Nuclear Instruments and Methods in Physics Research B26 (1987) 507-511 North-Holland. Amsterdam

507

PROTON INDUCED GSHELL X-RAY PRODUCTION CROSS-SECTIONS FOR Pb IN THE ENERGY RANGE 225-400 keV

Harsh MOWN, Pa&t S. SINGI-I, D. SING& HR. VERMA and C.S. RHURANA

l?epartment of Physics, Punjabi University, PatiaIa-147002, India

Received 21 July 1986 and in revised form 19 January 1987

The L-shell X-ray production cross-sections in lead (Pb) by proton impact over the energy range 225-400 keV, with an interval of 25 keV, have been measured. The thick target X-ray yields have been obtained using a HPGe detector. The experimental results for

BL,T QL.. “Ls and eL have been compared with perturbed stationary state theory with relativistic (R), energy loss (E) and Coulomb (C) corrections (ECP!&R theory). The comparison of L,, Lg and L, X-ray production cross-sections shows a fairly good agreement, except at the lowest energy. The L, X-ray production cross-sections are higher by = 20-304; than their theoretical estimates.

1. In~tion

The compilation of experimental L-shell X-ray pro- duction and ionization cross sections for proton impact by Sokhi and Crumpton [l] indicates that in recent years considerable attention has been paid to the study of innershell ionization in atoms by low energy protons (( 500 keV). Extensive investigations have been made for K-shell ionization [2,3] and the process is now reasonably well understood. However, the situation is not so clear for L-shell ionization because of the lack of experimental data on L-shell ionization and X-ray pro- duction cross-sections [l] and the large discrepancy between the experimental data and theoretical predict- ions [4,5]. The theoretical calculations for X-ray produc- tion cross sections involve large errors due to large uncertainty in atomic parameters (e.g. fluorescence yield, Coster-Kronig transition probabilities [6]), which are necessary to convert the theoretical values of sub-shell ionization cross-sections into X-ray production cross- sections.

Although the L-subshell ionization cross-sections and hence the L X-ray production cross-sections, for medium and high Z elements, are small at low proton energies, yet analytical work such as PIXE studies 17-91 can only be done if accurate experimental L X-ray production cross section data is available.

The theoretical models describing the ion-atom in- teraction have undergone a continuous evolution with the semi-classical approximation (lo], plane wave Born approximation [ 111, binary encounter approximation 1127 and the ECPSSR theory [13]. The ECPSSR theory, which is based on the perturbed stationary state for- malism and includes nonlinear effects due to energy loss and Coulomb deflection of the projectile is in substan-

0168-583X/87/$03.50 0 Elsevier Science Publishers B.V. (North-Poland Physics Publishing Division)

tial agreement with most experimental work involving protons of energy > 0.5 MeV corresponding to a re- duced ion velocity [f/S; > 0.4 [5].

The L-shell X-ray cross sections for Pb (lead) have been measured by Leite et al. 1141, Sokhi and Crumpton [lS] Sarter et al. 1161, Moheb et al. 1171 and Jesus et al. [IX] at various proton energies. Since at low energies, the nonlinear effects are large and experimental cross sections provide the most critical test of theory, there is a need to study the lower energy range more exten- sively. Keeping this in view, we have measured the L-shell X-ray production cross sections e.g. ur,, aLB, ut, and at, for thick Pb targets in the energy range 225-400 keV. The experimental results have been com- pared with the theoretical values based on ECPSSR theory.

2. ~~~rne~~ procedure and data analysis

The AN-400 Van de Graaff accelerator at the Nuclear Science Laboratories, Punjabi University, Patiala, was used to produce a proton beam of continuously variable energy up to 400 keV and beam current up to 50 PA. The particles after acceieration are passed through a 15O bending magnet to ensure that the beam which strikes the target is isotopically pure. The beam energy was calibrated using the 19F(p, cyy)160 resonance at 340.5 keV. The experimental setup, showing the target chamber and X-ray detection system is as indicated in fig. 1. The target chamber was evacuated to = 10W6 mm of Hg. The beam is centered through two collima- tors of internal diameters 2 mm and 1 mm, respectively.

