Nuclear resonance fluorescence with the 105-keV E1 transition of Gd155 using an ultracentrifuge

l1rj .D.2 : 1.E.4:

s.c

Nuclear Physics 16 (1960) 81-89;

© North-Holland Publishing Co., Amsterdam

Not to b. reproduced by photoprint or microfilm without written permission from the publisher

NUCLEAR RESONANCE FLUORESCENCE WITH THE I05-keV El TRANSITION OF Gd 155 USING AN ULTRACENTRIFUGE B. 1. DEUTCH, F. R. METZGER and F. J. WILHELM t Bartol Research Foundation of the Franklin Institute, Swarthmore, Pennsylvania tt

Received 14 December 1959

Abstract: Resonance fluorescence from the 105·keV level in Gd l 6 6 was studied with the centrifuge method. Assuming a branching ratio rolr = 0.69 for this El transition to the ground state, a mean life T)' = 6.0~: X 10-10 sec was calculated from the measured resonance scattering at different source velocities. This lifetime and Tl' of the 87-keV level measured by Vergnes are consistent with the EI lifetime predictions of the Nilsson model if the 87-keV orbital is [651!] and the 105-keV orbital is [643!J at a nuclear deformation <51'>1 0.26.

1. Introduction

A simplified version of the decay scheme given by Boehm and Hatch 1) for 1.7-year EU155 is reproduced in fig. 1. According to it the 87- and l05-keV gamma rays are El transitions. Using delayed coincidences, Vergnes 2) measured the lifetime of the 87-keV level and established an upper limit for the mean life of the l05-keV state. Taking into account the branching from these levels 1) and using theoretical conversion coefficients 3), one calculates from Vergnes'

....l..----"""TT"T- 0.105

----.-.-'-++- 0.0 B7 ---,.-L+--'--If-- 0.060

i-

....l..-_--''--'----'-_ 0

Fig. 1. Simplified decay scheme of Eu16s• Energies are given in MeV.

results partial gamma ray mean lives of 't'y(87) = 1.1 X 10-8 sec and 't'y(105) S 2.3 X 10-9 sec. Vergnes indicated that, in line with the experience in other t Now with the Franklin Institute Research Laboratories, Philadelphia, Pennsylvania. tt Supported by a grant from the National Science Foundation.

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strongly deformed nuclei 4), the lifetime of the hindered 87-keV EI transition agreed within one order of magnitude with the prediction of the Nilsson model"). Since there existed some ambiguity in the assignment of the proper orbitals to the two levels, and since it appeared to be of interest to test the Nilsson model for two hindered El transitions in the same nucleus, it was decided to measure the actual lifetime of the 105-keV level in Gd155 . Of the available resonance fluorescence techniques 6) the centrifuge method 7) was chosen because it allows one to change the resonance scattering in a well understood manner and thus enables one to separate it from the background radiation. It should be pointed out here that with a gadolinium scatterer and a EU155 source at rest a resonance fluorescence effect already exists at room temperature. It would, however, be difficult to establish the existence of this effect because it would involve the precise matching of two scatterers for all non-resonant scattering processes.

2. Source Preparation The source was prepared by irradiating 99.07 % enriched Sm 154 in the form of Sm 203 t in the Oak Ridge Laboratory Low Intensity Testing Reactor. 1. 7-year EU155 was formed from the 23-minute decay of Sm155 produced by the reaction Sm 154(n, y)Sm1.55. The EU155 had to be separated from the Sm because the maximum capacity of the centrifuge tubes was about 25 milligrams of material. For this separation, an ion exchange column 8-10) with Dowex 50-Xl2, 200-400 mesh cation exchange resin and with «-hydroxy-isobutyric acid as the eluant was used. The Sm20s was dissolved in 1M Hel and added to the resin column which had been ammoniated with 0.25M ammonium oc-hydroxy-iso-butyric acid. After the elution, the activity was dried, ashed, and added to the small centrifuge tube in a rubber matrix. After the decay of the 14-day EUi 56 formed in a second order reaction, the main remaining radioactive contaminants were the long lived isotopes EU152 and EU 154 which represented approximately 1 percent of the total activity.

