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Parity mixing in the 482 keV transition of 181Ta
Nuclear Physics Al37 (1969) 33-50;
@
North-Holland Publishing Co., Amsterdam
Not to be reproduced by photoprint or microfilm without written permission from the publisher
PARITY MIXING
IN THE 482 keV TRANSITION
OF “ITa
E. BODENSTEDT, L. LEY, H. 0. SCHLENZ and U. WEHMANN Znstitut fdr Strahlen- und Kernphysik der Universitiit Bonn
t
Received 25 July 1969 Abstract:
The parity impurity of the 482 keV gamma transition of ialTa has been investigated by a measurement of the circular polarization. The polarimeter construction is based on the polarization dependence of the Compton scattering cross section for magnetized iron. A 4n backscattering geometry is applied in order to reach a high circular polarization detection efficiency combined with a large transmission. The scattered radiation is detected simultaneously by six large plastic scintillators, each of which is connected to a fast scaling system. Possible reasons for systematic errors are carefully investigated. Systematic asymmetries of the polarimeter were measured by use of the 497 keV gamma radiation of roJRh. One does not expect a circular polarization for this radiation. We found for the circular polarization of the 482 keV transition in rslTa the value Pr = -(2.8*0.6)
The error is the standard deviation. previous experiments. E
RADIOACTIVITY
IsiHf
* 1O-5.
Our result is compared with theoretical predictions
[from
180Hf(n, r)]; measured Enriched targets.
y circular
and
polarization.
1. Introduction Feynman and Gell-Mann ‘) developed the theory of universal weak interaction. This theory predicts a weak parity violating nucleon-nucleon interaction. The existence of this force has the consequence that the total nuclear wave function JI is a mixture of a regular and an irregular part with opposite parities:
The amplitude 9 of the irregular part is estimated for heavy nuclei to be in the range: 10m7 < 9 < 10m6 [ref. “)I. I n g amma transitions between states of mixed parities the electric and magnetic multipoles of the same order interfere. The result is a circular polarization of the gamma rays: Py=-__-__
2
1+a2
(ML)
-
with 6
=
WLf 1))
t Present address: Interatom,
Bensberg, Germany. 33
2
I+62
’
RF,
34
E.
BODENSTEDT
et cd.
The factor R contains the special nuclear structure properties. In the case of the 482 keV gamma transition in 181Ta (3’ [402] --f 3’ [404]) one has a Ml/E2 mixture with a mixing parameter “) of 6 = 5.OkO.2. A measurable polarization is expected because the regular Ml transition is hindered by a factor of 3 - lo6 compared to the Weisskopf estimate. Wahlborn “) and Maqueda and Blin-Stoyle “) have calculated the expected circular polarization by using different approaches for the parity nonconserving potential. Both results agree and give approximately P* = - 1009
Z -10-4.
McKellar “) estimated the influence of the strong interaction on the weak nucleon nucleon force. He found that the amplitude is reduced by a factor of 2.5. Therefore the circular polarization of the 482 keV gamma transition is expected to be in the order of 1o-5 < Py < 10-4. In the last years several groups ‘-‘) have reported on measurements of the circular polarization of this transition. The results are not in good agreement and therefore one has not yet a clear evidence for the existence of parity impurities. We have tried to contribute by an independent investigation with slightly different techniques to the clarification of this fundamental problem. First results have recently been reported 1“). 2. Description
of the apparatus
The differential cross section for Compton scattering of circularly polarized gamma quanta is given in first order approximation by the formula 11) do/do = 3r,2(Uko)%o
+.Py&),
(1)
where r. = classical electron radius, k, = initial photon momentum, k = photon momentum after scattering, f = fraction of oriented electrons, P,, = circular polarization of the photons and 90 = 1 + co? 9 + (k, - k)( 1 - cos 9), & = -(l -cos 9) [(k, + k)cos 9 cos $ + k sin 9 sin + cos cp)]. The meaning of the different angles is illustrated in fig. 1. Iffis the fraction of oriented electrons in a scatterer of magnetized iron and N+ (or N-) denotes the scattering rate for both directions of magnetization the relative change of the counting rate defined by E = 2N+ -NN++N-’
‘%a
is proportional
PARl’l-3’
MIXING
35
to the circular polarization: E = VP, and q = 2f#+/&,.
In order to obtain a large polarization detection efficiency q it is essential to work at angles where the ratio &/bO has the largest possible value. Obviously the formula
Fig. I. Compton scattering from polarized electrons.
Fig. 2. (6,/dlo as a function of@ and y, y = 0”; contour lines in steps of 0.08; solid lines: positive, dashed lines: negative values.
for the Compton scattering cross section gives a maximum for this ratio for the angles 9 = 180”,
$ = 0” and 180”.
Since a construction of a scattering magnet which fulfills precisely these conditions seems to be unfeasible, we have studied the ratio Q = (p,-/+0 in the whole range of the angles 9 and I). This diagram has been calculated for the special energy Ey = 482 keV and is plotted in fig. 2. The absolute maximum reaches a value of Q = + 0.7854. It is interesting to note that there exists a second maximum with a value of Q = +0.3671 for the angles J, w SO”. 9 = 70”, This second maximum is usually used in circular polarization measurements with forward scattering geometry. I I
Fig. 3. Scattering magnet: (1) scattering foils, (2) magnet coils, (3) somce, (4) sc~ti~lators, (5) light pipes, (6) tungsten cones, (7) com~nsation coils, (8) mu-metal shieidings, (9) lead shields, (IO) threefold shielded photomultip~iers.
It should be mentioned that the formula for the circular polarization sensitivity r~ applies only for single scattering. If the thickness of the scatterer is so large that multiple scattering has also to be taken into account it turns out that the sensitivity q is normally slightly enlarged. Fig. 2 shows that the central maximum is so broad that a high sensitivity can be obtained even for angles which are quite different from the ideal values. We have constructed a polarimeter in backscattering geometry where we worked in the range of
‘*ITa
PARITY
MIXING
37
angles indicated in fig. 2 by the shadowed area. The design of the scattering magnet is shown in fig. 3. The whole magnet has rotational symmetry. In a backscattering geometry the radioactive source and the detectors for the backscattered radiation should be as close as possible. The minimum possible distance is limited by the dimensions of the absorber which is necessary to shield the detectors against direct radiation. We obtained a sufficient shielding by tungsten cones with a thickness of 9 cm. Plastic scintillators were used for the detection of the scattered radiation because they allow higher counting rates than sodium iodide detectors. Three plastic detectors are mounted on both sides thus allowing six independent measurements simultaneously. The three scattering foils on both sides of the magnet have a thickness of 3.5 mm and are magnetized separately. High quality armco iron of the type S3 from the “Vakuumschmelze Hanau” has been used for the scattering foils. The completely closed iron yokes were also made from armco iron (type ZSH from “Rheinstahl AG”). The space between the coils and the scattering foils is completely filled with lead in order to absorb the radiation which is not scattered in the foil. Both halves of the magnet are polarized in opposite directions. This keeps the stray field within the volume of the radioactive source smaller than 1 mG. The plastic detectors are connected to photomultiplier tubes of the type 56 AVP by plexi glass light pipes of about 2 m length. A threefold magnetic shield around the photomultiplier tubes reduces the stray field of the magnet at the position of the photomultiplier cathode to less than 10m6 G. Fast electronics (100 MHz) was used for the counting assembly. Because of the small energy of the Compton backscattered radiation 170 keV-200 keV and because of light losses in the 2 meter light pipes the photomultiplier pulses are rather small. We concluded from an analysis of the pulse-height spectrum that the pulse-height corresponds on the average to 1-2 photoelectrons. We discriminated against the dynode noise by using fast discriminators (type T 101 from EG & G). The output pulses of the fast discriminators were counted by fast scalers (100 MHz scaler from Borer and Co.). We worked usually with counting rates between lo6 and 10’ pulses per second for each detector. Due to this high counting rate the statistical accuracy reached in a counting period of 10 set is already better than 0.3 per mille. In order to eliminate influences of possible drifts of the electronics on the measurement of e.,it is necessary to change the direction of magnetization in very short intervals. We found that a 10 set interval is just sufficient. As a read out of the six counting rates at these short intervals would produce an unacceptably high dead time we accumulated the data for both field directions in two separate scalers for each of the six channels. In order to avoid counting asymmetries of the two scalers the primary counting rate was firstly reduced by 100 : 1 through two fast decades (fig. 4). The counting periods were controlled by fast timing circuits which are driven by a quartz stabilized 100 MHz master oscillator. We used a pause of 1 set after the reversion of the voltage of the magnet power supply. We have checked that this time interval is sufficient to build up the full magnetization after the reversion of the direction. This short remagnetization
38
E. BODENSTEDT
et al.
