The results of three separate experiments all dealing with the production of x-rays in projectiles moving in solids are discussed. The first experiment deals with the measurement of line widths of x-rays emitted from projectiles moving in solid targets. The effect of collisional broadening of x- rays is found to dominate the line widths giving greater than an order of magnitude increase in the measured line widths. The second experiment studies ''solid target effects'' in producing non-binomial distributions of characteristic K x-ray spectra in heavy ion-atom collisions. The third experiment studies aluminum K x-ray production in Ar$sup +$ $Yields$ Al collisions in very thin aluminum foils as a function of foil thickness. Parameterization of the observed non-linear dependence enables the lifetime of the argon 2p vacancy and total ionization cross sections for the argon L-shell in Ar $Yields$ Al collisions to be measured. (auth

Feldman, L. C.

Fortner, R. J.

Garcia, J. D.

Matthews, D. L.

Oona, H.

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Thia >• a preprint of a paper intended for publication In a journal or proceedings. Since changes may be made before publication, thia preprint is made available with the understanding that it wi l i not be cited or reproduced without the permission of the author. U C R L - 77310 PREPRINT H I.AWRENCE LIVERMORE LABORATORY University of Calilornia/Livermore.Calilornia Measurement of X-Rays Emitted from Pro jec t i les Moving in Sol id Targets R. J . Fortner D. L. Matthews L. C. Feldman J . D. Garcia H. Oona September 1975 -WOT1CE f ^,7} uninttHCd by Ihe L1 the L'mtri Sr«« ^ = = B RnciKh ind Dr«tlo T - : . ;™ s;,,r.;»;.«.. thrt. empluyt". S£SBBB bibiliiy »' rciponub i , 5 . "'."mu<». *«*»" "' ptnaii dudoitd, o Br^rr This paper was prepared fo r submission to 4th In ternat iona l Conference Beam-Foil Spectroscopy and Heavy Ion Atomic Physics Gat l inburg, Tennessee September 15 - 19, 1975 MEASUREMENT OF X-RAVS EMITTED FROM PROJECTILES MOViNG IN SOLID TARGETS* R. J. Fortner and D. L. Matthews Lawrence Livermore Laboratory Livermore, California 94550 L. C. Feldman Bell Telephone Laboratories Murray Hill, New Jersey 07974 J. 0. Garcia and H. Oona University of Arizona Tucson, Arizona 85721 ABSTRACT In this paper the results of three separate experiments all dealing with the production of x-rays in projectiles moving in solids will be discussed. The first experiment! deals with the measurement of line widths of x-rays emitted from projectiles moving in solid targets. The effects of collisionally broadening of x-rays is found to dominate the line widths giving greater than an order of magnitude increase in the measured line widths. The second experiment' studies "solid target effects" in producing non-binomial distributions of characteristic K x-ray spectra in heavy ion-atom collisions. The third experiment3 studies aluminum K x-ray production in Ar •+ Al collisions in very thin aluminum foils as a function of foil thickness. Parameterization of the observed non-linear dependence enables us to measure the lifetime of the argon 2p vacancy and total ionization cross sections for the argon L-shell in Ar + Al collisions. *V!ork performed under the auspices of the United States Energy Research and Development Administration , under contract Nc. W-7!*05-Eng-46. 2 1. COLLISIONAL BROADENING OF X-RAYS In Fig. lb and lc the neon K x-ray spectra for 3 types of colli­sions, 1.3 keV electrons incident on a neon gas target, and 90 keV neon ions incident on a neon gas target and a solid copper target. The gas target (Fig. 1c) spectra exhibit clean well resolved peaks and the origins of these peaks have been discussed extensively in the literature. Basically each line corresponds to an initial state involving one K-shell vacancy and fixed ionization in the L-shell. The peaks are designated as such in the figure where KL n corresponds to the x-ray transition from an initial state having one K-shell vacancy and n L-shell vacancies. However, these well resolved discrete peaks are