Fission fragment anisotropy for the 242mAm fission isomer by spin exchange pumping with polarized rubidium vapour

HB

NuclearInstruments and Methods in Physics Research B70(1992) 521-531 North-Holland

Fission fragment

anisotropy for the

by spin exchange pumping

with

242mAm

Beam Interactions with Materials&Atoms

fission isomer

polarized rubidium vapour

H. Backe, W. Lauth, W. Athenbach, M. Hain, M. Hies, A. Scherrer, A. Steinhof, S. Tölg and S. Ziegler Institut für Physik der Universität Mainz, Postfach 39 80, D-6500 Mainz, Gennany

The foundations of an experiment have been worked out with which, in principle, the spin, hyperfine constants and the isomer shift of the 14 ms fission isomer 242mAm can be measured. Such an _-xperiment would be based on the fission fragment anisotropy signal which has actually been observed in this work after spin exchange pumping with polarized rubidium vapour in an optical buffer gascell . A decrease of the count rate of (12f4)% hasbeen measured at 90° with respect to the quantization axis. From this result it is concluded that the nuclear spin of the 242mAm fission isomer must be larger than 1. The low-energy fission isomers originating from the 242 Pu(d, 2n)242mAm reaction have been post-accelerated with the aid of a 6 cm long 100 kV electrostatic accelerator unit in order to implant them through a0.4 pm thick entrance window into theoptical buffer gas cell . A neutralization efficiency of 13% of the americium fission isomers with an energy of about 1 MeV has been determined experimentally. 1. Introduction The optical spectroscopy of fission isomers has been a challenge since Bemis and coworkers [3] measured the isomer shift of the optical '" P7/2_8S7/2 (a = 6405 Â) transition in 240 '°Am (T1/2=0 .9 ms) from which a quadrupole moment Q = 35 t 2 eb was deduced [5,6] . In a later analysis, in which advantage was taken of results from muonic atoms [4], a somewhat lower quadrupole moment Q =29 .0 f 1.3 eb was reported. The goal of our optical spectroscopy experiment on 242'Am will be the observation of a resolved hyperfine pattern from which the nuclear :spin, the nuclear g factor and the intrinsic quadrupole moment can be determined model-independently. Such data should be of greatvalue to understand nuclearmatter in the state of high deformation [7] . However, it must be stressed that such experiments are extremely difficult . The production rate of fission isomers is very small, typically only in the order of a few per second . The half-lives of the fission isomers are very short (< 14 ms). Wellestablished in-beam methods like ß-RADOP (RAdiation Detected Optical Pumping) [8,9] can not be simply adapted for the optical spectroscopy of fission isomers. In the ß-RADOP experiments up to 106 alkali atoms were created directly in the optical cell . Also, at such large counting rates, small anisotropy signals are still detectable with statistical significance. The ß-lifetimes of the investigated nuclides were long enough to sur* This paper comprises part of the doctoral thesis of W. Lauth [1] and W. Achenbach [2].

vive neutralization, c.f. section 4. In addition, it is well known that the relaxation cross sections of alkalis in light inert gases are extremely small (< 10 -21 cm2 [10]). Therefore, small spin exchange rates of about 1000/s were sufficient to create an observable 0-anisotropy signal. As we will see below, americium fission isomers do not exhibit, in many respects, these nice features of the alkalis. In this paper we describe our arproach to the problem of an optical spectroscopy experiment on fission isomers with the fission-RADOP method . Preliminary results of this experiment have been presented elsewhere [111 . 2. Principle of radiation detected optical pumping (fission-RADOP)

It is assumed that a secondary beam of fission isomers is available with an energy high enough so that it can penetrate the entrance window of an optical cell filled with inert buffer gas. The buffer gas pressure and the remaining energy of the fission isomers after passage of the entrance foil are matched in such a way that the fission isomers come to rest in the buffer gas. A certain fraction of the recoiling ions neutralizes during the slowing down process, most of the remaining ions neutralize in charge exchange collisions with rubidium atoms (see section 4 of this paper). The gas acts at the same time as a storage medium for the fission isomers. If argon is used with a pressure of 30 mbar the diffusion time to the cell wall, the wallsbeing 1 cm apart, is on the order of 50 ms .

0168-583X/92/$05 .00 © 1992 - Elsevier Science Publishers B.V . All rights reserved

VIII . IONTRAPS/LASERS

52 2

H. Backeet al. / Fission fragment anisotropy of 242mAm fission isomer

Let us assume that the optical cell contains, in addition to the buffer gas, polarized rubidium vapour with a density of about 10 14/cm3. The polarization of the 5s valence electron of the rubidium atom can be transferred in spin exchange collisions to a 5f electron of americium and via the hyperfine interaction to the nuclear spin of the americium fission isomer. This process results in an anisotropy of the fission fragment distribution:

WI(O) = z +A 2P2(cos 0) +A 4P4(cos B) + ' " " . (1) The functions P(cos 0) are the Legendre polynomials with 0 the polar angle defined by the fission fragments and the polarization axis . The anisotropy parameters Ax are functions of the nuclear spin I, the projection of I on the nuclear symmetry axis K at the saddle point and the probability p1M of finding the nuclear spin I in a magnetic sublevel M [12] : AA = [(21 + 1)/2](IKI -K I AO) _ (- )"-M x (LW -M I AO)p rm .

