73Ge : A NEW HIGH RESOLUTION MÖSSBAUER NUCLIDE

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73Ge : A NEW HIGH RESOLUTION MÖSSBAUER NUCLIDE

L. Pfeiffer, R. Raghavan

To cite this version:

L. Pfeiffer, R. Raghavan. 73Ge : A NEW HIGH RESOLUTION MÖSSBAUER NUCLIDE. Journal

de Physique Colloques, 1974, 35 (C6), pp.C6-203-C6-208. �10.1051/jphyscol:1974622�. �jpa-00215779�

JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 35, Dtcembre 1974, page C6-203

7 3 ~ e : A NEW HIGH RESOLUTION MOSSBAUER NUCLIDE

L. PFEIFFER and R. S. RAGHAVAN Bell Laboratories, Murray Hill, New Jersey 07974, USA

RBsumB. - Nous signalons quelques expkiences demontrant I'effet de resonance sans recul de la transition 13,3 keV ^(z

4,3 ps de 73Ge. Nous avons observe cet effet dans des monocristaux en poudre, de m6me que dans des matkriaux monocristallins. La rksonance de 73Ge la plus Ctroite observee jusqu'ici, posstde une largeur de raie de 14 + ² pm/s. I1 s'agit de la resonance la plus ktroite observke jusqu'ici a la tempkrature ambiante.

Abstract. - We report experiments which demonstrate a recoilless resonance effect with the 13.3 keV transition (z

4.3 ps) in 73Ge. We have observed the effect in powder microcrystals as well as in single crystal materials. The sharpest 73Ge resonance observed thus far has a FWHM linewidth of 14 & 2 pmfs. This is the narrowest Mossbauer resonance observed so far at room temperature.

1. Introduction. - The high-resolution Mossbauer nuclides have long been the subject of considerable interest in Mossbauer spectroscopy. In this respect, substantial progress has been made in the well-known cases of 18'Ta and 6 7 ~ n in recent years. We report here the successful observation [I] of the Mossbauer effect in a new case ^: the 13.3 keV transition in 73Ge. This transition (see Fig. I), with a mean life of .r = 4.3 ps has long been considered very attractive because of the extreme sharpness of the resonance (EIT, x lot4), the conveniently long lifetime of the parent nuclide

1021 I I , I '

' 1

0

30

50

ENERGY (heV)

FIG. 1. - Energy spectrum of the diffused 73As source ⁱⁿ the Mossbauer geometry (see text) with 10 mg/cmz epitaxial 73Ge absorber, recorded with the 100 mm2 x 2 mm Si(Li) detector.

The energies of the emission lines are ⁱⁿ keV. The decay scheme of 73As + 73Ge is shown in the inset.

73As = 80 d) and the low energy of the y-ray which assures a high probability for recoilless transi- tions even at room temperature. The present ,results have been obtained in conventional y-ray transmission experiments at room temperature.

The principal difficulties in the observation of the 73Ge recoilfree resonance arise from the high total internal conversion coefficient of the transition (a, = 1 loo), the close proximity of its energy to the K-absorption edge of Ge (11.1 keV), and the low natural abundance of 73Ge (7.76 %). These aspects impose severe demands on the strength, radioactive purity and thickness of the source, the thickness and enrichment of the absorber and the energy resolution and the ability to handle high total counting rates of the detection system used to observe the 13.3 keV y-ray.

The extreme sharpness of the resonance enhances these demands by requiring materials of high crysta! quality for the source and absorber.

2. Experimental procedures. -- The 73As activity for these experiments was produced from the reaction 74Ge(p, 2r1)~~As. The optimal energy of the proton beam was determined by an activation experiment. A stack of natural Ge single crystal disks, each 100 pm thick and interposed with carbon absorbers of suitable thickness was exposed to a 33 MeV proton beam from the Princeton University Cyclotron. After suitable aging to allow short-lived activities to decay, the y-ray spectra from each of the thin Ge crystals was taken with a Ge(Li) detector. The areas of the 53 keV y-ray from 73As and the y-rays at 51 1, 596, 635 keV from 74As were measured and corrected for background, diffe- rential absorption, and the half-life of the parent iso- topes. With this normalization, the y-ray peak areas become a measure of the relative excitation cross-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974622

05-204 L. PFEIFFER AND R. S. RAGHAVAN

sections as a function of the energy of the proton beam.

