Nuclear resonance fluorescence in 69Ga

1.E.4: 3.A

Nuclear Physics A l l l (1968) 225--235; (~) North-Holland Publishintl Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

NUCLEAR RESONANCE FLUORESCENCE IN 69Ga H. L A N G H O F F and L. F R E V E R T t

!i. Physikalisches lnstitut der Universitiit Gb'ttinoen Received 15 January 1968 Abstract: Resonance fluorescence from the levels at 1107, 872 and 574 keV in SaGa has been observed using gaseous sources of SaGeCI~ and 68GeBr4. The lifetimes obtained from resonance scattering and self-absorption experiments are (g0r/gl)tl07 = 0.164-0.02 psec, (O0r/gl)sTs = 0.31 =t=0.06 psec and (Oar]O~)674 = 5 4-2 psec. The angular distribution of the resonantly scattered radiation is compatible only with spins ] and ~ for the 1107 and the 872 keV levels and [61 < 0.55 for the corresponding E2-M1 transitions. Resonance scattering was also observed with solid GeCI, sources. This yields information on the slowing down of the recoiling nuclei. The investigation of the y-spectrum emitted after the decay of eggs using a Ge(Li) detector revealed seven new y-transitions in addition to the 19 known y-transitions.

N U C L E A R REACTIONS SaGa(y, ),), E = 1107, 872, 574 keV; measured a(E; 0). SaGa levels deduced J, T,I.. RADIOACTIVITY egGe [from e°Ga(d, 2n)]; measured Er, It, 7,y-coin. SaGa deduced levels. Natural target, Ge(Li) detector.

1. Introduction Our knowledge of the character of the low-lying levels in 69Ga is rather poor. It might be presumed that the four levels at 574, 872, 1107 and 1337 keV arise from a coupling of the odd proton in a p~ orbit to a core vibration. This can be tested by the measurement of the y-transition probabilities. Since the levels decay mainly by y-transitions to the ground state, the resonance fluorescence method is applicable. In an attempt to excite levels in 69Ga with bremsstrahlung, Wilson and Booth 1) observed a transition of 1110 keV in the scattered radiation. The four levels are populated in the radioactive decay of 69Ge. Since the transition energy of the electron capture or fl+ transition to these levels is appreciable, gaseous sources of 69Ge would supply the suitable radiation for a study of resonance fluorescence from 69Ga. This technique was applied in the present investigation. 2. Resonance fluorescence experiments The 69Ge nucleus was produced by the 69Ga(d, 2n)69Ge reaction with deuterons of 25 MeV, Ga2 03 encapsulated in copper tubes was irradiated with about 30 # Ah. The irradiated powder and some Ge carrier were dissolved in hot concentrated KOH. After acidification with HCI, the germanium was precipitated with H2S. The pret Present address: Hahn-Meitner-lnstitute, Sektor Kernphysik, Berlin. 225

226

H.

LANGHOFF

AND

L.

FREVERT

cipitate was dried and then mixed with HgCl 2 or HgBr 2 . The mixture was transferred into a vacuum apparatus. After heating to about 200 °C, GeCl4, or GeBr4, respectively, were generated with high yield. These gases were collected in glass ampules. After sealing the ampules contained a total activity of about 7 mCi of 69Ge and negligible fractions of 6~Ge and 71Ge. For the resonance fluorescence experiments, the ampules were heated to about 80 °C in the case of GeC14 and to about 180 °C in the case of GeBr 4. At these temperatures, the activity was distributed evenly over the whole ampule, and it was concluded that the entire activity was in the gaseous phase. The pressure iv the sources was estimated to be 300 mm Hg. The resonance scattering experiments were performed in the usual ring geometry 2). Two scatterers, one containing 432 g of Ga2 O3 and the other an equal amount of Zn for comparison, were exchanged automatically every 15 rain. The scat-

|I

!

iI

" '""'

A/

t"

5

0

i

__

600

BOO

1000 1200

I~L~ Ear/keV]