M

(2)

As shown in fig . 1, a strong decrease in the counting rate at 90° will be observed. The depth of the minimum at 0 = 90° depends on the nuclear spin I of the fission isomer. Spectroscopic information can be obtained by polarization destructive methods. The polarization can be destroyed either by high-frequency saturation spectroscopy or by excitation of the americium atom with a single-mode cw laser. In the excited atomic state the polarization relaxes immediately resulting in an increase of the fission fragment counting rate at 90°. A laser scan would yield the required hyperfine pattern.

A Fig. 1. Angular distribution W,(B) of fission fragments originating from a fission isomer in the magnetic substate M = / [121 . At the saddle point K = I was assumed. A parameter is the nuclear spin I.

2 Pv2

2S1/2

Fig. 2. Relevant hyperfine levels for the D, transition of s5Rb with nuclear spin I= Z . The hyperfine splitting of the 2P1/2 term is scaled by a factor 5.6 with respect to that of the 2S1/2 term. Note that no transition from the F=3, mF = 3 level is possible with right-circularly polarized light .

The described spectroscopic method has several advantages. The experimental conditions can be permanently monitored in the presence of the anisotropy signal . No time consuming laser scans are required to find such a signal. Furthermore, the destruction of the anisotropy signal by laser excitation is always possible regardless of the de-excitation mechanism of the excited atomic state . The rubidium atoms can be polarized by optical pumping via the D, transition at 794.7 nm with a circularly polarized broad-band laser beam, i .e. a laser frequency profile that spreads out over all hyperfine components . The principle of the polarization process will be explained by the example of "Rb with nuclear spin I= z . The hyperfine levels are shown in fig. 2. With a circularly polarized laser beam, transitions are induced which obey the selection rules AF= 0, t 1 and Am F = + 1. The excited state may decay by electric dipole transitions back to the ground state . However, if the optical cell is filled with nitrogen, the radiation will be very effectively quenched in buffer gas collisions, i .e. the excitation energy is transferred to a nitrogen molecule . A very large quenching cross section 0r(P,/2 - S1 /2) = 0.58 x 10 -14 cm 2 was measured [101 for the rubidium d, transition in nitrogen. From this we calculate at T= 423 K a quenching rate Aqu-1, = 1.6 x 108/s in 25 mbar NZ which actually is a factor 4.5 faster than the spontaneous decay rate Asp = 1/-r = (28 its) -1 . The suppression of the fluorescent radiation is of great importance since a fluorescent photon can be absorbed also by a polarized rubidium atom which at least diminishes the polarization.

H. Backeet ai. / Fission fragment anisotropy of242mAm fission isomer

523

LASER AXIS

FINE STRUCTURE

J=71 2 51~75= 8~~~ Am1 ( GND)

HYPERFINE STRUCTURE

~1112 F=,11 912 ~--~\~--- 112 52 ---312 F ( l=2)

Fig. 3. Polarization of an americium fission isomer by spin exchange collisions . A nuclear spin I = 2was assumed. A spin exchange collision in the F = z hyperfine level may change MF by one unit . No further spin exchange collisions are possible from the F = z, MF = 1i sublevel.

F = 3, MF = 3 sublevel of the 2S,/2 ground state from

A radiative or quenching transition may end in an

this subject in section 6. It turns out that the relaxation cross section cannot be estimated with a sufficient

electron spin are coupled to its maximum F value. In this manner both the electron spin and nuclear spin are polarized. An external magnetic field B = 1 G in the direction of the laser beam axis is needed to

gases are known [14] . Since, on the other hand, the measurement of the americium depolarization cross section would be rather difficult due to the scarcity and toxicity associated with the high a-activity of ameri-

which no further pumping with circularly polarized light is possible. In this sublevel the nuclear spin and

reliability even though the relaxation cross sections of the chemical homologue europium in various buffer

maintain the polarization .

cium, we initiated a search for z fission fragment anisotropy without knowledge of that important num-

Polarization can be tranferred from a rubidium to an americium atom in a spin exchange collision. In such a collision the nuclear spin decouples from the electronic spin in both atoms. Furthermore, we can assume that the magnetic quantum number of the

nuclear spin will not change in the collision. This can be concluded from the fact that the hyperfine period of

the nucleus is on the order of 10 -s s while the collision lasts only about 3 x 10 - ' Z s (Kastler [13] formulated in analogy to the Frank-Condon principle, "In a sudden process which affects the electron configuration (a spectral transition or a molecular collision), the orientation of the nuclear axis remains unchanged"). For a spin exchange collision with a polarized Rb

ber.