The results of this experiment, which are shown in figure 2, indicate clearly that the optimal proton energy

INCIDENT PROTON ENERGY (MeV)

FIG. 2. - Relative production cross section for (p, n) and (p, 2n) on 74Ge measured as a function of proton energy.

Although the targets for this experiment were crystals of natural Ge, very little of the 73As or 74As activity is due to other

reactions.

for clean and efficient production of 73As is about 24 MeV. Targets of powdered Ge metal enriched in 74Ge to 97.7 % were bombarded with 24 MeV protons for 300 to 500 PA hours. After a three month period of aging, the 74As activity decayed to a low level, leaving a few millicuries of usable 73As activity in the enriched 74Ge powder.

Mossbauer sources were prepared by two different procedures : by thermal diffusion of the 73As activity from the irradiated 74Ge powder into single crystals of natural Ge, and by directly irradiating a 74Ge single crystal with 24 MeV protons. The recipe for making the diffused sources is as follows : The 74Ge target powder containing the 73As activity was crushed carefully in a mortar and pestle to remove the sintering which occurred during the bombardment by the 24 MeV proton beam. The powder was then transferred into a single crystal silicon container having a cavity 5 mm in diameter and 2 mm deep. The loaded cavity was covered by a 15 pm thick single crystal of high-purity natural Ge, which was in turn covered by a Si single crystal disk. The assembly was then annealed in flowing H, gas for 20 to 30 hours at 850

The radioactive 73As was thus sublimated from the powder onto the Ge covering disk eventually diffusing into it. In the other method of source preparation, a single crystal layer of 74Ge was grown epitaxially on a Si substrate. This epitaxial crystal was then irradiated directly with the 24 MeV proton beam for a dose of 300 pAh. The 73As activity was thus produced in situ in the Ge film.

Two types of Mossbauer absorbers were prepared for these experiments : a single crystal film of 73Ge epita- xially grown on a substrate of Si, and a p o ~ d e r absorber made from microcrystalline particles of 73Ge. The epitaxial films were prepared by vacuum deposition of the 73Ge under the following prescription : The Si substrates were crystals of (11 1) orientation with the dimensions 40 mm x 15 mm x 0.5 mm. These crystals were cleaved from p type 25 SZ cm Si wafers which had been mechanically polished and then gas-polished in HBr at 1 250 OC. The substrate crystals were supported in the vacuum deposition apparatus in such a way that they could be heated by electrical resistive heating. A charge of 150 mg of elemental Ge powder of enrich- ment 79.9 % 73Ge was placed in a vitrious carbon crucible, installed in the vacuum evaporation appa- ratus, and subsequently heated to the operating tempe- rature. The vacuum evaporation equipment was such that the source and the substrate could be screened from one another by a removable shutter. For each run with the shutter in position, the substrate was first preheated to 1 100 OC for 5 min in order to remove the surface oxide. The substrate temperature was then lowered to 950 OC and the source heated to 1 450

The shutter was removed and the deposition commenc- ed at a rate of 2 pmlmin. The substrate temperature was then lowered to 850 OC where it was maintained for the duration of the run. After 20 to 30 pm of Ge had been deposited, the shutter was reinserted and the substrate slowly cooled to room temperature to avoid rapid changes in layer strains.

Because of the difference between the thermal expansion coefficients of Si and Ge (4.2 x and 5.9 x 10-6/oC respectively) it is expected that the epitaxial Ge layer is unavoidably strained during the substrate cooling from the deposition temperature.

Deformation of the bonding in the Ge lattice due to the presence of such strains is undoubtedly relevant to the proper interpretation of the Mossbauer resonance spectra obtained with these films.

The microcrystalline powder absorbers were made by heating a 400 mg charge of elemental enriched 73Ge powder to 1 400 O C in flowing H, gas, causing the charge to melt into a ball. After slow cooling (5 OC/min) the resulting polycrystalline lump was crushed into small microcrystals. The microcrystals were then sorted by size by allowing them to settle for various times in water. Microcrystals of a diameter between 2 and 15 pm were selected in this way and then gently mixed with inert BN powder, and the mixture was packed in a disk-shaped holder case 1 mm deep with a bottom and cover of Be 0.38 mm thick separated by a brass supporting rim 2.2 cm in diameter. The Ge micro- crystalline content was adjusted to be 9 mglcm2.