Fig. 1. Spectrum of radiation scattered resonantly from Ga=O3. The background determined with a comparison scatterer of Zn has already been subtracted. The scattering angle was 130°. The lines at 574, 872 and 1107 keV originate from the excitation of the corresponding levels in SgGa.The dashed curve represents the spectrum of the incident radiation supplied by a gaseous GeCl4 source. tered radiation was detected by a 7.6 cm x 7.6 cm N a I detector which was shielded by 2 m m of lead to suppress low-energy radiation. Fig. 1 shows the spectrum of the radiation supplied by the GeCl 4 source and scattered by the Ga2Oa scatterer. The scattering angle was 130 °. The background observed with the comparison scatterer has already been subtracted. The three lines at 574, 872 and 1107 keV have to be ascribed to resonance fluorescence. The excitation of the 1337 keV level was not observed. The dashed curve indicates the spectrum of the incident radiation. In a second experiment, the measurement was repeated using a source which was solidified by cooling with a dry-ice alcohol mixture. The intensities of the lines at 1107 keV and 872 keV in the scattered radiation were reduced to 4.2_1.5 ~ and 17___4 ~o, respectively, of the amounts observed with the gaseous source. Since the

e°Ga NUCLEAR RESONANCE FLUORESCENCE

227

difference in Rayleigh scattering for the two scatterers at these energies is expected to be two orders of m a g n i t u d e smaller, it was concluded that this effect is due to reso n a n c e scattering caused by y - q u a n t a which are emitted before the recoil of the nucleus from the n e u t r i n o emission is slowed down. F o r the 511 keV radiation, which is emitted very intensely by the 69Ge source, we observed a very small difference in scattering with the solid source. F o r the analysis TABLE 1 Results of the resonance fluorescence investigation Er (keV)

1337

afGeCt4) (mb) start

<10

ascatt(GeBr4)

1107 126

4-6

218

872

574

+15

554-20

0.98 +0.06

0.844- 0.08

0.042 4-0.015

0.17+ 0.04

Oscan(GeCl,) °'scatt(GeClt)s°lJd

o'scatt(GeClt)gas Aa e(~,)

<0.07

<0.12

26.8

-t-2.5

# z / ' (meV) .q0

4.1

4-0.4

N(Er)exo(eV-1)

(1.04 -t-0.12)" 10-s

Nil: ~ r / ~(eVG e a t os)m

(1.6

go "r (psec)

(0.16 -t-0.02)

(0.31 4-0.06)

"/"exp gl l'vibr (E2)g o

10

16

gz

T(M l)s.p, go

7"exp

4.5

4-0.2) • I0 -~

28

q-5

2.1 4-0.4 (2.3 4-0.5)" 10-2 2.7 • 10-~

4

3.8" 10-s (5±3)

8+4

18

Bt

eGeCm,scattand a~e~r' are the cross sections for resonance scattering obtained with gaseous sources of GeCI4 and GeBr4, respectively, ~otld the cross section with a solid source of GeCI, and As the scatt anisotropy in the angular distribution of the resonantly scattered radiation. The level width/' and the lifetime T are deduced from the observed self-absorption e, gz and go are the statistical factors of excited and ground state, respectively. N(Er)exp the relative number ofT-quanta in the emission line which falls into an energy interval of I eV around the absorption line Er and N(Er)Ge the expected values for a gaseous source of Ge atoms. In lines 10 and 11, the experimental results for the line width Fex p and the transition probability Texp are compared with predictions of a pure vibrational model F(E2)v|br and the single-particle model omitting spin factors T(M l)s.p. • of the resonance spectra, this mismatch has been taken into account; it influences slightly the resulting intensity for the 574 keV radiation. I n order to investigate the a n g u l a r distributions o f the resonantly scattered radiation, m e a s u r e m e n t s were performed at the scattering angles o f 88 °, 115 °, 130 ° and 144 ° with gaseous a n d solid sources of GeCl 4. The analysis of these data showed that

