Triple focussing electron spectrum selector (TESS-II) with a pair of sector magnets

Nuclear Instruments and Methods 204 (1982) 101-108 North-Holland Publishing Company

TRIPLE FOCUSSING SECTOR MAGNETS

ELECTRON SPECTRUM

Y. N A G A I , H. F~.IRI, T. S H I B A T A , H . O H S U M I a n d Y. A D A C H I

K. O K A D A ,

101

SELECTOR (TESS-II) WITH A PAIR OF

S. N A K A Y A M A

*, H. S U Z U K I ,

Department of Physics, and Laboratory of Nuclear Studies, Osaka University, Toyonaka, Osaka, Japan H. SAKAI

Research Centerfor Nuclear Physics, Osaka University, Suita, Osaka, Japan Received 16 September 1981 and in revised form 26 April 1982

An achromatic geminate nuclear electron selector (AGNES) has been constructed for in-beam electron spectroscopy. It is essentially a pair of triple-focussing electron spectrum selectors (TESS). It consists of a pair of sector magnets with a field index n =0. Conversion electrons emitted at 90 ° and 180 ° with respect to the beam axis are transported achromatically through the pair of sector magnets to two focussing points. Electrons are triply focussed in radial, vertical and momentum axes, and their energies are analyzed by cooled Si(Li) detectors. It has a large solid angle of 50 m s r x 2 and a large momentum range of 57%. It is quite useful not only for measuring conversion coefficients and electron anisotropy but also for nuclear electron pairs.

1. Introduction I n - b e a m conversion electron spectroscopy c o m b i n e d with g a m m a - r a y spectroscopy has been playing a crucial role in d e t e r m i n i n g multipolarities of electromagnetic transitions in m e d i u m and heavy nuclei [1-3]. The recent advent of good Si(Li) detectors has m a d e it practical to use a magnetic field for transporting electrons a n d to analyze their energies by means of Si(Li) detectors [4-12]. Some groups developed miniorange type spectrometers [5] a n d others used solenoids [6-10] and sector magnets [4, 11, 12] to transport electrons. Ejiri et al. [4] have constructed a new type of electron spectrometer, TESS-I, for i n - b e a m e l e c t r o n - g a m m a spectroscopy. It uses a sector magnet with a field index n = 0 to focus electrons triply (radial, axial and m o m e n t u m axes) at a Si(Li) detector, where electron energies are analyzed. It is very powerful and suitable for in-beam e l e c t r o n - g a m m a spectroscopy with high precision [13,14]. The unique merits of the TESS-I are the following; (1) one gets very clean electron spectra because a proper baffle inserted at the intermediate d o u b l e focussing plane rejects b a c k g r o u n d s such as C o m p t o n scattered electrons; (2) electrons enter the Si(Li) detector perpendicular to the surface and one can get the full energy loss peak most effectively; a n d (3) * Permanent address: College of General Tokushima University, Tokushima, Japan.

Education,

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the small acceptance angle reduces the ambiguity due to the angular spread for anisotropically emitted electrons. T h e solid angle ( ~ 7 msr) and the m o m e n t u m range (AP/Pdv = 40%), however, are not large and one has to rotate the TESS-I a r o u n d the target point for the electron anisotropy measurement. The purpose of the present work is to design a n o t h e r type of triply focussing sector magnet, TESS-II. It has larger solid angle and larger momentum range than the TESS-1. A sector magnet of 280 ° deflection angle with intermediate radial focussing is used for the present TESS-It instead of the TESS-I magnet with intermediate double (radial and vertical) focussing. Thus the t a r g e t - m a g n e t distance in the TESS-II is m u c h shortened, resulting in a larger solid angle. A proper a r r a n g e m e n t of baffles inside the TESS-II has m a d e it possible to reject backg r o u n d electrons, which were cut off by the vertical slit of the TESS-I with the intermediate focussing property. M e a s u r e m e n t of the anisotropy of conversion electrons following nuclear reactions [2] is very i m p o r t a n t for getting the spin sequence. Thus a pair of magnets is used for simultaneous m e a s u r e m e n t of electrons emitted at 90 ° and 180 ° to the b e a m direction. The new TESS-It m a y be called A G N E S (Achromatic G e m i n a t e Nuclear Electron Selector). We proposed the design of the TESS-II in 1978, a n d the construction was approved at the program committee of the Research Center for Nuclear Physics, Osaka University. A similar achromatic m a g n e t system, ELSA [11], with enlarged m o m e n t u m

102

Y. Nagai et a L / Triplefocussing electron spectrum selector

b a n d appeared at the same time as we proposed this system. ELSA has a very large m o m e n t u m b a n d of A P / P - 140%, where AP=~ Pm,x- Pmi~, P = Pav but a small solid angle of 7.6 msr. The present TESS-11 ( A G N E S ) emphasizes the large solid angle of about 50 X 2 msr and simultaneous m e a s u r e m e n t of anisotropy of electrons. The m o m e n t u m b a n d of around A P / P - - 80 57% is not as large as for the ELSA, but is just adequate since low-energy &electrons below 100 keV have to be cut off anyhow. The m o m e n t u m b a n d is adjustable by a movable radial slit at the intermediate focal plane.

