Bremsstrahlung induced by proton and helium-3 ion impact

NUCLEAR

INSTRUMENTS

AND

METHODS

I32

(I976)

503-505;

©

NORTH-HOLLAND

PUBLISHING

CO.

B R E M S S T R A H L U N G INDUCED BY P R O T O N AND HELIUM-3 ION I M P A C T H. T A W A R A

Nuclear Engineering Department, Kyushu University, Fukuoka, Japan K. ISHII a n d S. M O R I T A

Physics Department, Tohoku University, Sendai, Japan C o n t i n u o u s X-ray spectra from a thin a l u m i n i u m target produced by MeV p r o t o n and helium-3 ion impact have been measured with a Si(Li) detector. T h e observed spectra can be well reproduced with a simple model where it is a s s u m e d that these c o n t i n u o u s X-rays originate from b r e m s s t r a h l u n g due to secondary electrons ejected f r o m the target. The calculated angular distribution o f these c o n t i n u o u s X-rays is not isotropic but generally intense at 90 °, which is also qualitatively in agreement with the experimental m e a s u r e m e n t .

1. Introduction

It is well known that sharp characteristic X-ray lines are always accompanied by continuous X-rays when energetic heavy ions impinge on a solid target. These continuous X-rays are thought to originate from various sources: (1) bremsstrahlung due to the secondary electrons ejected from the target atom; (2) bremsstrahlung due to the projectile; (3) low-energy photons from Compton scattered ),-rays. The first is dominant at low energy regions but its intensity decreases rapidly if the X-ray energy becomes higher than Tm=4meE/Mp, the maximum energy transferred to a free electron (its mass: me) from the incident ions with energy E and mass Mp 1). The estimated intensity of the second (nuclear bremsdo dEx I (bl keV.sr.z~) AI h}rgel

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2. Experiment

Proton and helium-3 ions from our Van de Graaff accelerator impinged on a thin self-supported aluminium target (155#g/cm2). X-rays produced were measured with a Si(Li) detector with a resolution of 205 eV at 6.9 keV. 3. Results and discussion

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strahlung) is found to be less than 1% of the first at the present energy range. It has also been recognized that Bremsstrahlung gives serious detection limits of detectable concentrations in trace element analysis. However, few quantitative investigations on continuous X-rays have been done s o farE'3). In the present work, we have measured continuous X-ray spectra and compared them with calculated spectra based on a simple model when proton and helium-3 ions impinged on a thin solid target.

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Fig. 1. Typical X-ray spectra at 90 ° from an a l u m i n i u m target, b o m b a r d e d by protons a n d helium-3 ions.

Fig. 1 shows typical X-ray spectra where no corrections are made for the absorption in a 10 #m Mylar window and air path and for the detection efficiency. The peaks at lower energy correspond to A1-K X-rays. It should be noted from this figure that not only A1-K X-rays but also continuous X-ray production can be well scaled with the square of the nuclear charge (Z~) of the projectile. In order to get the bremsstrahlung spectrum due to the secondary electrons we have to calculate the cross sections for ejection of electrons from target atom by ion impact as a function of the electron energy and ejection angle and to know the cross sections for bremsstrahlung production. For the electron production, we have used the formula of Bonsen and Vriens 4) which is based on the VIII. ELECTRON

AND X-RAY

EMISSION

504

H. TAWARA et al.

binary encounter approximation and takes into account the angular distribution of ejected electrons and the velocity distribution of bound electrons is taken to be hydrogenic. In fig. 2 are shown the calculated cross sections of electron ejection from an aluminium target by 4 MeV proton impact. The different curves of total cross sections correspond to the different mean energy of electrons. It is clear from this figure that electrons ejected from outer shells are dominant at the energy region below Try, while those from inner shells, especially from the K-shell, become dominant at an X-ray energy higher than Tm. Using the semi-classical formula for the cross section of bremsstrahlung given by Jackson 5) and taking into account the finite thickness of the target, the calculated bremsstrahlung spectra at 90 ° with respect to the beam direction are obtained as shown in fig. 3, where are included the observed continuous X-ray spectra with all necessary