228

H. LANGFIOFF A N D L. FREVERT

the angular'distributions for the 1107 keV and the 872 keV transitions are isotropic. Expressing the experimental data in terms of W(~) = I + A z P z ( 9 ) upper limits for A 2 of (A2)~o~ < 0.07 and (A2)a7 2 < 0.12 were obtained (table 1, line 4). The average cross-sections O'seatt for the excitation of the 1107 keV and the 872 keV levels were evaluated by averaging over the results for the different scattering angles and are listed in table 1, line 1. A correction of about 3 ~ takes into account the self-absorption in the scatterer. The cross section for the 574 keV level was obtained from the results for the scattering angles of 130 ° and 144 ° only. The decay modes (fig. 2) of the resonantly excited 1107 keV level via the 872 keV and the 574

S e69 (3g,sh/ 5¢ 20ZZ .. _ 2O25

1337 ~

l--a- ~

3,e is~:7 IIIl!lll

Illltllll

sz 0 37Ga69 Fig. 2. Level scheme of 6°Ga established by Temperley et al. B). The scheme was supplemented by the levels at 2044 and tentatively 1724 keV in order to explain the newly observed ),-transitions of 2044, 1724, 1407 and 1151 keV. The 1724 and 1511 keV transitions fit at two different positions into the decay scheme. keV levels lead to further small corrections on the cross sections for the 872 keV and the 574 keV levels. The evaluation of the level widths from the cross sections depends on the knowledge of the energy distributions for the emission lines N ( E ) . These are difficult to estimate, since the effect of the electron capture and the neutrino recoil on the complex molecule GeCI 4 is not known. Therefore, a self-absorption experiment was performed to determine the level widths. Since only a limited amount of gallium was available, a point geometry was chosen for this experiment. The scatterer, having the dimensions 7 x 7 x 5 cm 3 contained

e°Ga NUCLEAR RESONANCE FLUORESCENCE

229

290 g 6f Ga 20 3, the absorber consisted of 13.7 g/cm 2 of gallium metal. For comparison, an absorber and a scatterer of Zn were used. The two absorbers were matched with respect to their non-resonant absorption with sources of 6SZn, 54Mn and 69Ge. In the actual measurement, spectra were taken for the four different scattererabsorber combinations possible. The observed resonance absorption effects are expressed by aGaGa -- aGazn

e=l

aznGa -- aznzn '

where axy represents the number of counts observed in a run with absorber x and scatterer y. The experimental values are exlo7 = (26.8+_2.5)~'o,

es72 = (28_+5)~o.

Under the present experimental conditions, the thickness of the scatterer may be neglected in the analysis. Provided that the intensity in the emission lines N(E) varies only slowly with energy in the vicinity of the absorption lines, the partial level width F o is related to e by 3,4) (-- l)S+ 1 (trabsd) n

.ffil

n!(n+l) ~

'

where d represents the number of 69Ga nuclei/cm 2 in the absorber, and the cross section for resonance absorption, a~bs is given by gl Fo 22 O'ab s - -

4 go n½ A~'

g o and gl are the statistical factors of the ground and excited states, respectively, ;t the wavelength and Aa ~ As the widths of the absorption line caused by the thermal motion of the nuclei in the absorber or scatterer, respectively. Using Aa = 1.06 eV for the 1107 keV and A~ = 0.84 eV for the 872 keV transition, the experimental results yield for the partial widths (gl

F/go)x 107

=

4.0+_0.4 meV,

(giF/go)a72 = 2.1+_0.4 meV. The condition that N(E) varies slowly might not be fulfilled for the 1107 keV transition, since the energy available for the electron capture to this level is 5) 1130 keV. The influence on the result for gl F/go was estimated by the assumption that the source consisted of gaseous Ge atoms. For this case, the analysis of the experimental data can be performed exactly and yields a value of (g I F/go)11o7 = 4.1 _ 0.4 meV,.which is higher by only 2 ~ than the one given above. The result for the 872 keV transition remains unchanged.