2. Design of the T E S S - I I ( A G N E S )

2.1. Basic' magnet design and electron orbit analysis The basic design of the TESS-II ( A G N E S ) is similar to that of the TESS-I except that the TESS-I gives a double focussing image at the intermediate plane while the present TESS-II gives a radial focussing image at that plane. We give briefly the focussing property of the TESS-II following a similar expression as used in the previous paper [4]. We use a symmetry magnet with field index n = 0 because of its simplicity of construction. Because the energy analysis is made by a cooled Si detector and the magnet itself is used to transport cleanly the electrons, we do not d e m a n d a very good

focussing property and rather buy the simplicity for the magnet. This meets the non-dispersive (achromatic) condition [4,15,16],

fs(,)h(,) d, =fA'(')h(')d'+fx')h(')d,=O, (1) where t is the distance from the source measured along the central orbit, h(t) is the curvature, and s(t) is the deviation from the central orbit. A and B stand for the u p s t r e a m a n d the d o w n s t r e a m magnet with respect to the symmetry plane of each of the magnets. If electron orbits for the first magnet A are focussed in the radial ( r ) plane a n d parallel in the vertical ( z ) plane, then the total system c o m b i n e d with A and B satisfies the triply focussing conditions. These conditions are written as [4,17] Li r P

L(1 + tan U t a n ~ 0 ) + tan ½0 L(tan½,~-tanU)-I

Li~ = L(1 - (tan U ) ½ 0 ) + ½~ -~, p Ltan U- 1

= o,

(2)

(3)

where P is the mean radius of the electron orbit, Lit and L~: are the distance of the radial image from the b o u n d a r y of the magnet A and that of the vertical one, respectively. L is the distance from the source to the entrance of the magnet A, U is the entrance angle and ~0 is the deflection angle of the magnet A. Then one

Fig. 1. Schematic top view of the triply focussing magnet of the TESS-II (AGNES) and the electron orbit trajectories. P: magnet poles, D: distance pieces, B: baffles to define entrance and exit electron orbits, S: slit at the intermediate radial focal plane to define the range of electron momentum, PB: pole boundaries, E: effective field boundaries, CL: field clamps, A: field clamp adjuster, H: heavy metal to shield the detector against y-ray background, C: carbon liner, CU: copper plate to absorb Pb X-rays, T: target and SI: silicon semiconductor detector.

Y. Nagai et al. / Triple focussing electron spectrum selector

gets L

-- t a n ½0

1

P

tan U t a n ½ 0 + 1

tan U"

(4)

Because of the symmetry condition, the source distance L ' from the b o u n d a r y of the m a g n e t B is L ' = L and the exit angle is the same as the entrance one U. The magnifications are M ~ - + 1 and M~ = -- 1. Since the magnet gap g is not m u c h smaller than p, one has to correct for the extended field (soft edge) effect. Thus the effective field b o u n d a r y has to be shifted somewhat far from the actual pole boundary. It is o b t a i n e d from the measured magnetic field. The effective angle U~ is approximately given for the clamped Rogowski shape as [18] U~ = U

[O.35g(l+sin2U)/(pcosU)]

X [1 -- 1.5 g ( t a n U)/O].

(5)

T h e solid angle f~ is then given as = Scos U~/L 2,

(6)

where S is the area of the e n t r a n c e slit at the pole b o u n d a r y . The values L, f~ and U~ are calculated for the case of g/o : 0.4. They are plotted as a function of 0 in fig. 2. The values for the TESS-I are also shown for comparison. It is clear in fig. 2 that the solid angle of the TESS-II with the smaller entrance angle is larger

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t h a n that of the TESS-I with the larger entrance angle. N o requirement of the vertical focussing at the intermediate plane reduces the distance of the source to the e n t r a n c e of the magnet and that of the focussing point to the exit of the magnet. Thus the TESS-II gives a larger solid angle c o m p a r e d to that of the TESS-I with the vertical focussing at the intermediate plane. In the present case we adopt the value q, = 280 ° as previously, and thus the actual entrance and exit angles are b o t h U = 39 °, and the source distance is L / p = 1.7. p is 70 m m and g is 28 mm. A constant curvature (circle) of the pole b o u n d a r y is a d o p t e d instead of two straight lines of a c o n s t a n t exit angle for convenience of construction and for better m o m e n t u m focussing as in ref. 11. In practice the solid angle is defined by the baffle used to reject 6-electrons as discussed in sec. 3. The actual electron orbit analysis was carried out by using the c o m p u t e r code " R A Y T R A C E " [19]. The fringing field was measured as shown in fig. 3 a n d was t a k e n into account in the ray-trace analysis. The electron orbits and the basic c o m p o n e n t s of the TESS-II are shown in fig. 1.