time could be reached only by using a large voftage peak in the moment of reversion. After 50 magnetization cycles the data were punched on paper tape and also printed out by an electric typewriter. The next run was always started with changed magnetization direction. From the sequence of counting rates obtained in this way N:, N;,
N,-, N,f , N;, N;, . . .,
we calculated the relative change E by the formula:
This formula corrects already in second order approximation for long time drifts and radioactive decay. Groups of subsequent 400 values of .awere used to calculate a mean value and a rms deviation. It turned out that the latter exceeded the statistical error by roughly 25 “/ We included in the computer program also a calculation of the frequency distribution of the E values. These frequency distributions were used to eliminate single extruding points. scaler
Fig. 4. Block diagram of a counting circuit.
3. Test measurements The performance of the complete experimental set up was checked by a number of test measurements. These tests concerned the following properties of.the apparatus: (i) Since the expected circular polarization is extremely small it is essential that intrinsic asymmetries of the apparatus which produce systematically different counting rates for both directions of magnetization are kept as small as possible. Therefore we investigated possible sources of asymmetries for the different parts of the apparatus. Apparently the accuracy of the information obtained is limited by the counting statistics. It was not possible to exclude systematic asymmetries of the apparatus by these test measurements with sufficient accuracy. The final measurement of the circular polarization of the 482 keV transition of lslTa was therefore combined with the
l*%a
PARlTY
MIXING
39
measurement of the asymmetry ratio for a gamma line of another isotope with about the same energy where one is sure that no circular polarization should exist. (ii) For the final evaluation of the circular polarization it is necessary to know the polarization sensitivity of the polarimeter. We calculated the polarization sensitivity by Monte Carlo calculations taking into account the geometry of our polarimeter and we also have tried to measure the polarization sensitivity by using the circular polarized internal bremsstrahlung of a 32P source. (iii) We have determined the transmission CLof the polarimeter. It is important to obtain a rather high transmission. Otherwise it is necessary to apply inconveniently strong sources in order to use the full counting capacity of the system. We start with the discussion of the asymmetry tests. Asymmetries can be produced by the scaler system, by the photomultiplier tubes, by the scintillators, and finally by the scattering magnet itself. The scaler system was tested by putting a sufficiently strong source into the magnet. Then the asymmetry ratio was measured in the normal mode of operation but without exciting the scattering magnet. After a continuous run of four weeks no measurable asymmetry was observed in any of the six scaler systems, the average of the asymmetry ratic being E = (O.O&7.4) - lo- ‘* The photomultiplier tubes are known to be very sensitive to magnetic fields. We estimated from data which were published by the producer on the magnetic sensitivity of the tubes that our shielding should be sufficient. We performed a test run in which again the magnet was not powered but a magnetic field of twice the strength of the magnet stray field was produced at the position of the photomultiplier assembly by means of a big coil. The current in the coil was reversed every ten seconds. In a continuous run of three weeks no detectable asymmetries were observed, the average for the six scalers being E = (4&7)* lo-‘. Large asymmetries are easily produced by the scintillators. We used plastic scintillators of the material Naton 136 and found that the light output of this scintillator material is influenced by magnetic fields. Already weak magnetic fields enlarge the light output and therefore increase the counting rate. We investigated this property by using a radioactive source of “’ Hf mounted in the normal position in the polarimeter magnet. The scattering magnet was switched off and a small magnetic field was produced at the position of the scintillators by use of the compensation coils (7) shown in fig. 3. Fig. 5 shows the result of a measurement of the relative change of the counting rate as a function of the magnetic field. It is surprising to see that the highest sensitivity exists at the magnetic field zero. Here the counting rate increases by about 1.5°/00 if the magnetic field increases by 1 G. The magnetic field dependence of this effect is nearly the same for all six detectors and shows that saturation is
40
E. BODENSTBDT
f?t al.
reached at a magnetic field strength of about 100 G. We do not yet understand the mechanism of this interesting phenomenon. BN
15
I t
N,AH
I lo-‘/lnOe )
RELATIVE
CHANGE OF COUNTING RATE
Fig. 5. Relative change of counting rate as a function of the magnetic field at the scintillator position (errors calculated from the values for the six detectors).
Fig. 6. Measured asymmetry E as a function of the magnetic field, no earth field compensation.
The magnet produces a stray field of 50 G at the position of the scintillators. We reduced this field by compensation coils to about 1 or 2 G. No effect on the counting rate should happen if this remaining field has the same strength for both directions of magnetization. The symmetry is disturbed, however, by the earth magnetic field
“‘Ta
PARITY
MIXINO
41
which remains unaltered when the magnetization of the magnet is reversed. We observed, indeed,. a measurable change in the counting rate when the direction of the magnetic field at the position of the scintillators was reversed. In this test measurement the magnet was switched off and the field at the position of the scintillators was produced only by the compensation coils. The observed asymmetry in the counting rate as a function of the strength of the magnetic field is plotted in fig. 6. The measured curve exhibits the expected shape. The largest asymmetry factors are observed for the smallest field strength. We have therefore compensated the earth magnetic field carefully by use of three pairs of large Hefmholtz coils. A new run proved that the asymmetry was produced mainly by the earth magnetic field. The field compensation reduced the asy~etry by roughly two orders of magnitude. There are two possibilities for a further reduction of this asymmetry: (i) The stray field of the magnet is not compensated by the compensation coils, but instead enlarged to about 100 G by driving the compensation coils in the wrong direction. Now one should have reached nearly the saturation where the detectors become insensitive against the magnetic field. (ii) One uses the compensation coils in the right direction and uses in addition a mu-metal shield around the scintillators. In this way we could reduce the absolute magnetic field at the position of the scintiliators to < 0.1 G and differences in the absolute magnetic field for both directions of magnetization to less than 0.2 mG. In the final measurement of the circufar polarization of the 482 keV line of lslTa we have applied both methods. The long time test runs gave a slightly better symmetry for the second method. There are different possible sources of asymmetries produced by the scattering magnet: By inhomogeneities in the iron and by remanent magnetization a small difference in the factorfcan be produced for both directions of magnetization. At least in first order approximation this should not produce differences in the cross section for the Compton scattering process, if the gamma radiation is not circularly polarized (see eq. 1). An asymmetry could be produced, however, by differences in the stray magnetic field at the position of the scintillators. A second possibility comes from mechanical motions caused by magnetic forces. Even very small asymmetric motions would change the solid angles for the Compton scattering process sufficiently large to produce measurable asymmetries in the counting rates. We tried to avoid effects of this kind by the very compact magnet construction. Similar effects could be produced by magnetostriction. But also in this case asymmetries can be caused only by differences in the magnitude of the magnetostriction for both field directions. In order to keep asymmetries in the magnetization as small as possible we used a highly stabilized power supply for the magnet. One of the most dangerous sources for systematic errors is probably the so called
42
E. BODENSTEDT
t?f al.