corrections made and also those calculated by Folkmann et al.2). Small peaks near 2.3 keV might be due to impurities. Generally speaking, the present calculations give a better fit to the observed spectra than those of Folkmann who assumed isotropic angular distributions of ejected electrons and also of bremsstrahlung production and assigned about 50% uncertainty to his calculation. It is noted that the calculated cross sections for bremsstrahlung become smaller than the observed values as the X-ray energy becomes higher than Tm and the impact energy lower, where bremsstrahlung is produced mainly by secondary electrons ejected from the inner shells. In the observed X-ray energy range,

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Fig. 3. Comparison o f the observed and calculated X-ray spectra at 90 °. The solid and dotted lines are the present calculation and that of Folkmann, respectively.

BREMSSTRAHLUNG

bremsstrahlung at higher impact energies is produced mainly by electrons ejected from outer shells, while that at lower impact energy is by those from strongly bound inner shells. It is also found that the calculated angular distribution of bremsstrahlung due to the secondary electrons has the following form: da -

where 0L is the observed angle with respect to the beam direction. A and B are constants depending on the cross section of bremsstrahlung and of electron ejection and X-ray energy. In fig. 4 are shown the calculated angular distribution of bremsstrahlung, normalized to that at 90% at 4 MeV proton impact for some X-ray energies. With infinite X-ray energy, the angular distribution becomes of the form of pure sin 2 0L. To test the present calculation, we also measured intensities of bremsstrahlung at 45 °, 90 ° and 135 ° for 1.5 and 4 MeV proton impact. Ratios of intensities at 45 ° and 135 ° to those at 90 ° are shown in fig. 5, together with the calculated values. The agreement between the observed and calculated values at 135 ° and 4 MeV impact is very good but the observed intensity ratios at the forward angle (45 °) are larger than the calculated values at 1.5 and 4 MeV. This might be due to the so-called retardation effect which shifts the angle of the maximum cross section to forward angles with increasing electron energy6). It is concluded from the present work that: 2 sE8('~,,8,)

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1) bremsstrahlung spectra produced by heavy ion impact calculated with the present model can reproduce the observed spectra; 2) this type of bremsstrahlung is not isotropic, as shown both in the observed and calculated spectra; 3) it is clear from the anisotropy of bremsstrahlung that trace element analysis using proton induced X-ray techniques should be made at backward angles, rather than at 90°; 4) there are some discussions that the energy dependent anisotropy of continuous X-rays is a strong evidence for molecular orbital (MO) X-rays from a quasi-atom formed during heavy ion-atom collisionsT). However, this method of identification of MO X-rays should be carefully made because the angular distributions are expected to be similar in MO X-rays and bremsstrahlung; 5) measurements of angular and energy distributions of ejected electrons, especially with energies higher than Tm, would be interesting and useful for understanding not only bremsstrahlung but also basic collision mechanisms.

0.4 References

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0L Fig. 4. T h e calculated angular distribution o f b r e m s s t r a h l u n g at 4 M e V proton impact. T h e p a r a m e t e r is the X-ray energy.

1) E. Merzbacher a n d H . W . Lewis, Handbuch der Physik (Springer Verlag, Berlin, 1958) vol. 34, p. 166. 2) F. F o l k m a n n , C. Gaarde, T. H u n s a n d K. K e m p , Nucl. Instr. a n d Meth. 116 (1974) 487. a) F. F o l k m a n n , J. Phys. E 8 0 9 7 5 ) 429. 4) R. F. M. Bonsen and L. Vriens, Physica 47 (1970) 307. s) j. D. Jackson, Classical electrodynamics (J. Wiley, N e w York, 1962) p. 511. 6) H. K. T s e n g a n d R. H. Pratt, Phys. Rev. A 3 (1971) 100. 7) B. MUller a n d W. Greiner, Phys. Rev. Lett. 33 (1974) 469. VIII. ELECTRON

AND X-RAY

EMISSION