2.2. Coils and magnets A r r a n g e m e n t s of the coils are shown in figs. 3 and 4. A m a x i m u m magnetic field of 0.57 T can be obtained at the m a x i m u m current of 100 A and electrons up to 15.5 MeV can be measured. As discussed in sec. 2.1 the fringing field extends to some extent b e y o n d the magnet pole boundary. It depends on the current at the fixed position of the field clamp (see fig. 2). The position of the field clamp can be adjusted by means of the screw A (see fig. 1) to give the same field shape when the current is high. By reversing the current of one of the two pairs of coils, positrons can be measured in that magnet,

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104

Y. Nagai et aL / Triple focussing electron spectrum selector

COLD FIN( Si(Li) S S D FIELD CLAt~ 'OKE BEAI

MAGNET-~

Fig. 4. Top view of the TESS-II (AGNES) and the Ge(Li) detector. Electrons emitted at both 90 ° and 180 ° to the beam direction are detected. Here only electron orbits emitted at 180 ° are shown. The Ge(Li) detector is set at 45 ° to the beam direction.

while electrons are measured in the other. Thus the magnet pair can be used for the measurement of elect r o n - p o s i t r o n pairs. 2.3. Vacuum system

A sorption p u m p for rough p u m p i n g and a 120 1 ion p u m p for keeping the system in good clean vacuum are used to get oil-free circumstances for the cooled Si(Li) detector. Two liquid nitrogen cold traps are additionally used to get rid of vapours from the upstream- and d o w n s t r e a m - b e a m tubes of the A G N E S . The pressure inside the A G N E S is 4 X 10 7 Torr. A top view of the total system is shown in fig. 4. 2.4. Detectors for electrons and gamma-rays

A small Si(Li) detector with a diameter of I I m m a n d a depth of 2 m m is used to analyze the electron energy. The detector is cooled by liquid nitrogen through a cold finger. The surface of the detector is covered with a thin aluminized Mylar window to reject low energy &rays (atomic electrons) and to avoid vapor deposition on the detector surface. A n overall resolution of 2.6 keV

for 666 keV electrons is obtained for an in-beam experiment. As the Si(Li) detectors are close to the target, a heavy metal of 6 cm thickness is used between them to attenuate direct y-rays from the target. The surface of the heavy metal is covered with a 1 m m thick Cu plate to attenuate X-rays of the heavy metaL. A gamma-ray is set at 45 ° to the beam.

3. Performance The focussing properties of the A G N E S were investigated by using conversion electrons from radioactive sources of 88y and 169yb. These isotopes were produced in-beam at the b e a m spot of the target for the A G N E S . The radial- and momentum-focussing properties were checked by moving the Si(Li) detector in the m e d i a n plane (z = 0) a r o u n d the calculated focussing point. The axial focussing property was checked by observing the electron yield at the calculated focussing point ( r = 0 , z = 0 ) as a function of the height of the source from the median plane. They are shown in figs. 5a and b. The instrument focussing is checked by observing the intensity of the 388 keV K-conversion elec-

105

Y. Nagai et al. / Triple focussing electron spectrum selector I

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1

I

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5

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0

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1

I

5

i0

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Fig. 5. Radia] and axial focussing at the final image. (a) Electron yield as a function of the distance (r, z - 0 ) between the Si(Li) detector and the calculated focussing point (r =0, z =0) (radial focussing) in the median plane. (b) Electron yield at the calculated focussing point as a function of the height (z) of the radioactive source from the target position (z --0) (axial focussing).

tron in SSy as a function of the strength of the magnetic field of the A G N E S and it is shown in fig. 6. As shown in fig. 6b, the spectrometer has a constant detection efficiency over the m o m e n t u m range of AP/Pav = 57%. The A G N E S is set at the F beam line of the Research Center for Nuclear Physics, Osaka University. A 2 m m ~ spot of the beam is obtained at the target position. For the in-beam electron study we used narrow slits to reduce 8-electrons. We also used baffles between the entrance slit and the target that cut off the low-energy electrons with non-normal orbits. The m o m e n t u m range with a constant detection efficiency was AP/Pav = 44% as shown in fig. 6a. This was measured by using 311 keV K-conversion electrons in |74Hf produced by the 174Yb(c~, 4n) reaction. Since 6-electron have huge cross-sections it is hard to detect low energy conversion electrons in case of in-beam measurements. As the high energy 6-electrons are strongly forward-peaked, low energy electron spectra at backward angles are cleaner than those at 90 ° . Using the above mentioned baffle system and narrow slit together with the enlarged solid angle, we got rather clean low energy spectra even by the present TESS-II without the intermediate vertical focussing. In-beam conversion electron and gamma-ray spectra following 50 MeV alpha particle b o m b a r d m e n t of the 139La target are shown in fig. 7. They are measured,

