angular distribution of au–lα and -mαβ x-rays induced by 40 mev c4+
Post on 26-Jun-2016
212 Views
Preview:
TRANSCRIPT
ARTICLE IN PRESS
0969-806X/$ - se
doi:10.1016/j.ra
�Correspondfax: +91 22 228
E-mail addr
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
References
Berezhko, E.G., Kabachnik, N.M., 1977. Theoretical study of
inner-shell alignment of atoms in electron impact ionisation:
angular distribution and polarisation of X-rays and Auger
electrons. J. Phys. B 10, 2467–2477.
Hitachai, A., Awaya, Y., Kambara, T., Kanai, Y., Kase, M.,
Kumagai, H., Takahashi, J., Mizogawa, T., Yagishita, A.,
1991. Angular distribution of Ti K X-rays and Sn L X-rays
induced by 6MeV nucleon N-ion impact. J. Phys. B. 24,
3009–3018.
Jitschin, W., Hippler, R., Shankar, R., Kleinpoppen, H.,
Schuch, R., Lutz, H.O., 1983. L X-ray anisotropy and L3
subshell alignment of heavy atoms induced by ion impact.
J. Phys. B 16, 1417–1431.
Kumar, A., Garg, M.L., Puri., S., Mehta, D., Singh, N., 2001.
Angular dependence of L3 XRF cross sections following
selective L3 sub-shell photoionization in Pb. X-ray spectro-
metry 30, 287–291.
Mehlhorn, W., 1968. On the polarization of characteristic
X radiation. Phys. Lett. A 26, 166–167.
Mitra, D., Sarkar, M., Bhattacharya, D., Sen, P., Kuri, G.,
1998. Measurement of the anisotropy parameters for the M
X-rays of gold induced by 3–9MeV carbon ions. Nucl.
Instrum. Methods 145 (3), 283–287.
Yamaoka, H., Oura, M., Takahiro, K., Takeshima, N.,
Kawatsura, K., Mizumaki, M., Kleiman, U., Kabachnik,
N.M., Mukoyama, T., 2002. Angular distribution of Au and
Pb L X-rays following photoionization by synchrotron
radiation. Phys. Rev. A 65, 062713 and references therein.
top related