not observed for the neon K x-ray spectra emanating from ions moving in solid copper targets (Fig. lb). The peak of this spectra occurs in the region of the normal KL" peak; however, the peak extends in energy over a very broad energy range with essentially no structure detectable in the spectra. This spectra is skewed toward the high energy side, probably indicating the existence of states of excitation higher than KL°. On the low energy side of KL° the spectra extends to lower energies indicating that the individual lines are considerably broadened. In Fig. 2 the boron K x-ray spectra for 2 types of boron-carbon collisions are presented. The first spectra is for a methane gas target. Basically the x-ray lines are from boron coming primarily from electron transitions in one, two, and three electron atoms. The second spectra is for a solid diamond target. The most obvious difference is the large carbon K line which dominates the solid tar­get spectra but is not observed in a gas target. This peak occurs in the solid targets because of recoiling carbon atoms and subse­quent C-C collisions. A more important difference with respect to this paper is the x-ray transitions emanating from the boron pro­jectile. For the solid target only two broad contir'ious x-ray bands in the region where boron K x-ray transitions are expected are observed. The identification of the two bands is straightforward. The low energy band, centered near 190 eV, is consistent with cal­culations of the 2p •+ Is x-ray transitions in atoms having initial states involving one K-shell vacancy and one, two or three electrons in the L-shell. The second higher energy band, centered near 245 eV on the low energy tail of the carbon K x-ray is consistent with 2p •* Is transitions in atoms having two K-shell vacancies in the initial state and one, two or three L-shell electrons. The signifi­cant point to be extracted from both Fig. lb and Fig. 2 concerns the widths of the x-ray lines obtained from the projectiles moving in solid targets. In both figures the data obtained from gas target spectra exhibit clearly resolved peaks. However, when the x-ray spectra are measured using the same instrumental resolution for ions moving in a solid target all the sharply resolved x-ray struc­ture disappears and only broad x-ray bands are resolved. This Kl? Ktl KLZ Kl? KIZ KL5 I I I I I I -~\ • V v Â J I I L 850 900 Photon Energy (eV) Figure 1 4 ft/ ~I 1 I I 1 1 1 - 1 -Methane (Gas! ; \h.j\r A Ty ^ • * * • *»*»•* •*** ' - * < "Diamond (Solid) 200 2 5 0 Pholon Energy (evl Figure 2 broadening of the x-rays in the solid is the x-ray analog to the coll isional broadening of spectral lines observed in dense plasmas. I t is the result of a disruption in the radiation f ie ld of the inner shell vacancy induced by multiple collisions which take place prior to the f i l l i n g of the x-ray transit ion. An estimate of the size of the coll isional x-ray broadening can be obtained by unfolding the x-ray spectra in Fig. lb and 2. In this unfolding of the data we used Gaussian l ine shapes and centroid energies as observed in gas target measurements. This unfolding of the Ne* •* Cu spectra indicated a width of 12 + 3 eV for the peaks KL° to KL4 respectively. The boron K x-ray spectra were unfolded using the double K-shell vacancy peak from Fig. 2b. The spectra indicated a width of 12 ± 5 eV and equal intensities of the KzL.O and KM.1 x-ray peaks. 5 2. NON BINOMIAL DISTRIBUTIONS OF HEAVY-ION INDUCED NEON K X-RAY SPECTRA In Fig. la the neon K x-ray spectra produced in collisions of neons ions incident on a graphite (carbon) thick target. The spec­tra was observed only after long bombardment of neon ions and thus is attributed to Ne - Ne collisions taking place in the carbon solid. This spectra is expected to have two components: one from a mov­ing neon projectile, and the second from implanted neon atoms at rest in the graphite. The projectile spectra is broadened as in Fig. lb and the target spectra shows peaks. The dashed line in Fig. la indicates our estimate of the projectile