230

H. LANGHOFF AND L. FREVERT

If Wilson'and Booth 1) have excited the 1107 keV level with their bremsstrahlung, their results and the present results may be compared. Using (Fo/F)lx o7 = 0.97 and W(~)) = 1, as it results from the angular distribution measurement, the result of Wilson and Booth yields (9tF/go)11o7 = 10.8___4.0 meV. The two results differ by almost two standard deviations. The average cross section for resonance scattering ascau by a thin scatterer is given by 3) o ~ . = O~b~ _ N(E) expdE, with j'_+~ N ( E ) d E = 1 and E, the resonance energy. Provided N ( E ) is a slowly varying function of E in the vicinity of E r , the integration yields O'scat t

= o.bs~ zts ~ N(Er).

Using the results of the self-absorption and the scattering experiments, N(E~) was evaluated to be N(Er)11o7 = (1.04+0.12). 10 -2 eV -1 and N(Er)a72 = (2.3+0.5) • 10 -2 eV -1. The velocity distribution of the nuclei at the moment of the v-decay determines N ( E ) . Three assumptions leading to different velocity distributions may be considered. (i) The recoil of the nucleus from the neutrino emission is already dissipated over the whole molecule when the y-decay occurs. Then the Doppler shift imposed by the velocity of the recoiling nucleus on the emitted y-quantum is not sufficient to compensate the recoil losses of the y-quantum at the emission and the absorption; no resonance fluorescence should be observed from the two levels. (ii) The recoil energy exceeds the binding energy of the radioactive atom in the GeCI 4 molecule, and the radioactive nucleus leaves the molecule with a loss of its initial velocity. Only a very small resonance effect from the 1107 keV level should have been observed, since even a small velocity loss effects the cross section for the 1107 keV level appreciably. (iii) After electron capture, the vacancy in the K- or L-shell is filled up rapidly, and several Auger electrons are emitted. The radioactive atom becomes highly ionized, and the binding to the CI atoms weakens or even breaks before recoil energy is transmitted to them. Hence, the recoil of the neutrino is preserved approximately on the radioactive nucleus, the velocity could even be increased by the fragmentation of the molecule as has been observed by Metzger 6) for the similar molecule ZnCI 2. Assuming in view of Off) that the nucleus at the moment of the 7-decays has still the velocity received by the neutrino, N(E~) can easily be calculated. For the 1107 keV transition, N(E~) is no longer slowly varying and

N,(e)

=

® N ( E ) e x p - (e-E'] \ - - ~ / 2dE

had to be calculated in order to compare the theoretical values with the experiment.