A thick lead target was mounted at 4S” to the beam direction and the hyperpure Ge low energy photon

508

Proton beam

H. Mohan ei al. / PXXE cross sections for Pb

Guard ring

L/I /f/‘/J”’ “‘4 CO- axial F

connector

L Scattering cham

-Kapton w\ndow

detector

ber

Fig. 1. Experimental setup for measurement of L X-ray production cross sections, . CI is the current integrator. HpGe is the high

purity germanium detector, MCA is a multichannel analyser.

.

.

-~- .

120 1LO 160 180 200 220 2LO 260 280 300 3LU

. LP . . .:. . . ’

. :.

- Channel number F

Fig. 2. A typical L X-ray spectrum of Pb bombarded with 400 keV protons.

H. Mohan et al. / PIXE cross sections for Pb 509

(HpGe LEP) detector placed at right angle to the beam thick target X-ray yields NLx(E) were measured for outside the target chamber, was used to detect the L proton energies from 225400 keV at an interval of 25 X-rays. The H+ beam current on the target was mea- keV by recording the spectra at various energies and sured by current integrator model Elcor-A309F, which computing the area under each L, peak (i.e. L,, L,, Lg, has high sensitivity, accuracy, low drift and an internal Lr) after subtracting suitable background. The number calibrating source. A guard ring at - 300 V was placed of protons Np were measured by the current integrator in front of the target to prevent secondary electron Elcor-A 309F. The values of pr_ and X(E) have been emission from the target. The beam current employed taken from Storm and Israel [20] and Andersen and was 10 nA to 1 PA, so as to obtain X-ray yields Ziegler [21] respectively. The d NL.( E)/d E values were compatible with the input characteristics of detecting obtained by differentiation of a polynomial fitted curve electronics and thus avoiding the problem of dead time. as explained by Khan and Crumpton [22].

The fluorescent L X-ray spectra from the target were recorded with a 36 mm $I x 13 mm HPGe detector (resolution 360 eV at 5.9 keV) coupled to an Ortec Model 7100 multichannel analyzer. A typical spectrum for Pb at 400 keV proton energy is shown in fig. 2. This spectrum consists of four peaks of L X-ray groups, corresponding to L,, L,, LB and L, which are well separated from each other.

3. Results and discussion

The efficiency of the HpGe detector was obtained by placing the radioactive sources of 241Am and 57Co (ac- tivities certified to an accuracy of + 3.5%) at the target position. The observed efficiency curve thus takes care of the detector solid angle and the absorption of radia- tions in the air, chamber window and detector etc. The intensities of different radiations for 24?4m and 57Co sources were taken from Dias and Renner [19].

The experimental results of the L X-ray production cross-sections, oL (E), x are given in table 1. The errors in the measured values of L X-ray production cross sections, Au, (E), were calculated by adding the errors for the diff&ent parameters (e.g. pL,, NL,, eL , d NL,( E)/d E, S(E), Np etc.) quadratically. The stat& tical error for counts under the L,, Lg and L, X-ray peaks is Q 1% while for L, X-rays it is 2-3X. The uncertainty in the detection efficiency is of the order of 5%.

The L,, L,, L, and L, thick target production cross sections, QE) at various proton energies, were derived from the X-ray yield NLx(E), using the well known formula [ll]

The theoretical L X-ray production cross sections based on ECPSSR theory, are also given in table 1 for comparison. The theoretical values have been obtained by using the following relations giving by Close et al. [23]:

OL, = (ULJi3 + ~L,fi2fi3 + ULJ23 + UL,)O3%

uL. = (“L,fi3 + uL,fi2f23 + uL,f23 + uL~)w34u7

uLB = fJL,@& + ((7LJi2 + ~L*)~ZF,&v

+(%*fi3 + ULJi2f23 + ULJ23 + ULJO3E+

UL,@) = & dNL (E) Lx P

& S(E) +PL,NL,(E) 9 1 where n is the number of target atoms per gram, Np is the number of protons which impinge on the target, eL, is the absolute efficiency of the detector. pL, is the mass absorption coefficient of the target material for its own characteristic L, radiations and S(E) is the stop- ping power of the element for protons of energy E. The

