i g h e r than t h e o r e t i c a l ones i n t h e low energy r e g i o n b u t a r e i n agreement w i t h ours f o r p r o t o n energies around 0.8 MeV.

References

[I] Gray (T.J:), i n Methods of EqerimentaZ Physics, P. Richard ed., Academic Press, N.Y., 1980.

[2] Benka (O.), Geretschlager

(M.),

Kropf (A.), Paul (H.), Semrad (D.), J. Phys.

B: Atomic and MoZecuZar Phys. 1976, 9, 779.

[3] Rosato (E.)

,

Phys. Rev. A , 1983, 28,-2759.

141 Kropf (A.), Ph. D. Thesis, L i n z , 7 9 8 2 (umpublished).

151 Sokhi (R.S.), Crumpton

(D.),

At. Data and Nuclear Data TabZes, 1984, 3 0 , 49.

[6] Cohen (D.D.)

,

Harrigan (M. )

,

A t . Data and WucZear Data Tables, 1984,

33,

255.

[7] Krause (M.O.)

,

J . Phys. Chem. Ref. Data, 1979, 8, 307.

[8] S c o f i e l d (J.H.), A t . Data and Nuclear Data ~ a b z e s , 1974, - 14, 121.