3. Experimental Procedures A cut-out view of the experimental apparatus is shown in fig. 2. The 5 in. diameter Al rotor of the ultracentrifuge was shaped for minimum gamma ray absorption and allowed the use of sources with up to 0.01 ml volume. GdzOa and Sm20a scatterers of Ii in. diameter and 0.080 in. thickness were used at a position 15 em from the mean position of the source. The detector, a 6 mm thick NaI(Tl) crystal mounted on an RCA 6342 photomultiplier tube, was located 9 em from the mean position of the scatterer. A -h in. copper absorber t Obtained from the Stable Isotopes Division of the U. S. Atomic Energy Commission.

NUCLEAR RESONANCE FLUORESCENCE

83

in front of the crystal preferentially attenuated the low energy portion of the scattered radiation. Gold and lead shielding, placed between source and crystal in the manner indicated, absorbed the high energy gamma rays from the EU152, 154 contaminant. Both the vacuum vessel of the centrifuge and the detector were in addition encased in lead to reduce the background counting rate. Since, for an angular distribution of the form W(6) = 1+a2 cos28, the differential cross section at a scattering angle of 125 degrees is equal to 1/47& times the total cross section, independent of the value of a2 , this angle was chosen for the experiment.

a

I

, inches

Fig. 2. A cut-out view of the experimental apparatus.

A 10 HP compressor provided the air pressure for the driving aswell as the lifting systems of the centrifuge which had been built according to a design by Beams et al. .11). Once the source in the tip of the rotor had reached the desired speed, a simple reducing valve kept the air flow constant enough to maintain the source velocity to better than ± 1 %. The angular velocity of the rotor was measured by observing with a frequency meter the signal from a pick-up coil surrounding a small magnet attached to the rotor shaft. In order to improve the signal-to-noise ratio of the scattering experiment,

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the detection equipment was gated so that counts were accepted only when the gamma rays striking the scatterer were emitted essentially in a tangential direction to the path of the source. The gate was operated by light signals reflected from a polished surface of the magnet mentioned above. The gate width corresponded to a 60 degree rotation; the duty cycle (i) was checked with a stationary source of esl3? by comparing the "gated" counting rate with the counting rate unrestricted by a gate. The correct angular position of the gate was assured by visually aligning the source and the light beam that provided the initiating gate signal.

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70

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30

20

10

122 keV

conlom rnont

20

25

30 BASELINE

35

40

45

(VOLTS)

Fig. 3. The pulse height spectrum of the direct radiation from a Eu U 5 source. The decomposition into the main gamma lines is indicated by the dashed curves.

The pulse height spectrum of the direct EUl55 radiation is shown in fig. 3. The decomposition of this spectrum into 87-, 105-, and 122-keV lines is indicated by dashed lines. The 122-keV gamma ray is the main radiation from the EUl52 contaminant. The shapes of the lines and their positions were derived from the pulse height distributions observed for the 97- and 279-keV gamma rays emitted by Cd1 07 and Hg203, respectively. The shaded area represents the

NUCLEAR RESONANCE FLUORESCENCE

86

channel accepted by the single channel analyzer in the scattering experiment. The l05-keV radiation is responsible for (77±7) % of the direct beam counting rate in this channel. Measurements were carried out with the EU165 source moving towards the scatterer at a velocity of 230 m/sec ("forward velocity"), with the source at rest, and finally with the source moving away from the scatterer at a velocity of 294 m/sec ("reverse velocity"). When the source is moving towards the scatterer, the emission line is Doppler shifted so that it overlaps the absorption line completely, thus giving rise to the maximum resonance scattering effect. With the source moving away from the scatterer, the effect of the Doppler shift is to separate the two lines and to reduce the resonance effect practically to zero. With the source at rest, the intermediate situation of partial overlap exists.