6°Ga NUCLEAR RESONANCE FLUORESCENCE

231

The values obtained under these assumptions, N(Er)llo7 = (1.6+__0.2). 10 -2 eV -1 and N(Er)872 = 2.7 10 -2 eV -1, are only slightly higher than the experimental data. An incomplete fragmentation or collisions of the recoiling nucleus with its own CI fragments could be responsible for the observed difference. However, it might be pointed out that these numbers are rather insensitive to a small velocity gain of the radioactive nuclei by the fragmentation process. A clear indication of a velocity gain would have been the observation of resonance scattering from the 1337 keV level, since in this case the neutrino energy is not sufficient to restore the resonance condition. No resonance scattering has been observed. Because of the limited intensity of the 1337 keV quanta in the excitation radiation, the experimental upper limit for O'seatt < 10 mb is not very sensitive. The reason for the failure of this experiment might be that in the Coulomb fragmentation a relatively small amount of energy is transferred to the radioactive nucleus because of the low mass of the C1 atoms. Therefore, in a further experiment the CI atoms were replaced by Br atoms. However, also with GeBr 4 sources, no resonance scattering from the 1337 keV level was observed. The cross section for resonance scattering from the 1107 keV level with the GeBr 4 source was the same as with the GeCI 4 source, while for the 872 keV level it was smaller by 16___8 ~o (table 1, line 2). These results are consistent qualitatively with the concept that the radioactive nucleus moves with its original velocity at the moment of the y-decay except for the rather few cases where the fragmentation was incomplete or the nucleus has collided with the fragments of the molecule. The cross sections for the 1107 keV resonance are the same for both sources, since after any incomplete fragmentation or collision the nucleus has lost so much energy that resonance scattering is no longer possible. In contrast to this situation, the resonance condition for the 872 keV transition is still fulfilled for a Ga*CI fragment (however not for a Ga*CI 2) or after a Ga*-C1 collision has taken place. For the heavier bromine, this is no longer true, hence, the cross section obtained with the GeBr 4 source is smaller than with the GeCI, source. In the decay of 69Ge to the 574 keV level, the transition energy is higher than to the 1107 keV and the 872 keV levels and the influence of the CI or Br atoms should even be smaller, at least for the predominant decay mode by electron capture. By neglecting them again completely, N(Er)574 was evaluated to be N(E~)574 = 3.8" 10 -2 eV -1 assuming a E.C.//3 + branching ratio of 3.8. The cross section for resonance scattering then yields a lifetime of go'c/gl = 54-3 psec for the 574 keV level. 3. Discussion For a discussion of the results, assumptions about the statistical factors gl/go are necessary. The levels at 1107, 872 and 574 keV are fed directly in the decay of 69Ge by allowed electron capture and 13+ transitions. Since 69Ge has lo) spin and parity ~-, spin and parity of these levels should be ~}-, { - or ~-. Hence, the possible values for gt/go are 1.0, 1.5 or 2.0.

232

H. LANGHOFF AND L. FREVERT

With the crude assumption that these levels are pure vibrational states built on the ground state, the partial level widths for the E2 transitions Fvibr(E2) were calculated using the average experimental B(E2) values it) for the neighbouring doubly even nuclei of B(E2)ex = 0 . 1 5 . 1 0 -4B cm 4. The comparison of these theoretical values with the experimental results in table 10, line 10, shows that the experimental widths for the 1107 and the 872 keV levels are larger by at least a factor of 5. Therefore it seems to be very unlikely that these levels decay by pure E2 transitions but rather by mixed M 1 + E2 transitions. Hence, spin assignment ~ may be excluded, These conclusions are confirmed by the results of the angular distribution measurement. The experimental upper limits for the anisotropy are well below the anisotropy expected for an excited level with spin ~ of A 2 = 0.22 and A 4 = 0.13. The remaining spin values of ½ and ~ are compatible with the experimental results if the E2/M1 amplitude ratio J6[ < 0.55. This indicates that the 1107 keV and the 872 keV transitions have predominantly M 1 character. A strong excitation of the 1107 keV level was observed in a recent 68Zn(d, n)69Ga stripping experiment 12). It was concluded from the angular distribution of the neutrons that the proton was captured in a l p = 1 orbit. Therefore, spin ½ is the most likely assignment to the 1107 keY level leading to ~11o7 = 0.16___0.02 psec and - 0 . 4 5 < 61107 < -0.10. The experimental transition probabilities for the 872 and the 1107 keV transitions Texp are smaller by a factor of 4 compared with the predictions of the Weisskopf formula for M1 transitions T(Ml)sp, if spin factors are neglected (table 1, line 11). Similar fast M 1 transitions have been observed in the odd Cu isotopes. The high M 1 transition probability shows that these levels cannot be explained by pure core vibrations. A similar conclusion for the 1107 keV level was obtained from the results of the stripping experiment 12). The behaviour of the 574 keV transition seems to be different. Assuming pure M1 multipolarity, the transition is hindered by a factor of 18. A direct source of information on the lifetimes would be the relative cross sections for resonance scattering obtained with solid and gaseous sources, if the slowing down of the recoiling nuclei in the source was understood. In the simplest description 13.14), the recoiling nuclei move with their initial velocity vrc¢ until they collide with the neighbouring atoms after having passed a characteristic distance d. In the collision, they lose so much energy that ?-quanta emitted thereafter are no longer scattered resonantly. This description leads to aso,id/a~a~ = l--exp--(d/o~c¢T) and may be applied favorably to the 1107 keV transiticn, since v~¢ = 5.2. 105 cm/sec is just sufficient to re-establish the resonance condition. The experiment yields a collision-free distance of d11o~ = 0.38. 10 -8 cm. The result agrees with comparable data for ~52Sm [ref. 4)], d = 0.44. 10 -8 cm and for '87Re [ref. ~5)] d = 0.3 • 10 -8 cm. For the 872 keV transition, v~,¢ = 6.3 105 cm/sec exceeds considerably the velocity necessary for resonance fluorescence, v~,~ = 4.0 • 10 s cm/sec. The ?-quanta emitted after the first collision might still be scattered