Either of two y-ray detectors were used at various

times in this work. The first detector which was used t o

accumulate the spectrum shown in figure 1, is a Si(Li)

crystal 2 mm thick and 100 mm active area with a rated

energy resolution of 240 eV at 5.9 keV using an ampli-

73Ge : A NEW HIGH RESOLUTION MOSSBAUER NUCLIDE C6-205

fier shaping time of 8 ps. This detector employed pulsed-optical feedback in the liquid N coolet FET preamplifier. The other detector was a Si(Li) crystal 5.3 mm thick with an active area of 80 mm2 ; its rated energy resolution is 195 eV at 5.9 keV with an amplifier shaping time of 10 ps. The preamplifier of this detector did not employ pulsed-optical feedback. These detec- tors performed essentially equally well in isolating the 13.3 keV y-ray from the other decay products of 73As.

The increased sensitivity of the thicker detector to the 53.4 y-ray was in some measure offset by the increased resolution and superior peak-to-Compton ratio of that detector.

With either detector we found it was imperative to use some type of electronic pile-up rejection system and some type of gated dc baseline restoration in the shaping amplifier. Figure 3 illustrates this. In the spec-

NO PILEUP REJECTION TOTAL RATE

-

WINDOW RATE = 520 6'

I (

ENERGY (eV)

FIG. 3. - Illustration of the value of electronic pile-up rejection for this work. The two 73As spectra were taken under identical conditions except that in the lower spectra pulse pile-up rejection was used. The prominent peaks are the 9.88 and 11.0 keV X-rays of Ge. The expected position of the 13.3 keV y-ray is marked by

a pair of glitches in the upper spectrum.

trum taken with the pulse pile-up rejection circuitry inoperative, the 13.3 keV y-ray is not even visible because of the noise pile-up from other y-rays. Unfortu- nately pile-up rejection methods are not effective in handling small amplitude y-ray pulses. In the typical circuitry in use today, it is impossible to set the pile-up discriminator threshold below 2 to 3 keV without triggering excessively on the thermally generated pre- amplifier noise. The result is that small y-ray pulses are below the pile-up discriminator threshold and are thus ignored. Pulse pile-up involving these small pulses is not in any way rejected by such circuitry. We have found that this small pulse pile-up problem can be ameliorated by inserting a filter before the detector window which selectively absorbs the lowest energy

y-rays. Figure 4 shows the value of this approach. In the lower spectrum of that figure, the detector entrance window is covered with low energy absorbing filter consisting of 0.75 mrn of Teflon followed by 1 .O mm Be metal. This filter has very little effect on the true 13.3 keV y-rays, but a significant effect on the pile-up noise under the y-ray peak.

NO X-RAY FILTER

WlTH X-RAY FILTER

1021 d

0 4 8 12 16 20 24

ENERGY

(keV)

FIG. 4. - Illustration of the value of a filter which absorbs low energy y-photons. The two 73As spectra were taken under identical conditions (using pile-up rejection) except that in the lower spectra a filter consisting of 0.75 mm Teflon and 1.0 mm Be was placed in front of the detector entrance window.

The Mossbauer experiments reported in this paper were carried in conventional y-ray transmission geo- metry. The arrangement of the experimental apparatus is shown in figure 5. The velocity spectrometer is a

MU METAL ABSORBER (ATTACHED DIRECTLY MAGNETIC SHLELD TO ORLYE HOUSING1

\

\ (1rnrn.TEFLONl

/

GRANITE TABLE

FIG. 5. - Schematic description of the experimental apparatus.

conventional electromechanical drive, consisting of an

oscillator that generates the desired velocity waveform,

a mechanical unit consisting of a drive coil and a sense

coil in a magnetic field, and an electronic feedback

circuit which compares the currents induced in the

sense coil with the desired velocity waveform. Because

of the very small Doppler linewidth of the 73Ge

recoilless resonance. it is essential to minimize all

C6-206 L. PFEIFFER AND R. S. RAGHAVAN

sources of electrical noise in the drive-electronics, so that such noise does not contaminate the mechanical motion of the drive.

In preliminary experiments two sources of exter- nally generated noise were found on the drive error feedback signal. Sharp transient spikes 1.5 mTr pp were observed on the error signal after each channel advance of the multi-scaler address in the multi-channel ana- lyzer. This source of noise was easily eliminated by removing all ground connections between the multi- channel analyzer and the velocity drive electronics, and by connecting :he start input from the multi-channel scaler to the velocity drive through a 100 pf isolation capacitor. The importance of this particular source of noise was emphasized to us by our early experiments.