3. The experimental setup Our experimental setup is shown in fig. 4. The 242mAm isomers were produced through the 212pu(d, 2n) reaction by using a 12 MeV deuteron beam supplied by the EN and MP tandem Van de Graaff accelerator of the Max-Planck-Institut für

Kernphysik in Heidelberg . The deuteron beam does not traverse the optical cell. This has the consequence that the recoil ions must be post-accelerated since their

atom, the magnetic quantum number of the BS7/2 term is altered by OM,, = + 1 . After the collision, nuclear and atomic spin recouple to the total angular momentum F. In this recoupling process the magnetic quan-

recoil energy is by far too low to penetrate even a very thin entrance foil of a buffer gas filled optical cell in which the spectroscopy will be performed. The fission fragments are detected by a pair of position-sensitive

while the angular-momentum quantum number F may acquire any value in the interval M, + M,, <_ F <_ I +J. Neglecting the influence of relaxation processes the

different components of the experimental setupwill be described in more detail.

tum number obeys the selection rule

AM,,=0,+ 1 [2]

atom would finally end in the quantum state I F =1 + J, MF = I + J ) from which no further spin exchange tran-

sitions are possible (see fig. 3) . In this state the nuclear spin I>_ 1 of the fission isomer is polarized along the magnetic field direction rc:;ulting in the observation of a fission fragment anisotropy signal. However, the significance of such a signal depends among other things strongly on the relaxation of the polarization in americium-nitrogen buffer gas collisions . We will return to

parallel plate avalanche counters (PPACs), details of which are described in ref. [15]. In the following, the

3.1 . The

2a2iott

target

All measurements to be described below were car242pUF~ target '. The out with 50 Wg/CM2 .,

ried

"' Produced by Geel Establishment, Central Bureau for Nuclear Measurements, 2240 Gee], Belgium. Vill . ION TRAPS/LASERS

H. Backeet al. / Fission fragment anisotropy of 242mAm fission isomer

524

PPAC

IOOkV

OkV

OkV

~-

2

3i S-

Fig. 4. Experimental setup for the fission RADOP experiment. Note that the 12 MeV deuteron beam enters the target assembly at an angle of 45° with respect to the dash-dotted line in a plane perpendicular to the plane of drawing. The exit foils for the fission fragments are polyimide foils with a thickness of 165 Wg/cm2 which were evaporated with 100 Wg/cm2 gold. Results were good also with UPILEX 7.511 polyimide foils which can be stretched to a thickness of about 400 Wg/cm2. The UPILEX foils are supposed to be less sensitive under the influence of alkali hydroxides than the normal KAPTON polyimide foils . To avoid Vg/c.2 . deterioration of these foils they were protected by self-supporting aluminium foils with a thicknessof 170

12 PuF~,, made from 99.744% enriched 242pu, was evaporated onto a 50 Wg/cm2 carbon backing. These targets behaved very stably in a 5 RA deuteron beam focussed to a spot with a diameter of about 2 mm . No significant loss of target material was noticed in many days of beam time.

collisions with the target atoms, the recoiling ions exhibit a broad energy and angular distribution . The electrostatic isomer accelerator must focus as many as possible of these recoiling fission isomers into the optical cell . To approach this goal the electrode configuration of the fission isomer accelerator was optimized

3.2. Thefission isomer accelerator system

with the aid of ray trace calculations in the potential generated by the electrodes [18,19]. The transport efficiency of the fission isomer accel-

The recoiling fission isomers have a primary energy E~ = 100 keV. At that energy they will be able to leave

erator and the size of the beam spot were measured with a somewhat modified experimental configuration

the target if the layer to be penetrated is not thicker than approximately 18 Ilg/cm 2 [16] . Conversion c1cc-

as that shown in fig. 4. The optical cell was replaced by

tron transitions of excited rotational levels in the second minimum and succeeding Auger cascades result in large noneyuilibrium ionic charge states, typically between 10 + and 35 + after two to three conversion electron transitions [17]. Caused by the many nuclear

a 262 Wg/cm2 catcher foil which was installed at a distance of 86 mm from the grounded electrode of the isomer accelerator just between the position-sensitive PPACs. The foil was thick enough to stop all fission isomers . In fig. 5 the number of fission events per collected beam charge is plotted as a function of the applied high voltage . The beam spot was reconstructed event by event from the position information of the fission fragments in the PPACs. From those coordinates the intersection

point with the catcher foil can be calculated. Fig. 6 shows the intensity profile of the fission isomer beam . Half of the total intensity is found in a circle with a diameter of 5 mm.

I-

Z W

3.3. The optical cell

W 0

0

20

40

60

VOLTAGE

80

[KV]

100

Fig. 5. Fission events percollected deuteron beam charge as a function of the high voltage of the fission isomer accelerator. Data are corrected for the solid angle acceptance of the PPACs.