69Ga NUCLEAR RESONANCE FLUORESCENCE

233

resonantly. Therefore, the large value of d872 = 3.6 g l / g o " 10-8 cm is not unexpected. Similar lengths for these situations have been reported by Ofer and Schwartzschild t4). In order to improve the description of the slowing down process, Kalus and Lades 14) considered a continuous slowing down of the recoil and assumed a time dependence for the velocity of v ( t ) = ore c cos ct. With the suitable choice of the parameter c = 1.54. l0 t 3 sec-t, the experimental energy distribution *) of the 945 keV y-quanta, which are emitted by lS2Sm embedded in Eu2Oa, is reproduced reasonably. In the present case, the ratio of the cross sections was used to determine the parameter c. The numerical evaluation yields Ctlo7 = (4.2+1.8)" 1013 sec -1, ( g l C/g0)872 =

(1.84-0.6)" 10 t a s e c - t .

Taking into account the rather large experimental uncertainties, the difference between the two values is no longer serious.

4. Investigation of the ?-spectrum The y-spectrum following the decay of 69Ge was investigated using a Ge(Li) detector with an effective thickness of 7 mm; 28 y-lines were observed. With the strong sources available, it was possible to follow the decay of these lines over several halflives. Except for the 1078 keV transition from the decay of 6aGe via 6SGa, no contaminations were observed, hence, the remaining 27 lines have to be ascribed to the decay of 69Ge. In table 2 the observed energies and intensities are compiled. From previous investigations using NaI detectors 5,7,8) and Ge(Li) detectors 9), 20 of these lines were known. The energies and intensities determined in the present investigation for these lines are in good agreement with the results of Temperley et al. 9). In addition, transitions at 419.0, 1151.0, 1407, 1450, 1487, 1724 and 2044 keV have been identified. Weak transitions of 1485 and 1727 keV also observed by Temperley et al. 9), were, however, not assigned to the decay of 69Ge. Coincidences between a 420 keV and the 1107 keV transition were reported by Schwerdtfeger et al. s), although no 420 keV transition was identified in the y-spectrum investigated with a Ge(Li) detector 9). In a further experiment, a two-dimensional coincidence analysis was performed using the Ge(Li) and a 10.2 cm x 10.2 cm NaI detector. In addition to the well-known coincidences 8'9), the less certain coincidences between y-quanta of 1107-419, 533-574, 764--574 and 1349-574 keV have been confirmed. Fig. 2 shows the decay scheme established by Temperley et al. 9). The scheme has been supplemented by the results of the present investigation. The 1450 keV transition is an additional mode to depopulate the 2025 keV level. Since the total transition energy of 69Ge is 5) 2237 keV, the new transition of 2044 keV originates from a level

234

H. LANGHOFF AND L. FREVERT

at 2044 keV. The new level de-excites by y-transitions to the ground state and presumably also to the 318 keV state by the 1724 keV transition. An alternative possibility to explain the 1724 keV transition is the assumption of a level at 1724 keV. The existence of this level is supported by the observed 1407 and 1151 keV lines as de-exciting transitions to the 318 and 872 keV levels. The weak transition of 1487 was not incorporated into the decay scheme. TABLE 2 Energies E~ and intensities I of the y-transitions initiated by the decay of

