Angular correlation measurements in 99Tc

p 1.E.3:

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Nuclear Physics 66 (1965) 545--552; ( ' ~ North-Holland Publishing Co., Amsterdam

1.E.4

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

ANGULAR C O R R E L A T I O N M E A S U R E M E N T S IN 99Tc P. D A R. A N D R A D E , ALICE MACIEL, C. S. MOLLER, J. W I R T H and F. C. Z A W I S L A K

Instituto de Fisica and Faculdade de Filosofia, Universidade do Rio Grande do Sul, P6rto Alegre, Brasil t Received 30 October 1964 Abstract: The delayed angular correlation of the °°Tc 741-181 keV gamma-gamma cascade has been measured in a liquid molybdenium source and no attenuation of the Ask(t) coefficients has been observed; As = +0.125-t-0.005 and A4 = --0.0104-0.008. A determination of the half-life of the 181 keV level has been made and the value obtained is in good agreement with e a r l i e r results, T½ = 3.59+0.05 nsec. Using the integral angular correlation method with an external magnetic field, the g factor of the 181 keV level has been measured to be g = + 1.44 -4-0.12.

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I RADIOACTIVlTY gDMo [from Mo(n, 7) ]; measured 77-delay, yy(O, H). "gTc deduced level#, l lSlHf [from Hf(n, 7)]; measured ?y(O, H). lSlTa deduced level #. Natural targets.

1. Introduction The level scheme of the nucleus 99Tc (fig. 1) has been studied by several authors 1 - 7). There are some discrepancies in the published results, mainly concerning the angular correlation measurements of the 741-181 keV cascade. The results for the g factor of the 181 keV gamma-ray reported by Raboy and Krohn 5) and by Bodenstedt et aL 6) are in good agreement but this is not the case for the angular correlation coefficients. On the other hand, the authors o f ref. 6) observed in a liquid source a time-dependent attenuation of the coefficient A2(t). In order to clarify these points, we decided to perform the series of measurements described below. In sect. 2 the equipment used is described; the results of the measurements are presented in sect. 3 and discussed in sect. 4.

2. Apparatus The delayed measurements were performed with a differential equipment consisting basically of a transistorized time-to-pulse-height converter and a 256-channel Nuclear Data Analyser. A detailed description of the apparatus has been given earlier s). The integral coincidence system is basically a "fast-slow" circuit. The detectors are two 6655-A RCA ten stage photomultipliers with NaI(T1) crystals mounted on t Work supported by the U.S. Army Research Office, by the Conselho Nacional de Pesquisas (Brasil) and by the Comiss~o Nacional de Energia Nuclear (Brasil). 545

546

P. D A R. ANDKADE e t a / .

Lucite light guides 16 cm long and 4 cm diameter. Both detectors were carefully shielded against external magnetic fields. The slow signals are derived directly from the ninth dinode of the photomultipliers to the amplifiers and single-channel analysers. At each anode, a 1000 12 load produces a fast signal which is fed, via a cathode follower, through a 200 f~ cable, to the fast coincidence circuitry. The fast coincidence circuit consists of two E180F limiter Mo~

Tc~

T~ T~

Fig. 1. Decay scheme of D°Mo, tubes, sharing a common shaping cable load to add the fast shaped pulses. After this, a fast diode discriminator, a trigger circuit and a delay multivibrator produce an output signal that has the correct timing to be fed into a triple slow coincidence with the outputs of the energy selecting channels. The resolution of the fast coincidence, which was 30 ns for the measurements described below, has been set using a proper length of C3T shaping cable. The slow coincidence has a resolution of 2 #s. The room temperature and mains voltage were stabilized so as to obtain an overall stability better than 0.5 %. 3. Measurements and Results

Several radioactive sources were obtained in the reactor of the Instituto de Energia At6rnica in S~.o Paulo, irradiating powdered MoOa. For the measurements we have

547

ANGULAR CORRELATION MEASUREMENTS

used MoO4(NH4) 2 in ammoniacal solution. The sources were prepared by attacking the MoO3 by hot concentrated N H 4 O H in excess. The ratio N H 4 O H / M o O 3 was always larger than 5. The distance between the source and the detectors was 6 era. The source containers were cylinders of 3 m m diam. × 6 m m height for the differential measurements and 5 m m diam. × 5 m m height for the measurements in the magnet.

t'

Lt 1

1

: 0

, 50

moo

150

200

ch.

