IEEE Transactions on Nuclear Science, Vol. NS-30, No. 2, April 1983 DEUTERON- AND ALPHA-INDUCED K-SHELL IONIZATION CROSS SECTION RATIOS IN THE Z=12-17 REGION * +§ J.S. Lopes , A.P. Jesus and M.F. da Silva§ * Departamento de Fisica, Faculdade de Ciencias da Universidade de Lisboa, 1200 Lisboa, Portugal +Centro de Fisica Nuclear da Universidade de Lisboa, Faculdade de Ciencias, 1200 Lisboa, Portugal iDepartamento de Flsica, Laborat6rio Nacional de Engenharia e Tecnologia Industrial, 2685 Sacavem, Portugal Abstract Experimental deuteron- and alpha-induced K-shell ioni- sation cross section ratios are presented for elements ranging from Mg to Cl (Z=12-17) in the incident energy range 0.2 to 0.45 MeV/u. The experimental ratios R= a /4cdare generally close but slightly higher than the theoretical ratios calculated according to the pro- cedure of Brandt and Lapicki; the largest differences occur for Al and Si where the experimental values are 15-20% too high. The experimental ratios show roughly the same dependence on reduced projectile velocity, C= (v1 /vK)(2/0K),as the theoretical ones in the reduced incident velocity range used in the present work, E=0.45 to E=1. This is in opposition to the results previously obtained in the velocity range =0.25 to i=0.55 for elements ranging from Ca to Cu (Z=i20-29), where the observed ratios are rather independent of the reduced velocity. Introduction In K-shell hole production by light ions with velocity V1 smaller than the target electron velocity vK, the ionising collision takes place inside the shell. When the collision occurs the electron orbit has already been modified in response to the increase in the Cou- lomb attraction, and this leads to an appreciable lowering of the ionisation cross section. The opposite is true when v 1>vK and the collision occurs mainly out- side the shell. A very direct way of studying thisbinding energy effect, first described by Brandt et al.1 , is to ex- perimentally determine the ratio of alpha- and deuteron -induced ionisation cross sections. These ratios are insensitive to other effects influencing the cross sections and thus allow an essentially independent check on the accuracy of binding effect calculations and correction procedures. Here we present results pertaining to elements ranging from Mg to Cl in the incident energy range 0.2 to 0.45 MeV/u; the corresponding reduced velocities span the range E= 0.45 to i= 1, where the projectile velocity approaches the velocity of the target electron. Experimental The experimental set-up has been described in detail 23 elsewhere . Alpha and deuteron beams from 0.2 to 0.45 MeV/u were obtained from the 2MV Van de Graaff accele- rator at Sacavem and analysed to within + 1 keV; an NMR fluxmeter was used to determine the beam energy. The targets were evaporated onto %15 vig/cm2thick C foils except for Al which was self supporting. Target composition and total thickness in units of 1017 atoms cm 2 were: MgO (1.6), Al (1.7), SiO2 (8.5), Zn3P2 (1.4), Sb2 53(2.0), CsCl (1.2). The x-ray production cross sections were determi- ned through the dead time and efficiency corrected numbers of simultaneously detected x-rays and parti- 3 cles . Where necessary cross sections were corrected for target thickness as described in Ref.3; these cor- rections were generally under 5% except for Si where they reached 11% for 0.2 MeV/u alpha particles. The same set-up, including targets and detectors, was used in the alpha and deuteron measurements so that errors due to solid angles and detector efficien- cy cancel out in the cross section ratios. Results The ratios R= a /4ad between the alpha- and deuteron -induced x-ray production cross sections at several incident energies per atomic mass unit are shown in table 1; the corresponding errors are generally esti- mated not to exceed 5%. We take the ionisation cross section ratios to be equal to those in table 1 by as- suming the fluorescence coefficients to be the same for alpha- and deuteron-induced ionisation in spite of their dependence on atomic configuration and,there- by, on multiple ionisation. The work of Richard et al. has shown that in alpha-induced ionisation in Al over the energy range covered in the present work only about 20% of the ionised atoms are left in a single hole state, while %50% are KL-two hole and %25% are 0018-9499/83/0400- 954$01.00C 1983 IEEE 954
Transcript
IEEE Transactions on Nuclear Science, Vol. NS-30, No. 2, April 1983
DEUTERON- AND ALPHA-INDUCED K-SHELL IONIZATION CROSS SECTION RATIOS IN THE Z=12-17 REGION
* +§J.S. Lopes , A.P. Jesus and M.F. da Silva§
*Departamento de Fisica, Faculdade de Ciencias da Universidade de Lisboa, 1200 Lisboa, Portugal
+Centro de Fisica Nuclear da Universidade de Lisboa, Faculdade de Ciencias, 1200 Lisboa, Portugal
iDepartamento de Flsica, Laborat6rio Nacional de Engenharia e Tecnologia Industrial, 2685 Sacavem, Portugal
Abstract
Experimental deuteron- and alpha-induced K-shell ioni-
sation cross section ratios are presented for elements
ranging from Mg to Cl (Z=12-17) in the incident energy
range 0.2 to 0.45 MeV/u. The experimental ratios
R= a /4cdare generally close but slightly higher than
the theoretical ratios calculated according to the pro-
cedure of Brandt and Lapicki; the largest differences
occur for Al and Si where the experimental values are
15-20% too high. The experimental ratios show roughly
the same dependence on reduced projectile velocity,
C= (v1 /vK)(2/0K),as the theoretical ones in the reduced
incident velocity range used in the present work, E=0.45to E=1. This is in opposition to the results previously
obtained in the velocity range =0.25 to i=0.55 for
elements ranging from Ca to Cu (Z=i20-29), where the
observed ratios are rather independent of the reduced
velocity.