SaGe

Ey(keV) a)

b)

I a)

b)

233.8+ 1.0 318.2+0.7 419.04-1.5 511 533.04-1.0 554.44-1.0 574.14-0.7 587.44-1.0 764.14-1.0 790.04-1.0 872.44-0.7 1051.74-1.0 1107.14-0.7 1151.04-2.0 1206.94-0.7 1336.74-0.7 1349.44-1.0 1407 4-2 1450 4-2 1487 4-2 1525.74-1.0 1573.14-1.0 1724 4-2 1891.34-1.0 1924.34-1.0 2024.54-1.0 2044.04-1.5

237 4-2 320 q-2

3.3 19.1 0.6

4.24-0.3 14 4-1

511 532 4-1 553 4-1 573.44-0.5 587 4-1 763 4-1 788 ± 1 871.84-0.5 1052 4-2 1107.24-0.5

2.4 8.7 176 2.8 2.4 4.3 141 4.4 372 0.9 3.5 43 2.2 0.2 0.4 0.8 2.2 1.7 0.5 3.8 1.1 4.8 0.2

1206 4-2 1335 4-1 1349 4-2

1485 1525 1576 1727 1893 1922 2023

4-2 4-2 4-2 4-2 4-2

1000 3.24-1.1 7.6-t-0.7 166 4-1 2.74-1.0 2.1+1.1 4.84-1.1 131 4-1 4.04-0.4 372 4-3 3.54-0.5 41 4-3 3.14-0.3

<2 1.74-0.5 1.74-0.3 ~ 1 2.64-0.4 0.84-0.1 3.44-0.5

a) Present work. b) Ref. o).

The authors are indebted to Professor Dr. A. Flammersfeld for his interest and support of this investigation. They acknowledge the help of Dr. W. Sch~tt in the early stage and the assistance of Dipl. Chem. G. Elter in the chemical preparation of the sources.

References 1) H. H. Wilson and E. C. Booth, Bull. Am. Phys. Soc. 12 (1967) 75 2) H. Langhoff, L. Frevert, W. Seh~tt and A. Flaramersfeld, Nucl. Phys. 79 (1966) 145

e°Ga NUCLEAR RESONANCE FLUORESCENCE

235

3) F. R. Metzger, Progress in nuclear physics, Vol. 7, ed. by O. R. Frisch, (Pergamon, London, 1959), p. 54 4) F. R. Metzger, Phys. Rev. 137 (1965) 1415 5) C. M. Huddleston and A. B. Smith, Phys. Rev. 84 (1951) 289 6) F. R. Metzger, Phys. Rev. Lett. 18 (1967) 434 7) R. H. Nussbaum and S. K. Suri, Phys. Rev. 10'5 (1957) 1272 8) C. F. Schwerdtfeger, A. V. Ramagya and A. C. G. Mitchell, Nucl. Phys. 49 (1963) 55 9) J. K. Temperley, D. K. McDaniels and D. O. Wells, Phys. Rev. 139B (1965) 1125 10) C. M. Fou, R. W. Zurmtihle and J. M. Joyce, Nucl. Phys. A97 0967) 458 1 l) P. H. Stelson and F. K. Mc(3owan, Nucl. Phys. 32 (1962) 652 12) V. V. Okorokov, V. M. Serezhin, U. A. Smotryauo, D. L. Talchenkov, I. S. Trostein and Yu. N. Cheblukav, Soy. 3. Nucl. Phys. 4 (1967) 697 13) K. Ilakovac, Proc. Phys. Soc. A67 (19S4) 601 14) S. Ofer and A. Schwarzschild, Phys. Rev. Lett. 3 (1959) 384 15) H. Langhoff, Phys. Rev. 135 (1964) 1 16) J. Kalus and H. Lades, Z. Phys. 196 (1966) 129