The optical cell, the top view of which is shown in fig . 7, is the central piece and at the same time the most complicated part of the whole experiment . Particular demands are made on the entrance foil for the fission isomers. The entrance foil must be vacuum tight meaning that the leakage late should not exceed 10 -°

H. Backe et al. / Fission fragment anisotropy of 242"'Am fission isomer N

c

W

a

200

400

100 0

b 4 .8 mm

4 300 c 200

11

-1

52 5

-s

1

-10 -s

0 5 y [mm]

10

w

100 0

-20

-10

0

10

20

X [mm] Fig. 6. (a) Intensity profile of the fission isomer beam . The x and y axes are perpendicular to the direction of the fission isomer beam with the y axis lying in the plane of drawing of fig. 4. (b) Projection of the intensity profile shown in (a) onto the x axis . Accelerator voltage 100 kV. mbar I/s and must not be affected by a temperature as high as 200°C . These requirements are fairly well met by a grid-supported polyimide foil which is actually very stable when thicker than about 35 hg/cm2 [20] . Unfortunately, polyimide foils disintegrate under the influence of alkali vapour and particularly alkali hydroxides at high temperature . We therefore tried to protect them against the deteriorating alkali vapour by a thin aluminium or gold evaporation and, in addition, by other foils of various materials like carbon, aluminium or polyimide . However, the entrance foil ruptured after a few hours of full operation under experimental conditions which sometimes resulted in the destruction of the a-active 242Pu target . This was one of the reasons that we finally terminated the fissionRADOP experiments and continued the optical spectroscopy on fission isomers with the radioactive decay detected resonance ionization spectroscopy (RADRIS) [21,22]. 0 1 2cm W

Fig . 7. Top view of the optical cell . The frame of the cell consists of aluminium (120x 120x36 mm) with a thickness of 10 mm . It can be electrically heated up to a temperature of 200°C. While degassing, a valve is opened and the cell is pumped for several hours by the main vacuum system . The fork-like electrodes are indicated by dashed lines .

3.4. The laser system The optical system is shown in fig. 8 . We used a copper vapour laser from Oxford Physics in combination with a dye laser from Lambda Physik. In the

Fig. 8 . Schematic representation of the optical system with which a circularly polarized laser beam at the Rb D, wavelength (A = 79-4.7 nm) was prepared . The copper vapour laser supplies laser pulses with a width of about 30 ns at a repetition rate of 6.5 kHz . The mean power is 18-20 W. With the dye Styryl 9 we obtained a mean power at the wavelength of the rubidium D, line of 1 .2 W . The spectral profile has a width of 7 GHz. Since the total hyperfine splitting of the D, transition amounts to 3.39 and 7 .66 GHz [10] for '5 11b and s7Rb, respectively, the laser profile covers all hyperfine components. The output of the copper vapour dye laser combination, installed outside the radioactive control area, was guided with a 60 m long and a 0.2 mm thick quartz fiber to the experiment. The laser beam leaves the quartz fiber unpolarized. In order to avoid a 50% intensity loss with polarization, the beam passed through a Wollaston prism . Two beams were created, at a relative angle of 20°, which were polarized linearly, but perpendicularly to each other. The polarization of one of the beams is rotated by 90° with a A /2 plate. Both beams pass thereafter a A /4 plate with which they are circu larly polarized. The inset represents the approximate pulse structure of the laser beam at the optical cell . VIII. ION TRAPS/LASERS

526

H. Backe et al / Fission fragment anisotropy of zaz "'Am fusion isomer

optical cell a cross section of about 30-50 mmZ is illuminated by the beams . The total laser power amounted typically to 600 mW . The spectral intensity in the pulse, PWwr = 9 W/(mmZ GHz) (assuming a pulse width of 30 ns, a repetition rate of 6 .5 kHz and a spectral width of 7 GHz) saturates the D, transition (A i,t =Bik at 10 mW/(mm Z GHz)). The energy in a single laser pulse of 77 pJ corresponds to a total number N = 3 x 10 1° of photons . That number is sufficient to pump about 10' ° rubidium atoms in a single laser shot. Our experimental setup shown in fig . 4 accepts fission fragments with angles 0 between 50° and 90° with respect to the quantization axis. Since we do not detect fission fragments around 6 = 0° a reference measurement is required . One possibility would be to record the experimental fission fragment angular distribution with the polarizing laser beam switched off but otherwise identical conditions . However, since we were not sure that thermal heating of the buffer gas by the absorbed laser power could result in turbulence ire the buffer gas and as a consequence in a changed coincident detection efficiency, we switched the polarization of the laser light periodically every 10 s between circular and linear states. Irradiation of linearly polarized light will not result in a polarization of the fission isomers but all conceivable effects of thermal heating, etc., will be preserved . 4. Neutralization rate measurements of the fission isomers The planned experiments must be carried out on (neutral) americium atoms. It is therefore of great importance to know both the fraction of atoms formed during the slowing-down process as well as the neutralization rate of ions in the buffer gas atmosphere . In order to study these questions, two fork-like electrodes were inserted into the optical cell as shown in fig. 7 . The electrodes were connected to a voltage source supplying a potential of + 100 and - 100 V, respectively. The electric field strength, which amounts to = 20 V/cm in. the middle between the electrodes; causes alll americium ions to move to the electrodes, arriving there in a time of about t = sp/(Euo x 1013 mbar) .