right-left asymmetry term in the Compton scattering formula. Because of the importance of this effect a short description is given in the foliowing. The right-left asymmetry in Compton scattering of unpolarized gamma radiation by transversally polarized electrons has been calculated 1961 by Miller and Wilcox i2). The effect results from a radiative correction to the Klein-Nishina formula. The amplitude of this spin dependent additive term to the Klein-Nishina cross section exhibits a strong energy dependence. The absolute magnitude of the spin dependent correction term is rather small. At the maximum it reaches a value of 6 * 10m4 of the normal Klein-Nishina cross section according to the calculation of Miller and Wilcox. This term is large enough, however, to produce serious disturbances if circular polarizations in the order of 10-s are to be measured. The spin dependent correction term has the form
where & = Compton wavelength, ;1 = wavelength of incident radiation, s = electron spin, ui, u2 = unit vectors in the directions of the momenta of incoming and scattered quanta, respectively, and u = fine structure constant. One obtains a change in the absolute Compton cross section for a reversal of the magnetization of the scattering foil and therefore a reversal of the vector s as soon as the magnetization vector does not coincide with the scattering plane defined by the incoming and outgoing photon. Scattering events of this type are always admitted in polarimeters for circular polarization of gamma rays because of the &rite size of the source and especially of the detector for the scattered radiation. In the case of axial symmetry the correction term vanishes on the average because both signs occur with the same probability. Slight misalignments of the source and the detector cause, however, asymmetries which produce finite values for the asymmetry ratio even for unpolarized gamma radiation. Our scattering magnet was carefully machined in order to obtain a maximum possible axial symmetry and our source was mounted carefully on the axes. The above mentioned strong energy dependence of this effect makes the comparison measurements only relevant if gamma rays of the same energy are used. A determination of the circular polarization sensitivity of the scattering magnet by direct measurement is difficult. We therefore preferred to calculate the sensitivity rIn order to make this calculation reliable a few test measurements were necessary. An average value for the fraction of oriented electronsf of the scattering foils was determined by measurements of the magnetization of the scattering foils at different radii. For this purpose small induction coils were mounted into the scattering foils at different positions and the induction signal was measured when the magnet was powered. Because of the conical shape of the scattering foils the saturation magnetization is only reached at the inner part of the foil andf decreases slightly with increasing radius. A second test concerned possible backscattering from the lead behind the scattering
=‘Ta
PARITY
MIXING
43
foils. For this purpose we used a well collimated gamma beam of a strong 13’Cs source. We simulated the scattering foil by a thin iron plate of the same thickness and in the same geometry and measured the spectrum of the backscattered radiation. Then we investigated how this spectrum changed when we put a thick piece of lead behind the scatterer. Measurements of this type gave the result that the background of radiation which is scattered from the lead can be neglected. A third test concerned possible backscattering effects from the central lead shield. This effect should be very sensitive to small shifts of the source in the axial direction. A measurement gave no detectable effect, thus indicating that the contribution of scattered radiation from the central lead shield is negligibly small. The final calculation of the circular polari~tion efiiciency:
was performed by averaging over the different angles for the geometry of our magnet. This was done by a Monte Carlo calculation which took into account also the absorption of the radiation in the scattering foil, The result of this calculation was rj = -4.8 %. This value was used for the calculation of our preliminary result. A more refined calculation which took into account also double scattering effects gave a slightly larger sensitivity: q = -6.25 “/,* These Monte Carlo calculations have been extended to other energies and they have also been used to determine the transmission a of the polarimeter. The results obtained for q and Mas a function of the gamma energy are plotted in fig. 8. We have performed a rough test for the polarization sensitivity by measuring the circular polarization of the internal and external bremsstrahlung of 32P. Theintensities of the IB- and EB-spectra were calculated according to refs. I39i “). These calculations show good agreement with experimental data of refs. ’ 5, ’ “). Theoretical values for the polarization of IB and EB are given in refs. I391‘). The active material was put into an aluminum capsufe of 4 mm wall thickness which stops the p-radiation. The capsule was surrounded by lead shields of different thicknesses (1; 3; 5 and 7 mm). The mean energy and the mean polarization of the bremsstrahlung spectra must increase with increasing lead thickness and one therefore expects that the measured asymmetry ratio should change in the same way. We measured the energy spectrum emitted from our source with a 7.6 x 7.6 cm NaI(TI) crystal. Fig. 7 shows the measured spectrum for a lead shield of 1 mm thickness. In the same figure the spectrum is unfolded and finally also the theoretical shape of the spectrum is inserted. There is only a discrepancy at low energies which can be explained by build up effects in the lead shield which increase the number of low-
44
B. BODENSTEDT et 01.
4
counting rote
=P
5.103,,
Bfe~stf~ with
lmm lead shietd
experimental spectrum
experimental spectr
ElkeV)
Fig. 7. Pulse-height
spectrum
of 32P bremsstrahlung
with 1 mm lead shield around the source.
tt?
I
-6 0 ,E $” -4 P x -3 ; r2 1
i
ldill
400
2000
E, LkeVl Fig. 8. Theoretical
polarization
sensitivity q and transmission of y-energy.
Q:of our polarimeter
as a function
45
'*'Ta PARITY MIXING
energy quanta. The experimental spectrum and the theoretical energy dependence of the polarization were used together with our calculated polarization sensitivity rl to calculate the expected counting rate asymmetry. These values are compared with the experimental results in table 1. The errors of the theoretical values are due to uncertainties produced by the bad knowledge of the build up effects. The agreement between experiment and calculation is satisfactory thus indicating that the calculated polarization sensitivity must be fairly correct. TABLE
Counting rate asymmetries lmm
1
for 32P bre~tr~lung
3mm
[o/o0]
5mm
7mm
*lb
6.6k1.3
8.111.6
9.5h2.0
10.7+2.1
%XP
7.0f0.2
8.3 kO.3
8.510.5
8.6f0.6
Finally we have checked the calculated transmission a = 1.8 % of the polarimeter by using a islTa source of known activity. The measured counting rate agreed fairly well with the prediction using the above calculated transmission factors. 4, Radioactive sources The larHf sources of a strength between 4 Ci and 8 Ci were produced in the following way. The compound HfOZ with an enrichment of 92 % ‘*‘Hf was irradiated in a flux of 1014 neutrons/cm’ f set in the Kernforschungszentrum Karlsruhe for several
lcm
Fig. 9. Source assembIy; (a) radioactive
material,
(b) aluminum,
(c) Iead.
weeks. A weak beta transition of 550 keV maximum energy in the decay of 18iHf can perturb the circular polarization measurement by the contribution of its bremsstrahlung. In order to keep this bremsstrahlung background small the hafnium oxide was mixed with spectroscopic pure graphite powder before the irradiation in order to decrease the effective nuclear charge of the source (400 mg graphite were mixed with 200 mg HfOJ. The source material was pressed into a cylindrical capsule of very pure aIuminum with 1.5 mm wall thickness and finally surrounded by a 9.5 mm thick lead shield, The construction of the source is shown in fig. 9. The main purpose of the lead shield is a reduction of the bremsstrahlung background. In addition it has the effect
46
E. BODENSTEDT
er
al.
to suppress the background of low-energy gamma lines of l”Ta. The expected counting rate asymmetry factor which is produced only by the circular polarization of the bremsstrahlung was calculated for different lead thicknesses, for two different effective atomic numbers, and for the upper and lower limit of the intensity of the 550 keV “t
effect
of
bremsstrahlung
i_.__ErnI 0 1 2 3 4 5 6 7 8 9 -: ----
1(408):93’/. : I (408)=98’/.
