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Radiation Physics and Chemistry 75 (2006) 1490–1492

www.elsevier.com/locate/radphyschem

Angular distribution of Au–La and -Mab X-raysinduced by 40MeV C4+

Ajay Kumar, D. Misra, U. Kadhane, A.H. Kelkar, L.C. Tribedi�

Tata Institute of Fundamental Research, Colaba, Mumbai 400 005, India

Accepted 27 July 2005

Abstract

The angular distribution of Au–La and -Mab X-rays in collisions with 40MeV C4+ ions has been measured with

Si(Li) detector. The measured X-ray yield is found to be anisotropic and the anisotropy parameter has been derived.

r 2006 Published by Elsevier Ltd.

Keywords: High-energy ions; X-rays; Angular distribution

1. Introduction

Ionization of atoms by charged particles and photons

leads to the alignment of target inner-shell vacancy with

the total angular momentum ðJÞ412and this is because

of non-statistical population in the different magnetic

substates (Mehlhorn, 1968). Auger electrons or X-rays

emitted in the subsequent decay manifest this alignment

through their anisotropic angular emission or through

polarization of the X-rays. The angular distribution of

the X-rays emitted after ionization of an atom is

described by the equation (Berezhko and Kabachnik,

1977)

dI

dO¼

Io

4p½1þ bP2ðcos yÞ� (1)

where P2(cos y) is the second-Legendre polynomial, y is

the angle between incident beam and emitted X-rays, Iois the total X-ray intensity integrated over all angle, and

dO is the solid angle subtended by the detector at the

target. Here, b is the anisotropy parameter and is the

e front matter r 2006 Published by Elsevier Ltd.

dphyschem.2005.07.011

ing author. Tel.: +91 22 22804545x2465;

04610, +91 22 22804611.

ess: lokesh@tifr.res.in (L.C. Tribedi).

product of kinematic term (a) and the degree of

alignment (A20) (Yamaoka et al., 2002).

Extensive measurements have been carried out for

L3-subshell (J ¼ 32) alignment by photon, proton and

electron bombardment (Yamaoka et al., 2002). To

observe maximum anisotropy in the L3 subshell X-rays,

the L X-rays have been measured by selective ionization

of L3 subshell using synchrotron radiation (Yamaoka et

al, 2002) and photons in the secondary excitation mode

(Kumar et al., 2001). The Ll, La and Lb X-rays were

found to be isotropic within experimental error.

Only a few measurements are available in the

literature on the study of L3-subshell alignment with

highly charged ions. Jitschin et al. (1983) show an

alignment in the L3-subshell in heavy ion collisions.

Hitachai et al. (1991) have studied the Sn La X-rays and

their satellites due to 6MeV/u N7+ ions using Si(Li)

detector and crystal spectrometer. The L X-rays and

their satellites were observed to be isotropic within

experimental error. The data for M-shell alignment is

scarce except a few (Mitra et al., 2001).

In the present work, we have measured the angular

distribution of Au–La (L3–M4,5) and -Mab (M5–N6,7;

M4–N6) X-rays in collisions with 40MeV C4+ions. The

anisotropy parameter has also been derived.

ARTICLE IN PRESS

20 40 60 80 100 120 140 160

60

80

100

Angle, θ (in degree)

AM

αβ /A

Lβ

Fig. 2. The Au–Mab X-rays yield relative to the Lb X-ray yield

for different emission angles. The solid line illustrates the trend

only.

A. Kumar et al. / Radiation Physics and Chemistry 75 (2006) 1490–1492 1491

2. Experimental details

The 40MeV C4+ ion beam was obtained from the

BARC-TIFR pelletron accelerator facility at Mumbai,

India. The mass and energy analysed ion beam

was made to fall on thin Au target (3 mg/cm2) on C

backing (of thickness �10 mg/cm2). Thin target was used

to avoid multiple collisions. The target was mounted

at 901 to the beam direction on a rotatable multiple

target holder assembly in an electrically isolated

chamber. The emitted target X-rays have been detected

at 201, 451, 601, 751, 1051, 1201 and 1551 angle,

with respect to the beam direction. A Si(Li) detector

having a resolution of 180 eV at 5.89 keV was used.

The detector has a Be window of thickness 25mm in

front of it. The detector was kept in the vacuum and the

chamber pressure during the measurement was

�2� 10�6mbar. Fig. 1 shows a typical M X-ray

spectrum of Au taken with 40MeV C4+ at 751 angle.

The Mg (M3–N5) X-ray peak area was subtracted from

the composite Mabg peak using fitting procedure and

found �11% of the total Mabg peak area. The back-

ground spectrum from C-foil shows negligible contribu-

tion in the Mab region whereas a substantial fraction of

Si X-rays was found in the Mx (M5–N2,3) region.

Apparently strong contribution of the Mx intensity

compared to Mab intensity (Fig. 1) could be due to

additional contribution of Si K X-rays which could not

be fully subtracted out. However, this doesn’t affect the

present study since we are interested in Mab line only.

The background subtracted X-rays peak area was

estimated using a multi-Gaussian least-square-fitting

programme.

2.0 2.5 3.00

1000

2000

3000Angle-75°

M3O1,4,5+ M2N4+ M1N3

Au-M γ

Au-Mαβ

Si-Kα+Au-Mζ

Cou

nts

Energy (keV)

Fig. 1. Typical M X-rays spectrum of Au induced by 40MeV

C4+ ions and a typical Gaussian fit (dotted line) of the

background subtracted spectrum.

3. Result and discussion

The X-rays originating from L1 and L2 (J ¼ 12)

subshells are expected to have isotropic emission. The

Lb X-rays were also found to be isotropic. In Fig. 2, we

have plotted X-ray intensity ratio (AMabXALb) of Mab

and Lb, corrected by detector efficiency, as a function of

emission angle. This ratio is independent of detector

solid angle error. Similar plot was obtained for La/Lb X-

ray yield ratio (not shown). These relative intensities of

La and Mab was further plotted as a function of P2(cos y)term, in order to derive the anisotropy parameter (b).The values of b were found to be �0.0770.02

and �0.0870.02 for La and Mab, respectively. The

quoted error for the measured b values is due to errors in

the counting statistics and the angular distribution

fitting.

Earlier, the b value of �0.23 has been measured for

Mab X-ray lines with 3–9MeV C ions (Mitra et al.,

1998). It is worth noting that present and earlier

measured values of b show opposite sign of anisotropy.

It could be due to incident beam energy difference as bvalue is highly specific to the collision velocity (Jitschin

et al., 1983). More measurement on the M-shell

alignment is needed with different projectile–atom

combination at various incident energies.

Acknowledgements

Authors are thankful to the pelletron accelerator staff

at TIFR, Mumbai for their skillful operation of the

machine.

ARTICLE IN PRESSA. Kumar et al. / Radiation Physics and Chemistry 75 (2006) 1490–14921492

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