4. Results The average values of the differences between the counting rates with the Gd 2 0 3 scatterer and the Sm 2 0 3 comparison scatterer for the three experimental source velocities are given in table 1. TABLE 1 The experimental values for the difference N(v) between the counting rate NGd with the Gd 2 0 . scatterer and the counting rate NS m with the Sm, 0 3 comparison scatterer for the three velocities v used in the Gd 1 55 study.

Velocity v (m/sec)

I +230 (forward) I 0

-294 (reverse)

I

I

N(v)

Counting rate difference NGd-NS m in counts/minute

=

O.60±0.12 O.40±0.O6 O.23±O.12

The errors given are purely statistical; they are large because the resonance scattering effect represented only a small fraction of the total counting rate. With the gate in operation, typical counting rates were of the order of 17 counts per minute. The results shown in table I were obtained in three independent runs. The data of a fourth run were not included in the average because an unexplained increase in the background counting rate occurred during this run; inclusion of this fourth run in the average would have resulted in a 30 per cent increase of the value for '1:"1' Since the residual resonance effect for the reverse velocity (-294 m/sec) was expected to amount to less.than two percent ofthe maximum resonance scatter-

86

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DEUTCH, F. R. METZGER AND F.

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WILHELM

ing effect (see eq. (1)), the counting rate difference with the reverse velocity represented essentially the mismatch of the scatterers for all non-resonant effects. In plotting the experimental points in fig. 4, 0.22 counts/minute were subtracted from the counting rates given in table 1.

5. Evaluation For the centrifuge experiment the effective resonance scattering cross section may be written in the form a R

_ 211+ 1 ),2 F 2 I 1/ Mc 2 - --- -- - 0 - V e 210+ 1 4y':n; r E y 2k(Ts+Tsc )

M(EyIMc-V)1 2k(T.+T.c)

,

(1)

where 11 and 10 are the total angular momenta of the excited state and the ground state, respectively, E y is the energy of the gamma ray and), the corresponding wavelength; F ois the partial width for the direct gamma ray transition to the ground state, and r the total width of the level; M designates the mass of the nucleus, k Boltzmann's constant; T; and T ac are the effective temperatures 6,12) of the source and the scatterer, respectively; v is the relative velocity of source and scatterer, c the velocity of light. For the I05-keV transition in Gd1 55 a value Fo/F = 0.69 was calculated on the basis of the decay scheme of Boehm and Hatch 1) and the theoretical conversion coefficients of Rose 3). A Debye temperature of 250° was assumed in the calculation of the effective temperatures T« = Tac = 308°. Using eq. (1), the counting rates expected for different relative velocities v and different values of F o were calculated for the geometry of fig. 2. For this purpose the contributions of different sections of the scatterer for the subdivisions of the source path had to be summed. The electronic attenuation of the incoming and outgoing radiation was taken into account using a total cross section 13) at = SAX 10-22 cm'' per Gd atom. The counting rate versus velocity curves expected for mean lives 7:1' = nlTo of 4, 6, and 12 X 10-10 sec, are compared in fig. 4 with the experimental results. The curve calculated for "1' = 6 X 10-10 sec agrees well with the experimental points. Taking into account the statistical errors, our final value for the mean gamma lifetime is .'1' =

(6~~) X 10-10 sec.

The dashed curves in fig. 4 are those calculated for the upper and lower limits of the lifetime. It might be mentioned that the assumption of a Debye temperature of 250 was checked in a separate experiment. In this experiment, with the source at rest, the resonance scattering with source and scatterer at liquid nitrogen temperature was compared with the scattering at room temperature. From the 0

87

NUCLEAR RESONANCE FLUORESCENCE

0.6

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.'"'" '" ..J

U

::>

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0.1

200 REVERSE

VELOCITY

FORWARD

VELOCITY

(METERS/SECOND)

Fig. 4. The resonance scattering effect as a function of the velocity of the source. The heavy line represents the theoretical counting rate for 't"y = II X 10- 10 sec for the geometry used. The dashed curves are the theoretical counting rates for the upper and lower experimental lifetime limits. The experimental points are shown for comparison purposes.

absence of a marked effect of the crystalline binding 12, 14) it can be concluded that the "Debye" temperature of our source-scatterer combination must be smaller than 255°,

6. Discussion For the comparison of the experimental lifetimes with the predictions of the Nilsson model the theoretical lifetimes were calculated as a function of the nuclear deformation (j for the two ways in which the orbitals [651-1-J and [642tJ can be assigned to the 87- and 105-keV levels. The wave-functions of Nilsson 6) were used. The results of these calculations are shown in fig. 5 where the experimental ranges of lifetimes as reported in this paper (105 keY) and by Vergnes 2)

88

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DEUTCH, F. R. METZGER AND

F.