, 1(550)=5’/. , 1(550):5%.
lead
tzckness
, Zelf=60 , Z,ff=20
Fig. 10. Calculated asymmetry effect of bremsstrahlung of lslHf around the source.
as a function of lead thickness
beta-radiation. The result of this calculation is shown in fig. 10. Thus we expect for our source a bremsstrahlung background in the measured asymmetry ratio of only: E x 10-7. Among the other hafnium isotopes which are contained in our target material only 174Hf gives rise to a disturbing activity because of its high-activation cross section. The intensity ratio of the 343 keV transition in 17$Lu to the interesting 482 keV transition in “‘Ta is estimated to be roughly 1 : 300. Therefore a perturbation by this line can be neglected. The test measurements were done by use of the 497 keV transition in lo3Rh. The lo3Ru source was obtained by neutron irradiation of lozRu and the sources were surrounded by a 5 mm lead shield in order to keep the contribution of the bremsstrahlung of the weak 722 keV beta transition negligibly small.
-5.
Nl
N3
N2
2N,-K,jO6 N++K
N+-N-
2N.x
Kf
10 -
Nl
N2
N3
Fig. t f . a) Measured counting rate asymmetries for lsiTa (run III) for all six detectors; b) measured counting rate asymmetries for lo3Rh (run III) for all six detectors, the last point in each series is the mean value.
E. BODENSTBDT et al.
5. Polarization
measurements
We have performed four runs of polarization measurements with ‘sITa for about four weeks each. All these runs were followed by control measurements with lo3Rh. In the first run the compensation coils were powered in the wrong direction thus producing an alternating magnetic field at the position of the scintillators of about 100 G. In the other three runs the compensation coils were powered in the right direction and the stray field was shielded in addition by a mu-metal cap as described in sect. 3. Figs. 1laand 1lb show as an example the results of the 3rd run. Each point corresponds to a counting time of six days and contains about 400 E-values calculated as described in sect. 2. For a source strength of 5 Cur we obtained a statistical counting rate in each scaler of about 8 * lo6 sec- I. In the evaluation a dead time correction was included. TABLE 2 Measured asymmetry ratios E = ~~~-.q+~[lO-~] Run no.
Detector 1
II
Sl s2 s3 Nl N2 N3
0.7111.82 7.99f1.95 3.44kl.65 2.85h2.00 4.04kl.70 3.89rt1.70
1.6251.08 0.56k1.16 1.43+1.00 4.3111.3s -0.07+1.02 -0.35*1.01
weighted mean of the run
3.7Sf0.74
1.02*0.43
III
IV
3.2Okl.22 2.02f1.41 1.59~1.10 -0.90*1.30 0.1611.11 0.2111.13 1.6SkO.49
Weighted mean of the scaler
2.67kl.14 3.2Okl.34 1.88k1.06 0.41hl.15 1.23f1.05 1.49*1.08
2.23 &to.62 2.57+0.69 1.84+0.57 1.3310.68 0.8SrtO.58 0.8210.58
1.75 +0.43
1.5510.33 weighted mean of the 24 single results
It is noticed from fig. 1lb that the mean values obtained in the control measurement for the six independent counting circuits deviate from zero in different directions and with different magnitudes. We do not know the true reason for this apparative asymmetries. Most probably they are produced by stray magnetic field effects or by the right-left asymmetry of the cross section for Compton scattering. It must be mentioned that all error intervals are rms deviations and not statistical errors. The differences
for the four runs together with their rms deviations are shown in table 2 separately for all six circuits. A plot of this data is given in ref. lo). The mean value of the 24 independent measurements is & = (1.55f0.33)
* 10-6.
rsiTa
PARITY
MIXING
49
The error is again the rms deviation and exceeds the error expected from error propagation by a factor of 1.3. In the calculation of the circular polarization we included a small correction for a background of the low-energy gamma transitions in the decay of ‘*‘Ta and obtain finally for the circular polarization of the 482 keV transition: Py = -(2.81:0.6)
- lo-‘.
6. Comparison with theory and previous experiments A comparison of our result with the different theoretical predictions gives an especially good agreement with a calculation of the expected circular polarization by McKellar “) who obtained Py = -(2_tl)* 10-5. Our measurement is not in agreement with the most precise results of other groups: Py = -(0.67+0.07)
- 1O-5
and P, = -(0.39+0.18)* 10-5, obtained by Lobashov ‘*) and by Vanderleeden et al. “), respectively. The reason for this discrepancy is not yet understood. Possible systematic errors might be introduced by underestimating the bremsstrahlung background or by performing a comparison experiment with a different gamma energy as in this case the right-left asymmetry is not eliminated.? The weakest point of our measurement is a small long time drift which was indicated by the fact that the rms deviation of the mean value of the asymmetry ratio E exceeds the error expected from error propagation by a factor of 1.3. In order to eliminate this uncertainty we have started a new experiment where the ‘*rHf source and the lo3Ru comparison source are changed in shorter intervals. For this purpose we constructed a sluice way which allows easily to remove the sources without interrupting the operation of the magnet. We hope to certify in this way that the hafnium source as well as the comparison source find the polarimeter in exactly the same working conditions. This work was supported by the Bundesministerium fur wissenschaftliche Forschung. The numerical calculations were performed on the IBM 7090 computer of the Gesellschaft fur Mathematik und Datenverarbeitung in Bonn. + Note added in proofi A recent measurement by H. Diehl, G. Hopfensitz, E. Kankeleit and E. Kuphal gave I’?(482 keV) = -(1.3f0.7) . lo-$ which is only slightly smaller than our value. We are grateful to the authors for this information before publication.
50
E. BODENSTEDT
et al.