Fig. 2. Curve a is the single spectrum of 9*Mo. Curve b is the coincidence spectrum with the 181 keV gamma.

3.1. SINGLE SPECTRUM, COINCIDENCE SPECTRUM AND LIFETIME The coincidence spectrum with the 181 keV gamma-ray was measured with the differential equipment referred to in sect. 2. The single spectrlJ~n and the coincidence spectrum are shown in fig. 2. With the same equipment we also measured the half life of the 181 keV level. The value obtained for the half life after a least-squares fit to the experimental points of three independent determinations is T~ = 3.595-0.05 nsec. The error sources are the following: less than 0.5 % for the stability, less than 1% for the calibration of the delay cables and 1% for the weighted least-squares fit. In table 1 our result is compared with earlier measurements.

548

P. D A R. ANDRADE et aL TABLE 1

Half life of the 181 keV level T~ nsec

Equipment

Ref.

3.5-4-0.3

integral

e)

3.574-0.05

integral

s)

3.59-4-0.05

differential

present work

3.2. DELAYED ANGULAR CORRELATION The differential angular correlation of the 741-181 keV gamma-gamma cascade was measured during five mean lives and no attenuation was found (see fig. 3). A 2 (t)

,0

~'L

0.,

!Iti........

0.05

9 = 0.05 ns

0,01

I

-6 ~ 4 - 2

0

6 8 I0 12 14 16

ns

Fig. 3. Delayed angular correlation of the 741-181 keV cascade. The A=(t) and Ao(t) coefficients are plotted against time. The resolution curve was obtained with the prompt Compton gammas of a s°Co source at energies of 181 and 741 keV. Special care was taken in order to avoid the admixture of the 741-141 keV transition with the measured cascade, using lead, copper and tantalum absorbers in the detector o f the lower energy radiation. The 141 keV transition is much stronger than the i81 keV one, and. an incorrect setting changes appreciably the angular correlation function. To obtain the minimum admixture we have measured the angular correlation of the 741-181 keV cascade with different settings, i.e. different admixtures of the two abovementioned cascades, and we have obtained experimental values for the A2(t ) coefficient in a range from 7 to 12.5 ~o. The measured angular correlation

ANGULAR CORRELATION MEASUREMENTS

549

coefficients with minimum admixture (less than 1%), corrected for the solid angle of the detectors but not for the absorption in the source material are A2 = +0.125+0.005,

A 4 =

-0.010+0.008.

These results are compared with earlier results in table 2, TABLE 2 Angular correlation coefficients for the 741-181 keV cascade A2

A4

Equipment

Ref.

+0.0674-0.004

+0.020±0.005

integral

6)

+0.1184-0.011

--0.0034-0.008

integral

e)

--0.070

integral

~)

+ 0.126

integral

2)

differential

present work

+0.1254-0.005

--0.010!0.008

3.3. N U C L E A R g FACTOR

With an external magnetic field and the liquid non-perturbed source, we also measured the g factor of the 99Tc 181 keV level by the integral rotation method. A conventional water-cooled electromagnet was used; the magnetic field strength was

1.0E

0.850"9E~

0.75

W(O,H)

~

W

(0)

0.65 I I I I I I I I I I I I I I I I i i i 9(3° 110° 130° 150° 170° 1900210° 230° 250° 270° : 8

Fig. 4. Curve a is the directional angular correlation of the 133-482 keV cascade in lS~Ta. Curves b and c are shifts of the same angular correlation pattern in an external magnetic field of 6 1004- 70 G applied in both directions normal to the plane of the detectors.

P. DA R. ANDRADEe t

550

al.

measured with a test coil and a fluxmeter. In order to test our equipment we measured the weU-known g factor o f the 482 keV level in lStTa using the integral rotation method. The source used was HfF2 dissolved in 22.5N H F and the results in fig. 4 105 W(O)

1.0C

0.95

0.90 0.85

t I 90 °

I 120 °

I 150 °

I 180 °

I 210 °

I 240 °

/ 2_0 °i 0

Fig. 5. Directional angular correlation pattern for the 741-181 keV cascade in the magnet without field. W(O,H]

/

Loo

o.95

ag.~ I

90 =

I

iZO =

I 150 °

I 180"

I 210 °

I 240 °

I I, 270 °

O

Fig. 6. Shifts of the angular correlation pattern for the 741-181 keV cascade with an external field o f 16 0004-250 G applied in both directions normal to the plane of the detectors.

give, with a measured field of 6 1 0 0 + 7 0 G, a value o f g = + 1.17+0.07 in agreement with the k n o w n g value for this level. The integral angular correlation o f the 741-181 keV cascade in 99To measured in the magnet without field is shown in fig. 5; the full curve is a least-squares fit to

ANGULAR CORRELATION MEASUREMENTS

551

the experimental points and the obtained coefficients, corrected for the solid angle of the detectors but not corrected for the absorption in the source material are .42 ---- 0.097+--0.007,

A 4 = --0.010+0.010.