(IL, = UL,%4T. + (fJLJi2 + ULJ@2Fzy,

where uL1, uL, and uL3 are the subshell ionization cross sections for the L,, L, and L, subshells respectively;

Table 1 Measured L X-ray production cross-sections and their comparison with calculated values based on ECPSSR theory

Proton energy

@eV)

oL. (mb)

Exp. Theory

cLa (mb)

bP. Theory

oL, (mb)

hp. Theory

aL, (mb)

Exp. TheW

225 1.8kO.2 1.37 250 3.6kO.4 3.64 275 6.6 f 0.7 6.98 300 15.0*1.6 12.64 325 23.4*2.4 21.19 350 33.9* 3.5 32.34 375 46.2*4.7 47.0 400 67.1 f 7.0 66.78

0.7+0.1 0.57 1.91t0.2 1.66 3.8 + 0.4 3.22 6.1 f 0.7 5.83

10.3 + 1.2 9.79 16.2 + 1.7 14.86 21.6k2.3 21.71 31.9* 3.3 30.65

0.09 + 0.01 0.08 0.28 f 0.04 0.24 0.53 f 0.08 0.47 0.91* 0.12 0.84 1.3 *0.2 1.4 2.2 +0.3 2.11 2.8 +0.4 3.08 4.2 +0.6 4.3

0.11 f 0.02 0.08 0.26 f 0.05 0.20 0.45 kO.08 0.38 0.88 f 0.15 0.68 1.40f0.25 1.14 2.0 *0.4 1.74 2.7 f0.5 2.53 3.8 +0.7 3.59

510 i-I. Mohan et al. / PIXE cross sections for Pb

I I 1 I I I I I I

200 250 300 350 LOO

- Proton energy (keV) ----SF

Fig. 3. Ratio of experimental and theoretical (based on ECPSSR theory) L-shell X-ray production cross sections in Pb as a function of proton energy in the range 225-400 keV.

al? q, and ws are the fluorescent yields for the three L-subshells; fiz, f2s and fi3 are the Coster-Kronig transition probabilities of L, -+ L,, L, 4 L, and L, -+ L, respectively. The radiative transition probability F,, is the fraction of X-rays originating from the L, sub- shell and contributing to the L, peak of the L X-ray spectrum. In the present calculations, the values of al,, based on ECPSSR theory, are taken from the tables of Cohen and Harrigau [24], the F,, values from Scofield [25], while the values of wi and fji are taken from Krause [6]. Due to large uncertainties in the values of wi and fij, the theoretical values for L X-ray production cross sections aL_, o,+ at_ 1 and uL, obtained by the

above mentioned relations yield an error of = 25-30%. A comparison of the experimental L,, L,, L, and L,

X-ray production cross sections with the corresponding theoretical values (table 1, fig. 3) indicates that there is, in general, a good agreement for L,, LB and L, X-ray production cross sections within the experimental un- certainties, from 250 to 400 keV, while the L, X-ray production cross sections indicate higher values than the corresponding theoretical estimates. The L,, L, and L, X-ray production cross section at 225 keV, however, are higher by = ZO-30%. It will be interesting to carry out the measurements at energies below 225 keV to investigate the nonlinear effects.