contribution ob­tained by normalizing the spectra shown in Fig. lb to the low energy part of the spectra in Fig. la. The difference spectra which would be associated with the implanted neon target is shown in Fig. Id. Notice the difference in the two neon-neon spectra for a gas target (lc) and a solid target (Id). The x-ray line intensities from the gas target spectra are binomial in character and for the solid tar­get spectra they are not. He can attribute this difference to the capture of target electrons into excited states when the neon is in the carbon solid. These electrons can auto-ionize filling L-shell vacancies prior to the K x-ray decay which then results in fewer number of L-shell vacancies as observed in the x-ray spectra and a non-binomial distribution. 3. LIFETIME STUDIES OF Ar-2p VACANCIES TRAVELING THROUGH SOLIDS We report studies on the production of aluminum K x-rays in Ar-Ai collisions using varying thicknesses of thin aluminum foils. It has been suggested that the mechanism for aluminum K vacancy production requires two collisions; the first collision produces an argon 2p vacancy in the argon projectile and the second colli­sion transfers this 2p vacancy to the Is level of the aluminum. The reasons for interest in these studies is that in very thin foils, i.e., thickness less than vt where v is the velocity of the ion and x is the lifetime of the argon L vacancy, the aluminum K yield is not a linear function of thickness and the shape of the curve will depend on the lifetime of the argon 2p vacancy. Thus, parameterization of this data enables us to infer an average life­time for the argon 2p vacancy traveling through a solid. The experiments used thin foils of carbon of thickness ^lO yg/ cm' with thin evaporated layers of aluminum ranging from 60 A* to 333 A- In the first experiment the yield of aluminum K x-rays was measured for the argon beam incident, first on the carbon side and then on the aluminum side of the foil. Those yields are displayed in Fig. 3 as a function of energy. The aluminum yield for argon first traveling through carbon is considerably higher since there 6 t • C-AI foil A Al foil -J 1 I l_ A 8 12 IS 20 Projectile Energy (MeV) Figure 3 is a large steady state distribution of argon L vacancies estab­lished in this beam prior to penetration into aluminum. This observation confirms the basic explanation of Al-ls vacancy pro­duction mechanism discussed above. . In a second set of measurements the yield of aluminum K x-rays was measured as a function of aluminum thickness (T) by using dif­ferent foils and various angles to the beam. In this case the fraction of beam with L vacancies, fj(t) at depth t, now due to Ar-Al collisions may be written as f,(t) - NOVT 1+NCVT 0-e' (Nc ̂ where N is the atom density of Al, a the cross section for argon L vacancy production. Then the total yield of Al, Y 4 1 , is propor­tional to 'AT 7 « C-AI foil • Al foil 6000 S 3000 60 120 Elleciivc A!ymmum Thickness (A) Figure 4 'Al I f , { t ) d t « [T + e-(fe+ v4)T Ho + vT In Fig. 4 the yield of aluminum K x-rays as a function of f o i l thickness is shown. The data for the aluminum foi ls clearly shows the non-linear dependence of the aluminum K x-ray yield on fo i l thickness. Parameterization of these data eiabied us to determine independently both o and T. A summary of the values of T and a obtained for a variety of projecti le energies is shown in Table 1. TABLE 1 Energy (Hev) .3 .4 .5 1.0 1.5 2.0 1. R. J. Fortner, D. L. Matthews, J. D. Garcia and H. Oona (to be published). 2. R. J. Fortner and D. L. Ms^hows (to be published). 3. L. C. Feldman, R. A. Levesque, P. 0. Silverman and R. J. Fort­ner (to be published). (xl0" 1 7cm 2) (xl0"15sec) .35 1.50 (±.30) .59 2.15 (±.44) .90 4.8 (±.5 ) 1.58 11.0 (*2.5 ) 3.10 20. (±20) 4.10 REFERENCES 

Measurement of x-rays emitted from projectiles moving in solid targets

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Measurement of x-rays emitted from projectiles moving in solid targets

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