Introduction
In K-shell hole production by light ions with velocity
V1 smaller than the target electron velocity vK, the
ionising collision takes place inside the shell. When
the collision occurs the electron orbit has already
been modified in response to the increase in the Cou-
lomb attraction, and this leads to an appreciable
lowering of the ionisation cross section. The oppositeis true when v1>vK and the collision occurs mainly out-
side the shell.
A very direct way of studying thisbinding energy
effect, first described by Brandt et al.1 , is to ex-
perimentally determine the ratio of alpha- and deuteron
-induced ionisation cross sections. These ratios are
insensitive to other effects influencing the cross
sections and thus allow an essentially independentcheck on the accuracy of binding effect calculations
and correction procedures.
Here we present results pertaining to elements
ranging from Mg to Cl in the incident energy range 0.2
to 0.45 MeV/u; the corresponding reduced velocities
span the range E= 0.45 to i= 1, where the projectile
velocity approaches the velocity of the target electron.
Experimental
The experimental set-up has been described in detail23elsewhere . Alpha and deuteron beams from 0.2 to 0.45
MeV/u were obtained from the 2MV Van de Graaff accele-
rator at Sacavem and analysed to within + 1 keV; an
NMR fluxmeter was used to determine the beam energy.
The targets were evaporated onto %15 vig/cm2thickC foils except for Al which was self supporting. Target
composition and total thickness in units of 1017 atoms
cm2
were: MgO (1.6), Al (1.7), SiO2 (8.5), Zn3P2 (1.4),
Sb253(2.0), CsCl (1.2).
The x-ray production cross sections were determi-
ned through the dead time and efficiency corrected
numbers of simultaneously detected x-rays and parti-3
cles . Where necessary cross sections were corrected
for target thickness as described in Ref.3; these cor-
rections were generally under 5% except for Si where
they reached 11% for 0.2 MeV/u alpha particles.
The same set-up, including targets and detectors,
was used in the alpha and deuteron measurements so
that errors due to solid angles and detector efficien-
cy cancel out in the cross section ratios.
Results
The ratios R= a/4ad between the alpha- and deuteron
-induced x-ray production cross sections at several
incident energies per atomic mass unit are shown in
table 1; the corresponding errors are generally esti-
mated not to exceed 5%. We take the ionisation cross
section ratios to be equal to those in table 1 by as-
suming the fluorescence coefficients to be the same
for alpha- and deuteron-induced ionisation in spiteof their dependence on atomic configuration and,there-by, on multiple ionisation. The work of Richard et al.
has shown that in alpha-induced ionisation in Al over
the energy range covered in the present work onlyabout 20% of the ionised atoms are left in a singlehole state, while %50% are KL-two hole and %25% are
0018-9499/83/0400- 954$01.00C 1983 IEEE
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Table 1: Alpha- and deuteron-induced cross section
ratios R= a /4 d Errors are expected not to exceed 5%.
E/u Mg Al Si P S Cl
(MeV/u)
.20 .59 .65 .53 .55 .47 .50
.25 .67 .68 .62 .56 .56 .53
.30 .71 .78 .70 .63 .68 .56
.35 .84 .85 .77 .71 .68 .63
.40 .92 .91 .84 .68 .71 .61
.45 .96 .94 .88 .77 .70 .65
KL -three hole states. Corresponding values for other
elements are expected to decrease with atomic number
of the target and to be larger for alpha- than for
deuteron-induced ionisation4'5. However, from indivi-4dual wK values reported by Richard et al . for those
Al configurations, which do not differ by more than 5%,it may be expected that the average uK values corres-
ponding to alpha and deuteron ionisation are quiteclose, not affecting significantly the cross sectionratios.
The cross section ratios are also displayed in
figures 1 and 2, where the chemical symbol of each
element has been used to plot the respective experi-mental values at several reduced incident velocities.
< .70tzl
Discussion
We are aware of no other experimental results on the
ratio of alpha- to deuteron-induced cross sections per-
taining to the elements reported here but for those pu-blished by Brandt et al. for Al; however, these per-tain to lower values of the incident velocity. Rice et
al. have graphically presented results for the ratio
of alpha- to proton-induced cross sections for Mg, S
and Cl in the present velocity range. As referred to
below, Coulomb and energy-loss effects are small for
the cases studied here allowing one to use theoreticaldeuteron- to proton-induced cross section ratios, Od/Oato compare their results with the present ones. In
this way it is seen that there is fair overall agree-ment for Mg and S, whilst for Cl their results are
about 20% smaller than ours.
The purpose of this work is to study the bindingeffect in a region where the projectile velocity issmaller but already close to the velocity of the targetelectron. As discussed in a previous paper , by choo-
sing deuterons and alpha particles as projectiles one
is able to gain information on the binding effect withlittle interference from other low energy effects. In
the present work, Coulomb and energy-loss correctionsin the absolute cross sections, calculated according to
the procedure of Brandt and Lapicki9'10, do not exceed4% in Mg and 12% in Cl; these effects cancel in the
.90
Fig.1 Cross section ratio plotted against the reduced velocity for the elements Mg, S and Si. Thechemical symbol of each element is used to display the corresponding experimental values.Theoretical values for the ratios are shown by the lines pertaining to the various elements.
.90-
t
"'.70tt
.50 _
.50 .70 .90
Fig.2 Cross section ratio plotted against the reduced velocity for the elements Al, P and Cl. The
chemical symbol of each element is used to display the corresponding experimental values.
Theoretical values for the ratios are shown by the lines pertaining to the various elements.