References I) R. P. Feynman and M. Gell-Mamm, Phys. Rev. 109(1958) 193 2) 3) 4) 5) 6) 7)
F. C. Miohel, Phys. Rev. 133 (1964) B329 B. Richter, Diplomarbeit, Bonn 1969 S. Wahlborn, Phys. Rev. 138 (1965) B530 E. Maqueda and R. J. Blin-Stoyle, Nucl. Phys. A91 (1967) 460 B. H. J. McKellar, Phys. Rev. Lett. 20 (1968) 1542 F. Boehm and E. Kankeleit. Nucl. Phys. A109 (1968) 457 8) V. M. Lobashov, V. A. Naxarenko, L. F. Saenko, L. M. Smotritskii, and G. I. Kharkevich, Phys. Lett. 258 (1967) 104 9) D. W. Cruse and W. D. Hamilton, Nucl. Phys. Al25 (1969) 241 10) E. Bodenstedt, L. Ley, H. 0. Schlenx and U. Wehmann, Phys. Lett. 29B (1969) 165 11) H. Schopper, Nucl. Instr. 3 (1958) 158 12) S. C. Miller and R. M. Wilcox, Phys. Rev. 124 (1961) 637 13) G. Felsner, Z. Phys. 174 (1963) 43 14) S. J. Wyard, Proc. Phys. Sot. A65 (1952) 377 15) M. A. Hakem and M. Goodrich, Nucl. Phys. 31 (1962) 322 16) M. Goodrich, J. S. Levinger, and W. Payne, Phys. Rev. 91 (1953) 1225 17) C. Fronsdal and H. Uberall, Phys. Rev. 111 (1958) 580 18) V. M. Lobashov, V. A. Naxarenko, L. F. Saenko, L. M. Smotritskii, and G. I. Kharkevich, Proc. Conf. electron capture and high order processes in nuclear decays, Debrecen 1968, E&v& Lorand Phys. Sot. (Budapest 1968) Vol. III 19) J. C. Vanderleeden and F. Boehm, Bull. Am. Phys. Sot. 14 (1969) 587
@
North-Holland Publishing Co., Amsterdam
Not to be reproduced by photoprint or microfilm without written permission from the publisher
PARITY MIXING
IN THE 482 keV TRANSITION
OF “ITa
E. BODENSTEDT, L. LEY, H. 0. SCHLENZ and U. WEHMANN Znstitut fdr Strahlen- und Kernphysik der Universitiit Bonn
t
Received 25 July 1969 Abstract:
The parity impurity of the 482 keV gamma transition of ialTa has been investigated by a measurement of the circular polarization. The polarimeter construction is based on the polarization dependence of the Compton scattering cross section for magnetized iron. A 4n backscattering geometry is applied in order to reach a high circular polarization detection efficiency combined with a large transmission. The scattered radiation is detected simultaneously by six large plastic scintillators, each of which is connected to a fast scaling system. Possible reasons for systematic errors are carefully investigated. Systematic asymmetries of the polarimeter were measured by use of the 497 keV gamma radiation of roJRh. One does not expect a circular polarization for this radiation. We found for the circular polarization of the 482 keV transition in rslTa the value Pr = -(2.8*0.6)
The error is the standard deviation. previous experiments. E
RADIOACTIVITY
IsiHf
* 1O-5.
Our result is compared with theoretical predictions
[from
180Hf(n, r)]; measured Enriched targets.
y circular
and
polarization.
1. Introduction Feynman and Gell-Mann ‘) developed the theory of universal weak interaction. This theory predicts a weak parity violating nucleon-nucleon interaction. The existence of this force has the consequence that the total nuclear wave function JI is a mixture of a regular and an irregular part with opposite parities:
The amplitude 9 of the irregular part is estimated for heavy nuclei to be in the range: 10m7 < 9 < 10m6 [ref. “)I. I n g amma transitions between states of mixed parities the electric and magnetic multipoles of the same order interfere. The result is a circular polarization of the gamma rays: Py=-__-__
2
1+a2
(ML)
-
with 6
=
WLf 1))
t Present address: Interatom,
Bensberg, Germany. 33
2
I+62
’
RF,
34
E.
BODENSTEDT
et cd.
The factor R contains the special nuclear structure properties. In the case of the 482 keV gamma transition in 181Ta (3’ [402] --f 3’ [404]) one has a Ml/E2 mixture with a mixing parameter “) of 6 = 5.OkO.2. A measurable polarization is expected because the regular Ml transition is hindered by a factor of 3 - lo6 compared to the Weisskopf estimate. Wahlborn “) and Maqueda and Blin-Stoyle “) have calculated the expected circular polarization by using different approaches for the parity nonconserving potential. Both results agree and give approximately P* = - 1009
Z -10-4.
McKellar “) estimated the influence of the strong interaction on the weak nucleon nucleon force. He found that the amplitude is reduced by a factor of 2.5. Therefore the circular polarization of the 482 keV gamma transition is expected to be in the order of 1o-5 < Py < 10-4. In the last years several groups ‘-‘) have reported on measurements of the circular polarization of this transition. The results are not in good agreement and therefore one has not yet a clear evidence for the existence of parity impurities. We have tried to contribute by an independent investigation with slightly different techniques to the clarification of this fundamental problem. First results have recently been reported 1“). 2. Description
of the apparatus
The differential cross section for Compton scattering of circularly polarized gamma quanta is given in first order approximation by the formula 11) do/do = 3r,2(Uko)%o
+.Py&),
(1)
where r. = classical electron radius, k, = initial photon momentum, k = photon momentum after scattering, f = fraction of oriented electrons, P,, = circular polarization of the photons and 90 = 1 + co? 9 + (k, - k)( 1 - cos 9), & = -(l -cos 9) [(k, + k)cos 9 cos $ + k sin 9 sin + cos cp)]. The meaning of the different angles is illustrated in fig. 1. Iffis the fraction of oriented electrons in a scatterer of magnetized iron and N+ (or N-) denotes the scattering rate for both directions of magnetization the relative change of the counting rate defined by E = 2N+ -NN++N-’
‘%a
is proportional
PARl’l-3’
MIXING
35
to the circular polarization: E = VP, and q = 2f#+/&,.
In order to obtain a large polarization detection efficiency q it is essential to work at angles where the ratio &/bO has the largest possible value. Obviously the formula
Fig. I. Compton scattering from polarized electrons.
Fig. 2. (6,/dlo as a function of@ and y, y = 0”; contour lines in steps of 0.08; solid lines: positive, dashed lines: negative values.
for the Compton scattering cross section gives a maximum for this ratio for the angles 9 = 180”,
$ = 0” and 180”.
Since a construction of a scattering magnet which fulfills precisely these conditions seems to be unfeasible, we have studied the ratio Q = (p,-/+0 in the whole range of the angles 9 and I). This diagram has been calculated for the special energy Ey = 482 keV and is plotted in fig. 2. The absolute maximum reaches a value of Q = + 0.7854. It is interesting to note that there exists a second maximum with a value of Q = +0.3671 for the angles J, w SO”. 9 = 70”, This second maximum is usually used in circular polarization measurements with forward scattering geometry. I I
Fig. 3. Scattering magnet: (1) scattering foils, (2) magnet coils, (3) somce, (4) sc~ti~lators, (5) light pipes, (6) tungsten cones, (7) com~nsation coils, (8) mu-metal shieidings, (9) lead shields, (IO) threefold shielded photomultip~iers.