(3)

In eq. (3) s = 2.3 cm is the mean distance which the ions must pass to reach the electrodes, p = 20 mbar is the buffer gas pressure, in our example argon, and p  = 1 .7 cm`/(V s), the estimated mobility of americium ions in argon [23] . With these numbers a time t _-_ 1 ms was calculated, which is short in comparison. to the fission isomer half-life of 14 ms. Once a fission isomer has reached the electrode it can no longer be

300 iIn, 200 C G)

W

100 Ou -60

-40

-20

0

Z Imm]

20

40

Fig . 9. Stop distribution of 242mAm fission isomers in the optical cell after projection onto the fission isomer beam direction (z axis). A forerunner of the fission isomer accelerator shown in fig . 4 was used [181, accelerator voltage 50 kV. The optical cell was filled with 20 mbar argon, entrance foil thickness 38 Ilg/cm`. The measurements "charged+neutral" and "neutral" correspond, respectively, to the experimental situation with no voltage and U = ± 100 V at the fork-like extraction electrodes as shown in fig . 7. detected coincidently, resulting in a decrease of the fission fragment coincidence rate . This effect is clearly demonstrated in fig . 9 . Only about - of the fission isomers are neutral after they came to rest in the buffer gas. The shapes of the distributions "charged + neutral" and "neutral" are very similar. This fact indicates that the neutralization occurs during the slowing-down process rather than in the entrance foil . We now would like to describe the measurement of the neutralization rate of an americium ion in the buffer gas and its dependence on the alkali v-noun density . For these measurements the extraction voltage of the electrodes was periodically switched on and off. The fission events were recorded as a function of time in each such time interval . Typical time spectra are shown in fig . 10 . During the first 30 ms the voltage of the extraction electrode is switched off and the fission events originate from charged and neutral fission isomers . At the time t = 30 ms the extraction voltage is switched on and, as explained above, the fraction of charged fission isomers will be excluded from detection . This results in a sudden. Jecreasc uï the countrate a: t = 30 ms . In the subsequent time period 30 < t < 60 ms only the decay of neutral fission ,;: ntcrs will be detected . The measured spectrum in this time interval can be disentangled into two parts. The exponentially decaying part originates from the ions which were neutralized in the first time period, 0 < t < 30 ms . The constant contribution represents the fraction of fission isomers neutralized during slowing down. A mathematical analysis of the time spectra, which is described in ref. [24], yields the wanted neutralization rates. The spectra displayed in fig . 10 were taken at three different Cs vapour densities which are characterized

H. Backe et al. / Fission fragment anisotropy of 242mAm fission isomer

Fig. 10. Measurement of the neutralization rate of americium fission isomers in 20 mbar argon with Cs vapour. The fission event distribution is shown as a function of time . The extraction voltage of the fork-like electrodes as shown in fig. 7 was periodically switched off and on . The first 30 ms correspond to U = 0, the second 30 ms to U = f 120 V. The fore-runner of the isomer accelerator was used [18], U=50 kV. Cs oven temperature (a) 150°C, (b) 160'C, (c) 180°C. by the oven temperature. The real Cs vapour pressure is unknown . Nevertheless, the upper spectrum (a) represents a neutralization rate A N = (11 .8 ± 2 .8)/s which is small in comparison with the decay-rate AA . = 50/s of the 242mAm fission isomer, while in the lower spectrum the neutralization rate A N = (1 t 1) x 10 3 /s is much larger than the decay rate . Quite similar results were also found with rubidium .

527

larly polarized light and N, ; at irradiation of linearly polarized light, amounted to (Nc ;rc/Nj;n)j = 451/486 = 0 .928 ± 0.061, which only slightly deviates from one. In this experiment the polyimide foil deteriorated rapidly under the action of the rubidium vapour in combination with the laser beam. Therefore, in a second experiment we inserted in addition to the polyimide protecting foil (25 Wg/cm 2 + 4 Wg/cm2 aluminium) another self-supporting aluminium foil which served as a laser beam dump . Furthermore, we covered ,he region around the laser window with Teflon which :eacts strongly with rubidium . In this manner the rubidium vapour density should be decreased in the region of the laser window in order to avoid absorption of the laser beam already at a place where the detection efficiency of fission isomers is small . All other conditions were very similar to those in the previously described experiment . The experimental results of this second experiment are shown in fig . 11 . The upper part (a) represents the (not efficiency corrected) fission fragment angular distribution for irradiation by circularly polarized light. The corresponding reference spectrum taken when irradiating linearly polarized light looks quite similar. In fig . Ilb the ratio of the two spectra is shown in which the coincidence detection efficiency cancels out . Since the coincidence detection efficiency drops off rapidly below an angle of 0 = 60°, the integral counting rates in the spectra represent a measure of the fiss°on fragment anisotropy at about 90° . The experimental ratio amounts to (N~ ;rc/Nu,,)2 = 491/574 = 0.855 t 0.053 .