H. Mohan et al. / PIXE cross sections for Pb 511

Table 2 Total L-shell X-ray production cross sections in Pb at various proton energies

Proton Present work Other work ECPSSR energy (mb) (mb) theory

(keV) (mb)

225 2.7+ 0.3 2.1

250 6.04 0.6 5.7

275 11.4+ 1.1 11.05

300 22.9+ 2.3 18.7 f 3.4 [17] 19.99

325 36.4+ 3.7 3.52 350 54.3* 5.4 49.4 [16] 51.05 375 73.3+ 7.3 74.32

400 107 +11 125 * 23 [17] 105.32

The total L-shell X-ray production cross sections

(~L)total in Pb at 225-400 keV proton energies, have been computed from table 1. The present experimental results are presented in table 2 in comparison with those

from other workers [16,17] and the ECPSSR theory. The uncertainty in the present measurements is less as compared to the earlier work while there is a good agreement with theory at all energies, within the experi- mental uncertainties, in the range 225-400 keV, except

at 225 keV. As mentioned earlier the theoretical values for L

X-ray production cross sections involve large errors, the experimental results with an error of = 10% can there- fore be used with more certainty. The investigation for other elements are also in progress in this laboratory in order to test the validity of ECPSSR theory in the low energy region.

Two of us (H.M. and P.S.S.) are thankful to the University Grants Commission and the Council of Sci- entific and Industrial Research, Government of India, for financial assistance in the form of Junior Research Fellowships.

References

[l] R.S. Sokbi and D. Crumpton, At. Data Nucl. Data Tables 30 (1984) 49.

PI

[31

[41

151 [61 [71

PI

I91 WI

IllI

WI P31

P41

u51

I161

(171

P81

P91

WI WI

P21

1231

1241

PI

R.K. Gardner and T.J. Gray, At. Data Nucl. Data Tables 21 (1978) 515; 24 (1979) 281. H. Paul, At. Data Nucl. Data Tables 24 (1979) 243; Nucl. Instr. and Meth. 192 (1982) 11; B3 (1984) 5. T. Mukoyama and L Sarkadi, Nucl. Instr. and Meth. 190 (1981) 619; 197 (1982) 585. D.D. Cohen, Nucl. Instr. and Meth. B3 (1984) 47. M.O. Krause, J. Phys. Chem. Ref. Data 8 (1979) 307. R.T. Boro and S.J. Cipolla, Nucl. Instr. and Meth. 131 (1975) 343. T. Shiokawa, T.C. Chu, V.R. Navarrette, H. Kaji, G. Izawa, K. I&ii, S. Moxita and H. Tawara, Nucl. Instr. and Meth. 142 (1977) 199. G. Lmder, Nucl. Instr. and Meth. B3 (1984) 130. J. Bang and J.M. Hansteen, K. Dan. Vidensk. Selsk. Mat. Fys. Medd. 31 (1959) no. 13. E. Merzbacher and H.W. Lewis, in Handbuch der Physik, ed., S. Flugge, vol. 34 (Springer Verlag, Berlin, 1958) pp. 166-192. J.D. Garcia, Phys. Rev. Al (1970) 280; Al (1970) 1402. W. Brandt and G. Lapicki, Phys. Rev. A20 (1979) 465, ibid. A23 (1981) 1717. C.V. Barros Leite, N.V. de Castro Faria and A.G. de Pinho, Phys. Rev. Al5 (1977) 943. R.S. Sokhi and D. Crumpton, Nucl. Instr. and Meth. 192 (1982) 121. W. Sarter, H. Mommsen, M. Sarker, P. Schurkes and A. Weller, J. Phys. B14 (1981) 2843. H. Moheb, R. Bigaouette and W. Inman, Phys. Rev. A32 (1985) 3739. A.P. Jesus, J.S. Lopes and J.P. Ribeiro, J. Phys. B18 (1985) 2453. MS. Dias and C. Renner, Nucl. Instr. and Meth. 193 (1982) 91. E. Storm and H.I. Israel, Nucl. Data Tables 7 (1974) 566. H.H. Andersen and J.F. Ziegler, Hydrogen Stopping Powers and Ranges in all Elements, (Pergamon, New York, 1977). Md. R. Khan and D. Crumpton, Appl. Phys. 15 (1978) 335. D.A. Close, R.C. Bearse, J.J. Malanify and J.J. Umbarger, Phys. Rev. A8 (1973) 1873. D.D. Cohen and M.F. Harrigan, At. Data Nucl. Data Tables 33 (1985) 255. J.H. Scofield, At. Data Nucl. Data Tables 14 (1974) 121.