INNER SHELL STUDIES USING INELASTIC COINCIDENCE SPECTROSCOPY OF LOW ENERGY GAMMA RAYS

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INNER SHELL STUDIES USING INELASTIC COINCIDENCE SPECTROSCOPY OF LOW

ENERGY GAMMA RAYS

S. Manninen, K. Hämäläinen, T. Paakkari, P. Suortti

To cite this version:

S. Manninen, K. Hämäläinen, T. Paakkari, P. Suortti. INNER SHELL STUDIES USING INELAS-

TIC COINCIDENCE SPECTROSCOPY OF LOW ENERGY GAMMA RAYS. Journal de Physique

Colloques, 1987, 48 (C9), pp.C9-823-C9-826. �10.1051/jphyscol:19879145�. �jpa-00227256�

INNER SHELL STUDIES USING INELASTIC COINCIDENCE SPECTROSCOPY OF LOW ENERGY GAMMA RAYS

S. MANNINEN, K. H ~ M ~ L A I N E N , T. PAAKKARI and P. SUORTTI

Department of Physics, University of Helsinki, Siltavuorenpenger 20 D, SF-00170 Helsinki 17, Finland

Le spectre de diffusion inklastique des rayons gamma par les klectrons K de C'u a 6th niesurh avec rayonnementk 59.54 keV et une technique de coincidence dans la rhgion de tmnsfl.rement de moment, oh le rdle de l'knergie liaison~iante des klectrons est important. Compark avec une approximation d'impulsion les rksultats experimentaux montrent quelque structnre prhs dn bord d'absorption. Le spectre observk est compark avec des modbles thkoriques existant.

Abstract

Gamma ray inelastic scattering spectruni from C I ~ K-elect.rons has been measured using 59.54 keV radiation and coincidence technique in the momentum transfer region where t,he role of the elect,ron binding energy is important. Compared with an impulse approximatio~i, experinient,al result shows some structure close to the absorption edge. The observed spectrum is compared with existing theoretical models.

1 Introduction

Inelastic coincidence spectroscopy of gamma rays have been mainly used so far to measure total inelastic scattering cross sections for K electrons. Experimental results have been compared with Klein-Nisliina cross section, modified with the inelastic scattering factor and tlie effect of the electron binding can then be studied. Gamma ray energies used in these experiments have been of the order of 0.5 - 1.5 MeV. Recently Basavaraju et al. [I] have used 280 keV gamma rays to study also the energy spectrum of inelastically scattered photons by K-shell electrons. A careful analysis of their results showed an evidence of the infrared divergence of the scattering cross section predicted by Gavrila [2] and additionally they tried to find whether there is a shift ( s o called Conipton defect) in the Compton peak position due to the K-shell binding energy. Tllc study of the detailed structure of the scattered energy spectrum was not possible because of the low resolutioii of scintillation counters used in their experiment.

An interesting problem in these type of experiments arises when tlie energy transfer in inelastic scattering process is of the order of the K-shell binding energy. Suzuki [3] proposed that there is a change from Compton type of energy spectruni to Raman type of energy spectrum at around k a = 1 where

k

is the length of the scattering vector and a is the K-shell Bohr radius. Namikawa arid Hosoya [4] used GO keV gamma rays to measure the illelastically scattered spectrum from K electrons in F e and Cu and claimed to find both Conlpton and Raman type contributions in addition to a weak component interpreted as double Thonlson scattering. Their measurement was, however, heavily distorted by false coincidence events and their findings were criticized by

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879145

C9-824 JOURNAL DE PHYSIQUE

Manninen [5]. On the other hand Ohmura and Sato [6] used OPW method t o describe electron wave function i n t h e scattering state and their calculation gives a double peaked scattered spectrunl but it differs considerably with the results given by Namikawa and Hosoya. Recently synchrotron radiation a t primary energies of 70 keV and 62 keV have been used [Marchetti V. and Frank C., unpublished work] but no double peaked structure in C u K-electron spectrum was found, i n agreement with a calculation based on a Hartree-Fock-Slater approximation. Although t h e counting rate in a synchrotron experiment is very high the pulsed structure of t h e synchrotron beam makes the measurement of chance coincidences more problematic.