J. WILHELM

(87 keY) are also indicated t. If the calculated curves are taken at face value, the experimental lifetimes unambiguously assign the 87-keV level to the [651-1-J orbital and the 105-keV level to the [642tJ orbital. It is gratifying that for these two levels the agreement of experiment and theory occurs for the

10

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T exp, for 87 keY (VlfQnes J

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87 keY

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' - - - - -........- - - - - - ' - - - - - - ' - - - - - ' - - - - - ' 0.10 OHi 0.25 0.20 0.30

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(NUCLEAR

DEFORMATION)

Fig. 5. The mean gamma ray lifetimes for the 87- and l05-keV transitions in Gd 15 6 as a function oithe nuclear deformation 6. The solid curves represent the theoretical predictions of the Nilsson model for the most probable assignments of orbitals to the 87- and lOo-keV levels. The dashed curves correspond to the opposite assignment of orbitals. The experimental 't y for the l05-keV level has to be reduced by a factor of 1.5 if the lOo-keV orbital is [65ItJ.

same nuclear deformation 6 f'::::I 0.26, and that the assignment [521tJ for the ground state is consistent with such a deformation. The agreement between the experimental lifetimes and the predictions of the Nilsson model is impressive when one realizes that the ratio of the experimental t The lifetime for the lOo-keV transition indicated in fig. 0 is that calculated with II = t. For Jl = i the shaded area would have to be moved downwards, Le, the lifetime would be shorter by a factor of 1.5.

NUCLEAR RESONANCE FLUORESCENCE

89

lifetimes to the Weisskopf single particle values 15) for a nuclear radius R = 1.2 X10-13 A t em are R:! 2 X 103 for the 105-keV transition and ~ 2 X 104 for the 87-keV transition. We wish to thank Dr. S. Fallieros for helping us with the calculations using the Nilsson model, and for informative discussions. The generous advice of Prof. ]. W. Beams and Mr. F. Linke of the University of Virginia concerning the design of the ultracentrifuge is gratefully acknowledged.

References 1) F. Boehm and E. N. Hatch, Z. Physik 155 (1959) 609 2) M. Vergnes, Comptes Rendus 248 (1959) 1158 3) M. E. Rose, Internal Conversion Coefficients (North-Holland Publishing Co., Amsterdam, 1958) 4) M. E. Voikhanskii, JETP 33 (1958) 771 5) S. G. Nilsson, Mat. Fys. Medd. Dan. Vid. Selsk. 29, No. 16 (1955) 6) See, for instance, F. R. Metzger in Progress in Nuclear Physics, Vol. 7 (0. R. Frisch, editor) (Pergamon Press, New York, 1959) 7) P. B. Moon, Proc, Soc. A 64 (1951) 76 8) J. R. Grover (Brookhaven National Laboratory), private communication 9) R. K. Sheline (Florida State University), private communication 10) G. R. Choppin and R. J. Silva, Journ. Inorg. and Nucl. Chern. 3 (1956) 153 11) Beams, Linke and Sommer, Rev. Sci. Instr, 9 (1!J38) 248 12) W. E. Lamb, Phys. Rev. 55 (1939) 190 13) C. M. Davisson and R. D. Evans, Revs. Mod. Phys, 24 (1952) 79 14) R. Mossbauer, Z. Physik 151 (1958) 124 15) See, for instance, J. M. Blatt and V. F. Weisskopf, Theoretical Nuclear Physics (John Wiley and Sons, Iuc., New York, 1952) Chap. XII