It should be mentioned that the formula for the circular polarization sensitivity r~ applies only for single scattering. If the thickness of the scatterer is so large that multiple scattering has also to be taken into account it turns out that the sensitivity q is normally slightly enlarged. Fig. 2 shows that the central maximum is so broad that a high sensitivity can be obtained even for angles which are quite different from the ideal values. We have constructed a polarimeter in backscattering geometry where we worked in the range of
‘*ITa
PARITY
MIXING
37
angles indicated in fig. 2 by the shadowed area. The design of the scattering magnet is shown in fig. 3. The whole magnet has rotational symmetry. In a backscattering geometry the radioactive source and the detectors for the backscattered radiation should be as close as possible. The minimum possible distance is limited by the dimensions of the absorber which is necessary to shield the detectors against direct radiation. We obtained a sufficient shielding by tungsten cones with a thickness of 9 cm. Plastic scintillators were used for the detection of the scattered radiation because they allow higher counting rates than sodium iodide detectors. Three plastic detectors are mounted on both sides thus allowing six independent measurements simultaneously. The three scattering foils on both sides of the magnet have a thickness of 3.5 mm and are magnetized separately. High quality armco iron of the type S3 from the “Vakuumschmelze Hanau” has been used for the scattering foils. The completely closed iron yokes were also made from armco iron (type ZSH from “Rheinstahl AG”). The space between the coils and the scattering foils is completely filled with lead in order to absorb the radiation which is not scattered in the foil. Both halves of the magnet are polarized in opposite directions. This keeps the stray field within the volume of the radioactive source smaller than 1 mG. The plastic detectors are connected to photomultiplier tubes of the type 56 AVP by plexi glass light pipes of about 2 m length. A threefold magnetic shield around the photomultiplier tubes reduces the stray field of the magnet at the position of the photomultiplier cathode to less than 10m6 G. Fast electronics (100 MHz) was used for the counting assembly. Because of the small energy of the Compton backscattered radiation 170 keV-200 keV and because of light losses in the 2 meter light pipes the photomultiplier pulses are rather small. We concluded from an analysis of the pulse-height spectrum that the pulse-height corresponds on the average to 1-2 photoelectrons. We discriminated against the dynode noise by using fast discriminators (type T 101 from EG & G). The output pulses of the fast discriminators were counted by fast scalers (100 MHz scaler from Borer and Co.). We worked usually with counting rates between lo6 and 10’ pulses per second for each detector. Due to this high counting rate the statistical accuracy reached in a counting period of 10 set is already better than 0.3 per mille. In order to eliminate influences of possible drifts of the electronics on the measurement of e.,it is necessary to change the direction of magnetization in very short intervals. We found that a 10 set interval is just sufficient. As a read out of the six counting rates at these short intervals would produce an unacceptably high dead time we accumulated the data for both field directions in two separate scalers for each of the six channels. In order to avoid counting asymmetries of the two scalers the primary counting rate was firstly reduced by 100 : 1 through two fast decades (fig. 4). The counting periods were controlled by fast timing circuits which are driven by a quartz stabilized 100 MHz master oscillator. We used a pause of 1 set after the reversion of the voltage of the magnet power supply. We have checked that this time interval is sufficient to build up the full magnetization after the reversion of the direction. This short remagnetization
38
E. BODENSTEDT
et al.
time could be reached only by using a large voftage peak in the moment of reversion. After 50 magnetization cycles the data were punched on paper tape and also printed out by an electric typewriter. The next run was always started with changed magnetization direction. From the sequence of counting rates obtained in this way N:, N;,
N,-, N,f , N;, N;, . . .,
we calculated the relative change E by the formula:
This formula corrects already in second order approximation for long time drifts and radioactive decay. Groups of subsequent 400 values of .awere used to calculate a mean value and a rms deviation. It turned out that the latter exceeded the statistical error by roughly 25 “/ We included in the computer program also a calculation of the frequency distribution of the E values. These frequency distributions were used to eliminate single extruding points. scaler
Fig. 4. Block diagram of a counting circuit.
3. Test measurements The performance of the complete experimental set up was checked by a number of test measurements. These tests concerned the following properties of.the apparatus: (i) Since the expected circular polarization is extremely small it is essential that intrinsic asymmetries of the apparatus which produce systematically different counting rates for both directions of magnetization are kept as small as possible. Therefore we investigated possible sources of asymmetries for the different parts of the apparatus. Apparently the accuracy of the information obtained is limited by the counting statistics. It was not possible to exclude systematic asymmetries of the apparatus by these test measurements with sufficient accuracy. The final measurement of the circular polarization of the 482 keV transition of lslTa was therefore combined with the
l*%a
PARlTY
MIXING
39
measurement of the asymmetry ratio for a gamma line of another isotope with about the same energy where one is sure that no circular polarization should exist. (ii) For the final evaluation of the circular polarization it is necessary to know the polarization sensitivity of the polarimeter. We calculated the polarization sensitivity by Monte Carlo calculations taking into account the geometry of our polarimeter and we also have tried to measure the polarization sensitivity by using the circular polarized internal bremsstrahlung of a 32P source. (iii) We have determined the transmission CLof the polarimeter. It is important to obtain a rather high transmission. Otherwise it is necessary to apply inconveniently strong sources in order to use the full counting capacity of the system. We start with the discussion of the asymmetry tests. Asymmetries can be produced by the scaler system, by the photomultiplier tubes, by the scintillators, and finally by the scattering magnet itself. The scaler system was tested by putting a sufficiently strong source into the magnet. Then the asymmetry ratio was measured in the normal mode of operation but without exciting the scattering magnet. After a continuous run of four weeks no measurable asymmetry was observed in any of the six scaler systems, the average of the asymmetry ratic being E = (O.O&7.4) - lo- ‘* The photomultiplier tubes are known to be very sensitive to magnetic fields. We estimated from data which were published by the producer on the magnetic sensitivity of the tubes that our shielding should be sufficient. We performed a test run in which again the magnet was not powered but a magnetic field of twice the strength of the magnet stray field was produced at the position of the photomultiplier assembly by means of a big coil. The current in the coil was reversed every ten seconds. In a continuous run of three weeks no detectable asymmetries were observed, the average for the six scalers being E = (4&7)* lo-‘. Large asymmetries are easily produced by the scintillators. We used plastic scintillators of the material Naton 136 and found that the light output of this scintillator material is influenced by magnetic fields. Already weak magnetic fields enlarge the light output and therefore increase the counting rate. We investigated this property by using a radioactive source of “’ Hf mounted in the normal position in the polarimeter magnet. The scattering magnet was switched off and a small magnetic field was produced at the position of the scintillators by use of the compensation coils (7) shown in fig. 3. Fig. 5 shows the result of a measurement of the relative change of the counting rate as a function of the magnetic field. It is surprising to see that the highest sensitivity exists at the magnetic field zero. Here the counting rate increases by about 1.5°/00 if the magnetic field increases by 1 G. The magnetic field dependence of this effect is nearly the same for all six detectors and shows that saturation is
40
E. BODENSTBDT
f?t al.
reached at a magnetic field strength of about 100 G. We do not yet understand the mechanism of this interesting phenomenon. BN
15
I t
N,AH
I lo-‘/lnOe )
RELATIVE
CHANGE OF COUNTING RATE
Fig. 5. Relative change of counting rate as a function of the magnetic field at the scintillator position (errors calculated from the values for the six detectors).
Fig. 6. Measured asymmetry E as a function of the magnetic field, no earth field compensation.