5 . The fission fragment anisotropy signal With the experimental setup shown in fig . 4 we searched for a fission fragment anisotropy signal after optical spin exchange pumping with polarized rubidium. The polyimide entrance foil at the optical cell was 35 pg/cm 2 thick. It was evaporation coated on one side with 4 Wg/cm 2 aluminium. This foil was pretecied by a second polyimide foil of 21 Wg/cm2 which was coated on the side facing the cell with 10 wg/cm 2 aluminium. This foil acted at the same time as a reflector for the laser beam . In a first experiment a temperature T = 216°C of the rubidium oven and the temperature of the aluminium cell frame of 100°C were chosen such that about 25% of the incident laser power (480 mW) was absorbed by the rubidium vapour . The ratio of the detected fission fragments, Ncirc at irradiation of circu-

0

20

40

60

0 (deg]

80

Fig. 11 . Fission event distributions as a function of the angle 0 with respect to the laser beam direction . To,; = 208°C, T,,uo-amc =100'C, pia_ = 500 mW at the optical cell, absorption 25% . (a) Irradiation of circularly pola.,zed light, (b) ratio of the fission event distributions taken at irradiation of circularly and linearly polarized light . In the mean this ratio deviates from unity indicating evidence for a fission fragment anisotropy signal. VIII . ION TRAPS/LASERS

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H. Backe et al. / Fission fragment anisotropy of 242'Am fission isomer

Data were taken for two hours. This ratio clearly deviates from one. Under similar conditions we measured in two earlier experiments (Ncirc/Nl,n)3 = 124/136 = 0.91 ± 0 .11 (Tmen = 237'C, Plaser = 420 mW, absorption 30%) and (Ncirc/Nrn)4 = 53/81 = 0 .65 t 0.12. The weighted mean of all four measurements yields = 0 .876 f 0.036 and an (Ncirc/Nüin)rural = 1119/1277 anisotropy A = l - ( Ncirc/NI,) = 0.124 1 0 .036 . This result provides strong evidence of the existence of a fission fragment anisotropy signal . 6. Discussion In principle the following three reasons could be responsible for observing such a small fission fragment anisotropy signal : (i) The alkali vapour density might not have been high enough to neutralize the americium ions by charge exchange collisions. However, with the below estimated rubidiumi vapour density of n =(5 ± 2) x 10 13/cm3 , an assumed nonresonant charge exchange cross section 16 cm2 and a mean relative velocity between ?CE = 10the rubidium atom and the americium ion, Vrel = 3 .8 x 104 cm/s, we obtain a neutralization rate AN qCE n Ure, = 190/s, which is about a factor of 4 larger than the decay rate of the 242mAm fission isomers . (ii) The optical cell could be contaminated with water and oxygen . The I-ii 20 and 02 molecules react with rubidium. However, the resulting molecules could considerably contribute to the relaxation of both the rubidium and americium polarization . (iii) It could be that during the slowing down or thereafter, stable americium-nitrogen compounds are formed. The existence of the compound AmN is well known [25], but the reaction dynamics have not yet been investigated to our knowledge . The experiments necessary to elucidate the possible reasons require substantial effort and are beyond the scope of our possibilities. In the discussion we ignore these possible effects which might cause a deterioration of the anisotropy signal . In the following we would like to show that our measured anisotropy signal will allow one to determine a iower limit of the nuclear spin of the americium fission isomer. For that purpose we present in fig . 12 results of calculations of the anisotropy parameters A 2 and A 4, which have been carried out on the basis of a simple spin exchange model described in the appendix. We see that the anisotropy signal depends on various parameters, the range of which has to be discussed: (i) the spin polarization P of the rubidium atoms; (ii) the spin exchange rate 1/TsE=nRb QSE Vrel between rubidium and americium (n Rb = density of rubidium atoms, QSE = spin exchange cross section, vre, = mean

Fig. 12. (a), (b) Calculated anisotropy parameters A Z and A 4 as a function of the ratio of the relaxation time 711 and the spin exchange time r SE. A parameter is the nuclear spin I of the americium fission isomer. A rubidium polarization P = 0 .6 was assumed . (c) Weighted sum according to A = 0.728A, 0 .215A 4 which incorporates the detection efficiency of our fission fragment counter. The figure shows also the calculated anisotropy for the nuclear spin 1= 4 at the rubidium polarization P = 0.7 and 0.5 (dashed lines) and the experimental value of the fission fragment anisotropy A = 0.124±0 .036 (horizontal dash-dotted lines) . relative velocity between rubidiu :n and americium atoms); (iii) the relaxation rate 1/TR=nN2 o'R Vrel of americium in nitrogen buffer gas collisions (nNZ= density of nitrogen molecules, vR = relaxation cross section, ure, = mean relative velocity between americium atoms and nitrogen molecules), and the nuclear spin I of the americium fission isomer. For probing the rubidium vapour density and the rubidium spin polarization we measured the attenuation of a weak cw laser diode test beam, traversing the optical cell, as a function of the wavelength in the vicinity of the D 1 transition [26] . The polarization has been obtained from the difference signal recorded for circularly and linearly polarized light. In this measurement the pulsed and circularly polarized pumping beam remained steadily on resonance of the D 1 transition . From the results of these measurements we conclude that in the americium cell we maintained a rubidium vapour density of (5 t 2) x 10 13 /cm 3 and a spin polarization P = 0.62 t 0.10. The spin exchange cross section between rubidium and americium has also not been measured . However,