These coefficients are smaller than the ones given in subseet. 3.2 because o f scattering in the magnet and absorption in the source material. The measured shifts of the angular correlation pattern in an external magnetic field o f 16 000-t-250 G applied successively in both directions normal to the plane of the detectors are shown in fig. 6. The full curves are a least-squares fit giving g = + 1.44-t-0.12, where the quoted error is composed of the errors in the measurements of the magnetic field strength (1.5 ~o), in the half life (1.5 ~o) and in the weighted least-squares fit (8.5 ~ ) . The comparison of the result obtained with other determinations is given in table 3. TABLE 3

Nuclear g factor o f the 181 keV level

g factor

Method

Perturbation

Ref.

+ 1 . 5 4-0.20

IA

n o correction factor used

B)

+1.444-0.13

IR

correction factor used G2 = 0.81

6)

+1.444-0.12

IR

G, = 1

present work

4. Discussion of the Results

Table 2 shows some discrepancies in the A 2 ( t ) angular correlation coefficient for the 741-181 keV cascade. As remarked in subsect. 3.2 we obtained for this coefficient values between 7 and 12.5 ~ by changing the admixture ratio of the 741-141 keV cascade with the measured 741-181 keV one through small setting variations in the lower energy channel. Therefore, we suspect that an incomplete separation of the 141 and 181 keV levels is the cause of these discrepancies. As can be seen in fig. 3 the value measured for A2(t) in the 741-181 keV cascade by the differential method in a liquid source ( M o O , ( N H , ) 2 in a liquid solution) does not show any attenuation. Bodenstedt et aL 4) measured the delayed angular correlation for the same cascade using a similar source and found an attenuation for A2(t). For t -- 0, the value A2(0) = 0.118+_0.011 obtained by the authors of ref. 4) is in good agreement with our result (fig. 3). The presence of the reported attenuation for this coefficient may have been caused by a different chemical composi-

552

P. DA R. ANDRADEet aL

tion and/or different temperature of the source solution. As is known, depending on the ratio NH4 O H / M o O 3 in the source solution, we m a y have various chemical compositions such as (NH4)2MoO 4 (amrrtoniulll molybdate), (N]-I4)2MoO 7 (ammonium bi-molybdate), (NH4)6Mo 7 024 (ammonium para-molybdate) and others. In the measurement of the g factor, we used the liquid non-perturbing source. Table 3 shows the very good agreement between our value and the value given in ref. 6) with a correcting factor G2 = 0.81 +0.05 for the h 2 coefficient. As suggested earlier 1, 6), our measured A 2 and A 4 values are consistent with the spin assignment ½ to the 922 keV level, assuming that the 741 keV transition is 36Yo E2. But due to the large error in A 4 we cannot rule out the spin assignment of ½ to this level. Because of the low statistics in our measured coincidence spectrum (fig. 2), it has not been possible to establish the 950 keV g a m m a ray in coincidence 6) with the 181 keV g a m m a ray. The authors thank Gerhard Jacob and Th. A. I. Maris for m a n y valuable discussions, E. R. Fraga for the preparation o f the sources and Ildon Borchard for the help in the early stage of the experiment.

References 1) C. I_~viand L. Papineau, Compt. Rend. 739 (1954) 1782

2) 3) 4) 5) 6) 7) 8)

U. Cappeller and R. Klingdh6fer, Z. Phys. 139 (1954) 402 P. Lehmann and J. Miller, Compt. Rend. 240 (1955) 1525 S. Raboy and V. E. Krohn, Bull. Am. Phys. Soc. 112 (1957) 230 S. Raboy and V. E. Krohn, Phys. Roy. 111 (1958) 579 E. Bodenstedt, E. Matthias and H. J. K6rner, Z. Phys. 153 (1959) 423 I. S. Estu]in, G. M. Chcrov and Z. N. Pastukhova, JETP (Soviet Physics) 8 (1959) 51 D. E. Brand~io et aL, Nuclear Physics 56 (1964) 65