We found whenever the multi-channel ana!yzer was not properly isolated from the velocity spectrometer, that it became impossible to observe 'my recoilless resonance effect with 7%3e. The other source of noise on the error signal took the form of sharp spikes of peak amplitude 2 to 50 mV in sync with the 60 cycle 11 5 V line. This noise was eliminated by replacing the power supplies of the velocity spectrometer electronics with two 12 V automobile batteries. With the spectrometer battery- operated and properly isolated from the multi-channel analyzer, the external noise on the error signal was found to he less than 15 pV pp over the bandwidth dc to 1 MI-17.

The velocity calibration of the spectrometer was Iinearly extrapolated from 57Fe hlossbauer measure- ments of s7Co in Pd against sodium nitroprusside made over the velocity ranges & 10' pm/s to k lo4 pm/s.

The error associated with the velocity calibration is believed to be less than _+ 5 "/. The 7ero velocity channel of the spectrometer was determined by measu- rements of s7Co in Fe against Fe and also be extrapola- tion of the sodium nitroprusside line positions plotted as one ovei the velocity potentiometer setting. The zero velocity position is believed to be determined to within

f 0.5 channel.

The experiment was isolated from external acoustic and mechanical vibrations by operating the velocity spectrometer and the detector on a 20 cm thick granite slab which was itself suspended by an air suspension system for vertical isolation and by means of three 4.0 cm !ong pendu!ums on each leg for horizontal isola- tion. The fundamental frequency of the suspension system was 1.5 Hz vertical and 1.0 Hz horizontal, corresponding to more than a factor of 10 isolation at 10 Hz and above in the vertical mode and at 6 Hz and above in the horizontal mode.

From the magnetic moment and the spin of the nuclear ground state of 73Ge, one can calculate that a stray magnetic field of only 22 Oe on either the source or the absorber is sufficient to broaden the Mossbauer resonance by a natural linewidth. For this reason, the mu-metal shielding shown in figure 5 was installed around the source and the absorber. This shielding reduces the stray fields from the velocity transducer

magnets at the source and the absorber to less than 1.5 Oe.

3. Results and discussion. - The first velocity spec- trum which showed a recoilless resonance effect for the 13.3 keV transition of 73Cie is shown in figure 6. This

-

- ⁴⁰

VELOCITY

(prn/s)

FIG. 6. - First evidence for a recoilless resonance effect with the 13.3 keV transition of 73Ge. Source : 73As diffused in natural Ge.

Absorber : epitaxial 73Ge film on (111) Silicon.

spectrum on which we have already reported [ I ] was obtained under the fol!owing experimental conditions : The recoilless source was made by diffusing the 73As activity into a 10 pm thick (110) single crystal of high purity natural Ge. The recoilless absorber was a 20 pm thick (111) epitaxial layer of enriched 73Ge on a Si substrate. The total count rate in the 13.3 lie\' single channel analyzer (SCA) window was a 7 s-', of which 39 % were due to the y-ray signal. The reso- nance absorption dip may be characterized by the following parameters obtained by a least squares fit of a single Lorentzian function to the data points : depth = 1.0(1) % ; FWHM linewidth = 53(7) pm/s, center shift = f 7(2) pm/s. Accounting for the y-ray background and for thickness and velocity broadening effects, we obtain a corrected depth of 2.0(2) % and a corrected FWHM linewidth of 47(7) pm/s, or about 7 times the natural Mossbauer linewidth,

The results of our next two Ge resonance experi-

ments are shown in figure 7. For these experiments the

Mossbauer source of the first experiment was replaced

with a source made by bombarding 24 MeV protons at

300 pAh onto a 20 pm thick 74Ge film epitaxially

grown on a Si substrate. The absorber and all other

conditions in the experiment were identical with those

of the first experiment. The total count rate in the

73Ge : A NEW HIGH RESOLUTION MOSSBAUER NUCLIDE C6-207

I I I I / I I I I

SOURCE BEFORE ANNEAL I

99.0 1

600°C ANNEAL -1

I I I I I I I J

- 8 0 - 4 0 0 4 0 8 0 VELOCITY ( p m

FIG. 7. - Recoilless resonance experiments using a radiation damaged y-ray source. Source : 73As produced in situ by bom- barding an epitaxial 74Ge crystal films with 24 MeV protons.