H. Backe et aL / Fission fragment anisotropy of ` J2'"Am fission isomer

it is known from measurements at the chemical homolog europium that obviously the screening of the O and P electrons does not effect the spin exchange with the 4f electrons significantly [27]. Since the spin exchange cross sections of various alkalies are on the order of (1-2) x 10-14 cm2 [10] we assume QsE =(1.0 .5)x10-14 cm 2 and obtain ASE = (1 .8 t 1.2) x ±0 10 4/S or 'rSE = 56 ± 37 ws at 373 K. In order to get the ratio -r R/,rSE we need to know the relaxation cross section of americium in the nitrogen buffer gas. Relaxation cross sections of americium in buffer gases have not been measured. In ref. [28] the question was addressed whether the relaxation cross section can be estimated from measurements at the chemical homolog europium [14] and from the known wave function of the europium [29] and americium [30] atomic ground states . It was assumed that the relaxation is brought about by virtual electric dipole-dipole transitions [31] . The ratio of the cross sections QRAm/QREu = 5.6 can be calculated using (r 2 )Am/(r 2)Eu=2.0 [32] . For europium in neon a cross section QREu = 1.3 x 10 -20 cm2 was measured (extracted from the measurement displayed in fig. 5 of ref. [14]). Assuming that the polarization relaxes in nitrogen with a similar cross section we obtain o-RAum = 7.3 x 10 -20 cm 2. However, we would like to mention that it is not clear at all whether the straight-line approximation of the collision trajectory used in ref. [31] is a good approximation at such small cross sections. In addition even the relaxation mechanism is not known with certainty [28] . Therefore, the quoted relaxation cross section may be even much larger and we will consider o-RA. as a lower limit . The relaxation rate of the polarization of americium in nitrogen is AR >_ 1.9 x 10'/s or T R <_ 0.5 ms at a pressure of 25 mbar and an estimated temperature of 373 K. This yields together with the upper boundary defined by the error of the spin exchange rate TR/TSE < 28 . Together with the measured anisotronv A = 0.124 f 0.036 we finally conclude from fig. 12c that the nuclear spin of the Z4"Am fission isomer is greater than 1. This result is more or less expected since there are proton and neutron Nilsson orbitals close to the Fermi surface [33] (see also ref. [17]), which have enough angular momentum to couple in the odd-odd fission isomer 242mAm even to a spin as high as 7. 7. Conclusion and outlook It has been painted out in this paper that the nuclearspin of "'Am fission isomers can be polarized in a nitrogen buffer gas cell by spin exchange collisions with polarized rubidium atoms. A reduction of the fission countrate at 90° with respect to the quantization axis of (12 t 4)% has been observed experimentally.

529

This result suggests a nuclear spin of the 242m,&.M fission isomer ! > 1. In principle all requirements for hyperfine spectroscopy with polarization destructive methods are fulfilled . In practice such an experiment cannot be performed with the available experimental setup because unreasonably long beam times are demanded . However, inspecting fig. 12 we see that the significance of the anisotropy signal can still be improved by increasing the rubidium density and polarization employing e.g. a modern titanium-sapphire laser system . In addition the detection geometry for the fission fragments can be improved . Ideally, the whole solid angle should be covered by the detection system avoiding by this means the reference measurement with linearly polarized light. However, there remains to be solved the problem of the entrance foil instability under the influence of the rubidium vapour. This problem may require a considerable effort . In having this in mind we decided to pursue in the near future an alternative route to the optical hyperfine spectroscopy of fission isomers. This is the radioactive decay detected resonance ionization spectroscopy (RADRIS) method which relies also on optical spectroscopy it,, a bufer gas cell. However, the latter method has several advantages, among which are (i) that no rubidium is required in the optical cell, (ü) that it is not limited to americium, and (iii) that the signal is a normal countrate increase over an extremely :ow background level . With the RADRIS method, which has been developed in our laboratory for the ß-active 208 TI nuclide [21], a signal was observed recently at 242mArn [22] . Acknowledgements We are indebted to many persons of the MaxPlanck-Institut für Kernphysik at Heidelberg for their continuous support. In particular we would like to thank Prof. D. Habs and the accelerator group headed by Dr. R. Repnow. We would like to thank Dr. F. Ames and Dr. H. Rimke for their help in running the copper vapour dye laser system . Fruitful discussions with G. Huber, H.-J. Kluge and E.W. Otten are gratefully acknowledged . This work has been supported by the Bundesministerium für Forschung and Technologie under contract 06 MZ 188 I . Appendix: Calculation of the fission fragment anisotropy parameters ..ith a simple spin exchange model Assume that an americium atom is brought into an optical buffer gas cell with polarized rubidium . The spin polarization P of the rubidium vapour is P = VIII . ION TRAPS/LASERS

H. Backe et al. / Fission fragment anisotropy of 242 Am fission isomer

530

(s_)/s= 2(s, >, with (s,) the expectation value of the spin of the 5s electron with respect to the z' axis, and s = 2 . The expectation value ( F, ) = , (Iz ) are the expectation values of the z component of the electronic spin S = g and the nuclear spin I of americium, respectively . It is assumed that the electronic and nuclear spin systems of both rubidium and americium reach thermodynamic equilibrium . This equilibrium is characterized by distinct spin temperature parameters 13 and PA. for rubidium and americium, respectively, because of different relaxation rates. Neglecting the relaxation processes the maximum expectation value max/[I+(TSE/TR)\
Z) cotte [(S+ 21)PA .~ - 2 cotte[PAm/2]+