The magnet produces a stray field of 50 G at the position of the scintillators. We reduced this field by compensation coils to about 1 or 2 G. No effect on the counting rate should happen if this remaining field has the same strength for both directions of magnetization. The symmetry is disturbed, however, by the earth magnetic field
“‘Ta
PARITY
MIXINO
41
which remains unaltered when the magnetization of the magnet is reversed. We observed, indeed,. a measurable change in the counting rate when the direction of the magnetic field at the position of the scintillators was reversed. In this test measurement the magnet was switched off and the field at the position of the scintillators was produced only by the compensation coils. The observed asymmetry in the counting rate as a function of the strength of the magnetic field is plotted in fig. 6. The measured curve exhibits the expected shape. The largest asymmetry factors are observed for the smallest field strength. We have therefore compensated the earth magnetic field carefully by use of three pairs of large Hefmholtz coils. A new run proved that the asymmetry was produced mainly by the earth magnetic field. The field compensation reduced the asy~etry by roughly two orders of magnitude. There are two possibilities for a further reduction of this asymmetry: (i) The stray field of the magnet is not compensated by the compensation coils, but instead enlarged to about 100 G by driving the compensation coils in the wrong direction. Now one should have reached nearly the saturation where the detectors become insensitive against the magnetic field. (ii) One uses the compensation coils in the right direction and uses in addition a mu-metal shield around the scintillators. In this way we could reduce the absolute magnetic field at the position of the scintiliators to < 0.1 G and differences in the absolute magnetic field for both directions of magnetization to less than 0.2 mG. In the final measurement of the circufar polarization of the 482 keV line of lslTa we have applied both methods. The long time test runs gave a slightly better symmetry for the second method. There are different possible sources of asymmetries produced by the scattering magnet: By inhomogeneities in the iron and by remanent magnetization a small difference in the factorfcan be produced for both directions of magnetization. At least in first order approximation this should not produce differences in the cross section for the Compton scattering process, if the gamma radiation is not circularly polarized (see eq. 1). An asymmetry could be produced, however, by differences in the stray magnetic field at the position of the scintillators. A second possibility comes from mechanical motions caused by magnetic forces. Even very small asymmetric motions would change the solid angles for the Compton scattering process sufficiently large to produce measurable asymmetries in the counting rates. We tried to avoid effects of this kind by the very compact magnet construction. Similar effects could be produced by magnetostriction. But also in this case asymmetries can be caused only by differences in the magnitude of the magnetostriction for both field directions. In order to keep asymmetries in the magnetization as small as possible we used a highly stabilized power supply for the magnet. One of the most dangerous sources for systematic errors is probably the so called
42
E. BODENSTEDT
t?f al.
right-left asymmetry term in the Compton scattering formula. Because of the importance of this effect a short description is given in the foliowing. The right-left asymmetry in Compton scattering of unpolarized gamma radiation by transversally polarized electrons has been calculated 1961 by Miller and Wilcox i2). The effect results from a radiative correction to the Klein-Nishina formula. The amplitude of this spin dependent additive term to the Klein-Nishina cross section exhibits a strong energy dependence. The absolute magnitude of the spin dependent correction term is rather small. At the maximum it reaches a value of 6 * 10m4 of the normal Klein-Nishina cross section according to the calculation of Miller and Wilcox. This term is large enough, however, to produce serious disturbances if circular polarizations in the order of 10-s are to be measured. The spin dependent correction term has the form
where & = Compton wavelength, ;1 = wavelength of incident radiation, s = electron spin, ui, u2 = unit vectors in the directions of the momenta of incoming and scattered quanta, respectively, and u = fine structure constant. One obtains a change in the absolute Compton cross section for a reversal of the magnetization of the scattering foil and therefore a reversal of the vector s as soon as the magnetization vector does not coincide with the scattering plane defined by the incoming and outgoing photon. Scattering events of this type are always admitted in polarimeters for circular polarization of gamma rays because of the &rite size of the source and especially of the detector for the scattered radiation. In the case of axial symmetry the correction term vanishes on the average because both signs occur with the same probability. Slight misalignments of the source and the detector cause, however, asymmetries which produce finite values for the asymmetry ratio even for unpolarized gamma radiation. Our scattering magnet was carefully machined in order to obtain a maximum possible axial symmetry and our source was mounted carefully on the axes. The above mentioned strong energy dependence of this effect makes the comparison measurements only relevant if gamma rays of the same energy are used. A determination of the circular polarization sensitivity of the scattering magnet by direct measurement is difficult. We therefore preferred to calculate the sensitivity rIn order to make this calculation reliable a few test measurements were necessary. An average value for the fraction of oriented electronsf of the scattering foils was determined by measurements of the magnetization of the scattering foils at different radii. For this purpose small induction coils were mounted into the scattering foils at different positions and the induction signal was measured when the magnet was powered. Because of the conical shape of the scattering foils the saturation magnetization is only reached at the inner part of the foil andf decreases slightly with increasing radius. A second test concerned possible backscattering from the lead behind the scattering
=‘Ta
PARITY
MIXING
43
foils. For this purpose we used a well collimated gamma beam of a strong 13’Cs source. We simulated the scattering foil by a thin iron plate of the same thickness and in the same geometry and measured the spectrum of the backscattered radiation. Then we investigated how this spectrum changed when we put a thick piece of lead behind the scatterer. Measurements of this type gave the result that the background of radiation which is scattered from the lead can be neglected. A third test concerned possible backscattering effects from the central lead shield. This effect should be very sensitive to small shifts of the source in the axial direction. A measurement gave no detectable effect, thus indicating that the contribution of scattered radiation from the central lead shield is negligibly small. The final calculation of the circular polari~tion efiiciency:
was performed by averaging over the different angles for the geometry of our magnet. This was done by a Monte Carlo calculation which took into account also the absorption of the radiation in the scattering foil, The result of this calculation was rj = -4.8 %. This value was used for the calculation of our preliminary result. A more refined calculation which took into account also double scattering effects gave a slightly larger sensitivity: q = -6.25 “/,* These Monte Carlo calculations have been extended to other energies and they have also been used to determine the transmission a of the polarimeter. The results obtained for q and Mas a function of the gamma energy are plotted in fig. 8. We have performed a rough test for the polarization sensitivity by measuring the circular polarization of the internal and external bremsstrahlung of 32P. Theintensities of the IB- and EB-spectra were calculated according to refs. I39i “). These calculations show good agreement with experimental data of refs. ’ 5, ’ “). Theoretical values for the polarization of IB and EB are given in refs. I391‘). The active material was put into an aluminum capsufe of 4 mm wall thickness which stops the p-radiation. The capsule was surrounded by lead shields of different thicknesses (1; 3; 5 and 7 mm). The mean energy and the mean polarization of the bremsstrahlung spectra must increase with increasing lead thickness and one therefore expects that the measured asymmetry ratio should change in the same way. We measured the energy spectrum emitted from our source with a 7.6 x 7.6 cm NaI(TI) crystal. Fig. 7 shows the measured spectrum for a lead shield of 1 mm thickness. In the same figure the spectrum is unfolded and finally also the theoretical shape of the spectrum is inserted. There is only a discrepancy at low energies which can be explained by build up effects in the lead shield which increase the number of low-
44
B. BODENSTEDT et 01.
4
counting rote
=P
5.103,,
Bfe~stf~ with
lmm lead shietd
experimental spectrum
experimental spectr
ElkeV)
Fig. 7. Pulse-height
spectrum
of 32P bremsstrahlung
with 1 mm lead shield around the source.
tt?
I
-6 0 ,E $” -4 P x -3 ; r2 1
i
ldill
400
2000
E, LkeVl Fig. 8. Theoretical
polarization
sensitivity q and transmission of y-energy.