Dose

300 pAh. Absorber : epitaxial 73Ge film on (111) silicon. a) Before source anneal. b) After source annealed at

600 OC in flowing Hz gas for 1 hour.

13.3 keV SCA window was 19 countsis of which 45 %

were due to the y-ray signal. The spectrum at the top of the figure which shows no recoilless resonance effect was taken using the bombarded source after a three month aging interval but without a high temperature anneal. The lower spectrum was taken with the same bombarded source following a 1 hour anneal in flowing Hz gas at 600 OC. Clearly the recoilless resonance effect is highly sensitive to the proton radiation damage of the Ge crystalline environment and clearly also the 600 OC anneal is adequate to remove most of this damage. The corrected parameter values for the reso- nance are : depth

1.4(1) %, linewidth

50(7) pm/s and zero shift = + 7.1(1.6) pm/s. The values for linewidth and zero shift of this resonance are in general agreement with the values obtained in the first experi- ment, however, the corrected absorption depth is substantially smaller in this experiment. The reason for the reduced absorption effect with the epitaxial source has not been established. However, a likely hypothesis is that inelastic Compton scattering in the thicker epitaxial source converts resonant 13.3 keV y-rays into non-resonant y-ray noise in the SCA window.

The three resonant spectra displayed in figure 8 show our recent progress in reducing sources of line broaden- ing in the 73Ge problem. In the top spectrum of the

VELOCITY (,um/S)

FIG. 8. - 73Ge recoilless resonance experiments. a) (top) A reproduction for reference of figure 7b. Source : 73As in annealed epitaxial 73Ge on (111) Si. Absorber : epitaxial 73Ge on (111) Si.

b) (center) Source : 73As in annealed epitaxial 74Ge on (111) Si.

Absorber : microcrystalline 73Ge powder suspended in inert powered BN. c) (bottom) Source : 73As diffused into a 15 pm thick (110) natural Ge crystal. Absorber : microcrystalline 73Ge

powder suspended in inert powered BN.

figure, we have reproduced for reference figure 7 b just discussed. The center spectrum was obtained under experimental conditions similar to those of the top spectrum, except that the epitaxial absorber of the top spectrum was removed and replaced with an absorber made from size-selected microcrystalline particles, as described in Section 2. The thickness of the micro- crystal powder absorber was 9 mg/cm2 Ge. The total SCA count rate was 12 s - l , 41 % of which were due to 13.3 keV y-rays. The corrected least squares parameters of the resonance are : depth ⁼ 1.8(2) %, F W H M line- width

27(5) ym/s and center shift ⁼ + 5.4(1.3) ym/s.

Since the absorber is the only experimental difference

between the top and center spectrum, the sharper

resonance of the center spectrum suggests that the

C6-208 L. PFEIFFER AND R. S. RAGHAVAN microcrystals provide a significantly improved solid

state environment for the absorbing 73Ge nuclei over the epitaxial Ge film.

The lower spectrum in figure 8 was obtained under conditions comparable in all respects with the center spectrum except that the annealed epitaxial y-ray source of the center spectrum was removed and replaced with a new source of 73As diffused into a 15 pm thick crystal of natural Ge. The SCA count rate for this experiment was 14 s-', of which 49 % were due to the 13.3 keV y-ray. The corrected least squares parameters of the resonance are as follows : depth = 2.9(2) %, linewidth = 14(2) pm/s and center, shift = + 0.3(0.6) pm/s. The substantially reduced linewidth of the lower spectrum compared to that of the center spectrum shows that the source made by diffusing As into a natural Ge crystal provides a substantially more homogeneous environment for the 73Ge nuclei than does the source made by epitaxial deposition of Ge on Si. The absence of a center shift in the lower spectrum strongly points to crystalline strain in the epitaxial films as the origin of the center

shifts in the other spectra. Finally the substantially larger absorption depth of the lower spectrum compar- ed to the center spectrum is in agreement with our hypothesis of Compton inelastic degradation of the initially monoenergetic 13.3 keV y-rays in the thicker epitaxial source.

The resonance shown in spectrum 8c is the highest resolution Mossbauer resonance observed so far at room temperature. At 14 pm/s the resonant linewidth is 4 times more sharply defined than the experimentally observed resonance in 1 8 ' ~ a . Thus a new high-resolu- tion tool of general utility has become available for measuring very small changes in energy. It is especially fortunate to have such a powerful new tool available in Ge because of all the elements, Ge and Si are pro- bably the best studied and best understood solids. They offer clean, simple systems for testing ideas and looking for small and subtle new effects.

Acknowledgments. - The authors wish to thank Dr. A. G. Cullis for preparing the epitaxial Ge films and C. P. Lichtenwalner for technical help.

Reference

[I] See also RAGHAVEN, R. S. and PFEIFFER, L., Phys. Rev.

Lett. 32 (1974) 512.