The aim of the present work is t o try t o solve the discrepancies mentioned above by measuring the inelastic scattering cross section of Cu-K electrons using coincidence technique a t the momentulli transfer such t h a t k a 1.1. The experiment is optimized t o minimize false coincidence events. T h e measured energy spectrum is compared with the existing theoretical models. A short discussion is finally given for future prospects in this area.

2 Experiment

In t h e experiment gamma rays from a 900 nlCi 241Am source were scattered through a nlean angle of 132 degrees from a C'u foil (thickness 17 pm).Both CtiK-fluorescence x-rays and scattered gamma rays were detected using Ge detectors which had a resolution of 300 eV a t 8 keV and 500 eV a t 60 keV. Single channel analysers were used to set the energy windows for the G'uK x-rays (window 7.2 keV

-

9.6 keV) and gamma rays (30.0 keV - 65.0 keV). The outputs of single channel analysers were led t o a coincidence unit and its output t o the gate of the multicha~iriel analyser. Both pulser and time t o amplitude converter were used to set the timing of the counting circuit. In both counting chains crossover timing was used. The total counting rates i n the chosen energy windows were 1060 c/s (Cu-fluorescence) and 365 c/s (gamnla rays). On the basis of t h e time t o amplitude spectrum a 60 ns window was chosen for coincidence measurements. Counting rates on both sides of this window were analysed and to count chance coincidences a 200 ns offset was chosen. A careful check t o prevent parasitic scattering events (detector t o detector scattering, collimator t o detector scattering) was also performed.

In t h e actual measurement a series of coincidence and out of coincidence measurements was per- formed. T h e counting rate a t the coincidence turned out t o be 0.017 c/s and 0.013 c/s out of coincidence respectively. Altogether both measurements were repeated so that t h e total count- ing time was 2320000 s. After the subtraction of the chance coincidences the spectrum of true coincidences is shown in Fig. 2. The channel width of the multichannel analyser was 70 eV and each point i n Fig. 2 corresponds the sum of five adjacent channels. Including t h e relatively wide angular divergences in the scattering geometry, the total resolution function has a half width of approximately 1 keV.

3 Results and Discussion

Before any comparison between the present experiment and theory one should make a colnlne~it concerning the terminology. Compton scattering usually covers all those inelastic scattering pro- cesses in which the ejected electron goes t o the continuuln states in the metal or colnpletely out from the atom in other cases. The initial electron l n o ~ i i e ~ l t u m distribution broadens the scattered energy spectrum. If the energy transfer is smaller than the electron binding energy, there is a cut-off in t h e final energy spectrum but its shape is still dominated by the electron molnentuln distribution. In Raman scattering the elettron goes t o a vacant discrete state and the photon energy loss is determined by the energy difference between the initial and final states. Of course, if t h e energy given t o the ejected electron in a Compton scattering process is so small t h a t i t goes just above the Fermi surface of a metal, one should expect to see the effect of the structure of the density of empty electron states in the scattered spectrunl providing the resolution is goocl

In this approximation the interaction time is expected to be so short that the scattering takes place in a constant potential. Ribberfors [7] has shown that even close t o the absorption edge IA still gives surprisingly good results. Depending on the scattering angle, the scattered spectrunl in energy scale might look very different as can be seen in Fig. 1 in which the Compton profile of Ctc 1s electrons have been given at two scattering angles. Indeed the result corresponding GO degrees does not look like an usual Compton profile and this 1nig11t be the reason why a tern1 of Ranza7z scattering has been used [3] in this case instead of Coinpton scattering from bound electroias.