Q:of our polarimeter
as a function
45
'*'Ta PARITY MIXING
energy quanta. The experimental spectrum and the theoretical energy dependence of the polarization were used together with our calculated polarization sensitivity rl to calculate the expected counting rate asymmetry. These values are compared with the experimental results in table 1. The errors of the theoretical values are due to uncertainties produced by the bad knowledge of the build up effects. The agreement between experiment and calculation is satisfactory thus indicating that the calculated polarization sensitivity must be fairly correct. TABLE
Counting rate asymmetries lmm
1
for 32P bre~tr~lung
3mm
[o/o0]
5mm
7mm
*lb
6.6k1.3
8.111.6
9.5h2.0
10.7+2.1
%XP
7.0f0.2
8.3 kO.3
8.510.5
8.6f0.6
Finally we have checked the calculated transmission a = 1.8 % of the polarimeter by using a islTa source of known activity. The measured counting rate agreed fairly well with the prediction using the above calculated transmission factors. 4, Radioactive sources The larHf sources of a strength between 4 Ci and 8 Ci were produced in the following way. The compound HfOZ with an enrichment of 92 % ‘*‘Hf was irradiated in a flux of 1014 neutrons/cm’ f set in the Kernforschungszentrum Karlsruhe for several
lcm
Fig. 9. Source assembIy; (a) radioactive
material,
(b) aluminum,
(c) Iead.
weeks. A weak beta transition of 550 keV maximum energy in the decay of 18iHf can perturb the circular polarization measurement by the contribution of its bremsstrahlung. In order to keep this bremsstrahlung background small the hafnium oxide was mixed with spectroscopic pure graphite powder before the irradiation in order to decrease the effective nuclear charge of the source (400 mg graphite were mixed with 200 mg HfOJ. The source material was pressed into a cylindrical capsule of very pure aIuminum with 1.5 mm wall thickness and finally surrounded by a 9.5 mm thick lead shield, The construction of the source is shown in fig. 9. The main purpose of the lead shield is a reduction of the bremsstrahlung background. In addition it has the effect
46
E. BODENSTEDT
er
al.
to suppress the background of low-energy gamma lines of l”Ta. The expected counting rate asymmetry factor which is produced only by the circular polarization of the bremsstrahlung was calculated for different lead thicknesses, for two different effective atomic numbers, and for the upper and lower limit of the intensity of the 550 keV “t
effect
of
bremsstrahlung
i_.__ErnI 0 1 2 3 4 5 6 7 8 9 -: ----
1(408):93’/. : I (408)=98’/.
, 1(550)=5’/. , 1(550):5%.
lead
tzckness
, Zelf=60 , Z,ff=20
Fig. 10. Calculated asymmetry effect of bremsstrahlung of lslHf around the source.
as a function of lead thickness
beta-radiation. The result of this calculation is shown in fig. 10. Thus we expect for our source a bremsstrahlung background in the measured asymmetry ratio of only: E x 10-7. Among the other hafnium isotopes which are contained in our target material only 174Hf gives rise to a disturbing activity because of its high-activation cross section. The intensity ratio of the 343 keV transition in 17$Lu to the interesting 482 keV transition in “‘Ta is estimated to be roughly 1 : 300. Therefore a perturbation by this line can be neglected. The test measurements were done by use of the 497 keV transition in lo3Rh. The lo3Ru source was obtained by neutron irradiation of lozRu and the sources were surrounded by a 5 mm lead shield in order to keep the contribution of the bremsstrahlung of the weak 722 keV beta transition negligibly small.
-5.
Nl
N3
N2
2N,-K,jO6 N++K
N+-N-
2N.x
Kf
10 -
Nl
N2
N3
Fig. t f . a) Measured counting rate asymmetries for lsiTa (run III) for all six detectors; b) measured counting rate asymmetries for lo3Rh (run III) for all six detectors, the last point in each series is the mean value.
E. BODENSTBDT et al.
5. Polarization
measurements
We have performed four runs of polarization measurements with ‘sITa for about four weeks each. All these runs were followed by control measurements with lo3Rh. In the first run the compensation coils were powered in the wrong direction thus producing an alternating magnetic field at the position of the scintillators of about 100 G. In the other three runs the compensation coils were powered in the right direction and the stray field was shielded in addition by a mu-metal cap as described in sect. 3. Figs. 1laand 1lb show as an example the results of the 3rd run. Each point corresponds to a counting time of six days and contains about 400 E-values calculated as described in sect. 2. For a source strength of 5 Cur we obtained a statistical counting rate in each scaler of about 8 * lo6 sec- I. In the evaluation a dead time correction was included. TABLE 2 Measured asymmetry ratios E = ~~~-.q+~[lO-~] Run no.
Detector 1
II
Sl s2 s3 Nl N2 N3
0.7111.82 7.99f1.95 3.44kl.65 2.85h2.00 4.04kl.70 3.89rt1.70
1.6251.08 0.56k1.16 1.43+1.00 4.3111.3s -0.07+1.02 -0.35*1.01
weighted mean of the run
3.7Sf0.74
1.02*0.43
III
IV
3.2Okl.22 2.02f1.41 1.59~1.10 -0.90*1.30 0.1611.11 0.2111.13 1.6SkO.49
Weighted mean of the scaler
2.67kl.14 3.2Okl.34 1.88k1.06 0.41hl.15 1.23f1.05 1.49*1.08
2.23 &to.62 2.57+0.69 1.84+0.57 1.3310.68 0.8SrtO.58 0.8210.58
1.75 +0.43
1.5510.33 weighted mean of the 24 single results
It is noticed from fig. 1lb that the mean values obtained in the control measurement for the six independent counting circuits deviate from zero in different directions and with different magnitudes. We do not know the true reason for this apparative asymmetries. Most probably they are produced by stray magnetic field effects or by the right-left asymmetry of the cross section for Compton scattering. It must be mentioned that all error intervals are rms deviations and not statistical errors. The differences
for the four runs together with their rms deviations are shown in table 2 separately for all six circuits. A plot of this data is given in ref. lo). The mean value of the 24 independent measurements is & = (1.55f0.33)
* 10-6.
rsiTa
PARITY
MIXING
49
The error is again the rms deviation and exceeds the error expected from error propagation by a factor of 1.3. In the calculation of the circular polarization we included a small correction for a background of the low-energy gamma transitions in the decay of ‘*‘Ta and obtain finally for the circular polarization of the 482 keV transition: Py = -(2.81:0.6)
- lo-‘.
6. Comparison with theory and previous experiments A comparison of our result with the different theoretical predictions gives an especially good agreement with a calculation of the expected circular polarization by McKellar “) who obtained Py = -(2_tl)* 10-5. Our measurement is not in agreement with the most precise results of other groups: Py = -(0.67+0.07)
- 1O-5
and P, = -(0.39+0.18)* 10-5, obtained by Lobashov ‘*) and by Vanderleeden et al. “), respectively. The reason for this discrepancy is not yet understood. Possible systematic errors might be introduced by underestimating the bremsstrahlung background or by performing a comparison experiment with a different gamma energy as in this case the right-left asymmetry is not eliminated.? The weakest point of our measurement is a small long time drift which was indicated by the fact that the rms deviation of the mean value of the asymmetry ratio E exceeds the error expected from error propagation by a factor of 1.3. In order to eliminate this uncertainty we have started a new experiment where the ‘*rHf source and the lo3Ru comparison source are changed in shorter intervals. For this purpose we constructed a sluice way which allows easily to remove the sources without interrupting the operation of the magnet. We hope to certify in this way that the hafnium source as well as the comparison source find the polarimeter in exactly the same working conditions. This work was supported by the Bundesministerium fur wissenschaftliche Forschung. The numerical calculations were performed on the IBM 7090 computer of the Gesellschaft fur Mathematik und Datenverarbeitung in Bonn. + Note added in proofi A recent measurement by H. Diehl, G. Hopfensitz, E. Kankeleit and E. Kuphal gave I’?(482 keV) = -(1.3f0.7) . lo-$ which is only slightly smaller than our value. We are grateful to the authors for this information before publication.
50
E. BODENSTEDT
et al.
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