(A .4)

which is actually an equivalent for the electronic spin to eq . (VI .27) of ref. [10]. The occupation numbers of the nuclear Zeeinan levels follow as ptmt = jsinh(P Am/2) /sinh [ (1 + "21 ) PA.]) x exp(P A.M),

(A .5)

with which the anisotropy parameters AA can be calculated with eq . (2) . The anisotropy parameters A Z and 4 are plotted in figs. 12a and 12b assuming I = K at the saddle point. Information about the unknown nuclear spin I of the fission isomer can be obtained if the mentioned parameters are known. References

ill W. Lauth . Dissertation, Institut für Physik der Universität Mainz, 1991 (unpublished).

[2] W. Achenbach, Dissertation, Institut für Physik der Universität Mainz, 1986 (unpublished). [3] C .E. Bemis, J.R. Beene, J .P. Young and S.D. Kramer, Phys. Rev . Lett. 43 (1979) 1854. [4] W.M . Johnson, E .B . Shera, M .V. Hoehn, R .A . Naumann, J.D. Zumbro and C.E. Bemis, Phys. Lett. B161 (1985) 75. [5] J .R. Beene, C.E . Bemis, J .P . Young and S .D . Kramer, Hyperfine Interactions 9 (1981) 143. [6] J .R. Beene, C.E . Bemis, S .D . Kramer and J .P. Young, Proc. Conf. on Lasers in Nuclear Physics, Oak Ridge, TN, USA, 1982, eds. C .E. Bemis and H .K. Carter (Harwood Academic, London, New York, 1982) p. 171 . [7] G.A . Leander, Proc. Conf. on Lasers in Nuclear Physics, Oak Ridge, TN, USA, 1982, eds . C.E . Bemis and H.K . Carter (Harwood Academic, London, New York, 1982) p. 487. [8] H . Schweickert, H . Dietrich, R . Neugart and E.W . Otten, Nucl. Phys . A246 (1975) 187. [9] Ch. von Platen, J. Bonn, U. Köpf, R . Neugart and E.W. Otten, Z. Phys . 244 (1971) 44. [10] W . Happer, Rev. Mod . Phys. 44 (1972) 169. [ll] H. Backer, W . Achenbach, Th. Arndt, F . Ames, Th . Blönnigen, M . Dahlinger, D . Habs, M. Hain, M . Hies, I . Klaft, W . Lauth, H . Rimke, A . Scherrer, A . Steinhof, S . Tölg, N. Trautmann, J.B. Wilhelmy and S. Ziegler, Proc . 22nd Zakopane School on Physics, part 1 : Selected Topics in Nuclear Structure, Zakopane, Poland, 1988, p . 233. [12] R. Vandenbosch, Ann . Rev . Nucl. Sci . 2 7 (1977) 1 . [13] A. Kastler, in: New directions in Atomic physics, vol. II, eds. E .U . Condon and O . Sinanogu (Yale University, New Haven, London, 1972) p . 62 . [14] A . Sahm, J. Kowz!e!ci and G . zu Putlitz, Z. Phys . A281 (1977) 317 . [15] W. Lauth, Dip .omarbeit, Institut für Physik der Universität Mainz, 1985 (unpublished). [16] E . Luikkonen, V. Metag and G . Sletten, Nucl . Instr. and Meth . 125 (1975) 113. [17] V. Metag, D. Habs and H.J. Specht, Phys. Rep . 65 (1980) 1. [18] M . Hain, Diplomarbeit, Institut für Physik der Universität Mainz, 1984 (unpublished) . [19] A. Scherrer, Diplomarbeit, Institut für Physik der Universität Mainz, 1990 (unpublished). [20] J. Pauwels, J. van Craen, J . van Gestel and J . van Audenhove, Nucl. Instr . and Meth. 167 (1979) 109 . [21] W . Lauth, H . Backe, M . Dahlinger, I . Klaft, A. Scherrer, P . Schwamb, G . Schwickert, N . Trautmann and U . Othmer, in: Inst. Phys . Conf. Ser. n o 114 : Section 1, paper presented at RIS 90, Varese, Italy, 1990, p. 19 ; Phys. Rev. Lett., to be published. [22] H. Backe, Th . Blönnigen, U. Doppler, P . Graffe, D. Habs, M. Hies, Ch. Ilgner, H . Kunz, W . Lauth, H . Schdpe, P . Schwamb, W. Theobald and R. Zahn, First observation of a resonance ionization signal on 24zmAm fission isomers, paper presented at Int . Symp. Lasers in Nuclear Physics, RIKEN, 1991, to be published in Hyperfine Interactions. [23] L.M . Chanin and M.A. Biondi, Phys . Rev. 107 (1957) 1219 . [24] S. Ziegler, Diplomarbeit, Institut für Physik der Universität Mainz, 1986 (unpublished).

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VIII . ION TRAPS/LASERS