I I

In Fig. 2 Cornpton profile of C a 1s electrons, calculated in the IA is also glven. The binding energy cut-off a t (59.54 - 8.98) keV is included and the edge is slneared with the experiniental resolution function. Because the present measurernent is not an absolute one, the profiles have been normalized t o have the same area aftel the absorption correctioli is included in the theoretical profile. One can conclude that the overall behaviour is well described by the IA, aitliough the double peaked structure is, of course, missing.

a5-

Figure 1. Theoretical Compton profiles

J ( E ) of CuK-electrons calculated using the

:

impulse approximation a t two different scat- tering angles ( a ) 170' and (b) GO0. The in- cident energy hw1=59.54 keV and hwz cor- responds the free electron Conlpton shift, i.e. the peak of the Coinpton profile. When

-

6wl - hwz

<

8.98 keV which is the CuK- binding energy, Compton scattering is not energetically possible.

T h e O P W calculation [GI predicts a peak close to absorption edge which is identical, both co11- cerning the position and the half width, with the present observation. Also having a closer look to the synchrotron experiment [Marchetti V. and Frank C., unpublished work] a similar peak can be found when ka ^ZY 1. In the case of the earlier Am-experiment [4] a stronger peak a t higher energies is found which must be a result of false coincidences. Also there is no evidence for paranletric (double Thomson) scattering in the present data, in agreement with the recent calculation [S].

Additionally there is a second peak a t around 4.5 keV in Fig. 2 which is narrower and a t higher energy than in t h e O P W calculation. It is possible t,hat in spite of attempt,^ t o avoid det,ect,or- detector scattering there is a small cont.ribution left. Based on the experimental geo1net.r~ the false coi~~cidences arising from detector-detector scattering are centered a t 48.7 keV (elastic-ineladic)

a) 0 = 170"

and at 44.8 keV (inelastic-inelastic) and the corresponding electron recoil energies in x-ray detector are centered a t 10.8 keV and 9.0 keV. The first one is quite far from the x-ray energy window

_---

---- _-_---

2--

4

4 %

and elastic scattering is one order of magnitude smaller than inelastic one at these momentum transfers (sin

:/A -.

^3.7 in the scattering t o the x-ray detector), but the latter one can give some contribution. The other false coincide~lce sources, more important a t higher energies, like

-E* ( C u l 4

I u,

I,

bremsstrahlung produced by photo- and C:ompton electrons are negligible in t,he present case.

Based on the present experiment,al clat,a one can conclude t,hat t,he effect of the elect,ron binding t,o the Compton scattering process ca.n be seen as a deviation from t,he impulse approxima.t,ion.

011 the other hand neither the use of synchrotron radiat,ion (because of its pulsed structure) nor t,he present gamma ray sourc,e (because of t,he liinited in tens it,^) can solve the problem complet,ely.

Therefore a new spectrometer based on a high volt,age W x-ray tube is under construct.ion in our laboratory. Using a S i (440) reflection, for example, in transmission mode it is possible t o separate WK,, and obtain a focussed bean1 having more than l o 7 photons/s when W tube is operated a t 160 kV/10 I ~ A . Conlpared wit,h t,he present flux onto t,he sanlple it is about. the same

30 LO 50 60

b) $3 =60° ^{u, [keVI}

-

- 0 / ' / -

/

..- ,/'

E~[CUI

+

_I

0; woc

30 LO 50 60

C9-826 JOURNAL DE PHYSIQUE

but the essential improvement is extremely well defined geometry. Together with an evacua.ted sample chamber this will impro've the truelchance coincidence ratio and make false coincidence contribution negligible.

40000

a)

30000

-

m I-

10000 -

Figure 2 . The experimental Coinp-

ton scattering cross sections for Cu (a) ₄₀₀ and for CuK-electrons (b) at the scat-

tering angle of 132". Included also

in (b) is the theoretical cross section ³⁰⁰ calculated in the impulse approxima-

tion. The effect of the sample ab-

g

200 sorption is included in the theoreti-

5

cal curve and its sharp binding edge at hwl - hRK has been smeared with the total experimental resolution func- tion. The statistical error bars at sonie points include the chance coincidence counting rate.

30.0 35.0 LO.0 410 50.0 55.0 60.0

References

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