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106

Y. Nagai et al. / Tmple focussing electron spectrum selector

respectively, at 90 ° and 45 ° with respect to the beam• The measured spectra demonstrate fairly clean with a good signal to background ratio for in-beam experiments. The continuum c o m p o n e n t of the electron spectrum is mostly due to real internal conversion electrons.

The background tails due to direct y-rays from the target and the non-full-energy-loss of the incident electrons in the Si detector are not severe as illustrated by the yield (see A in fig. 7a) outside the selected electron m o m e n t u m window. Since the electron enters the Si

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Fig. 7. In-beam conversion e]ectron (a) and 7-ray (h) spectra following 50 MeV alpha particle bombardment of the 139Ea target. The assignments in the spectra are from ref. 20. The conversion electron spectra were taken at two magnet settings, one below channel 450 and the other above that.

Y. Nagai et al. / Triple focussing electron spectrum selector detector perpendicular to the surface as in TESS-I, the full-energy loss peak is as large as 90%. Signals produced by ,/-rays from the target a n d external conversion electrons p r o d u c e d by y-rays hitting the surface of the magnet pole give negligible contributions to the backgrounds. They were respectively checked by turning off the magnet current a n d by putting an a l u m i n u u m plate of 2 m m thickness between the target and the entrance of the magnet applying the magnet current.

4. Summary and concluding remarks A new type of triple focussing electron spectrometer, TESS-II, was designed and constructed. It works quite well for in-beam experiments as shown in fig. 7a. It consists of a pair of achromatic magnets and cooled Si(Li) detectors. The specifications of the TESS-II are summarized in table 1. In addition to the specifications the TESS-II can be characterized as follows: (1) It gives fairly clean electron spectra for in-beam measurements. The spectra are free from atomic electrons, low energy electrons produced by scattered y-rays a n d direct 3,-rays from the target. Thus the TESS-II m a g n e t selects internal conversion electrons by filtering out the various backgrounds. Because we select the necessary m o m e n t u m b a n d by the adjustable slit in the intermediate focal plane, we can get rid of the t r e m e n d o u s low-energy 8-ray background. Most of this is rejected by the slit. (2) One can measure simultaneously conversion electrons emitted at 90 ° and 180 ° , thus o b t a i n i n g the electron anisotropy. (3) It can be used for detection of low energy elec-

Table 1 Specification of the TESS-It (AGNES). Focussing properties

Triple in radial, axial and momentum axes at the detector; radial focussing at the intermediate focus

Field index Mean radius Air gap Source distance (image) Entrance angle (exit) Deflection angle Magnification Solid angle Momentum range AP/Pdv Maximum field Maximum electron energy Vacuum

0 70 mm 28 mm 119 mm 39 ° 280 ° M~=+I, M:--l 50 msr X 2 57 80% 5.7 kG 15.5 MeV 4 × 10 7 Torr

107

trons with a good peak-to-background ratio at the backward direction for in-beam experiments due to the strong forward peaking of high energy 8-electrons. (4) A definite small angular spread of the magnets leads to d e t e r m i n a t i o n s of internal conversion coefficients with little ambiguity for the effect of the angular distribution for i n - b e a m experiments. The a p p a r a t u s can thus be used for in-beam electron spectroscopy. E l e c t r o n - p o s i t r o n pairs following E0 transitions may also be measured. The good timing signal from the thin Si(Li) detector gives us more inform a t i o n on the lifetime of the nuclear excited state. We are greatly indebted to the late Prof. S. Y a m a b e for his continuous encouragement. Fruitful discussions with Prof. S. M o r i - o b u a b o u t the " R A Y T R A C E " program and with Dr. C. Kim a b o u t the Si detector are warmly acknowledged. We wish to give our sincere t h a n k s to Profs. M. K o n d o and A. Shimizu for their e n c o u r a g e m e n t and the R C N P cyclotron crew for their nice operation of the machine. One of the authors (S.N.) was a Yukawa fellow Osaka University in 1980. He is very grateful for the financial support. A part of the present test m e a s u r e m e n t was performed by using the cyclotron b e a m time of the Research Center for Nuclear Physics, Osaka University, u n d e r program n u m b e r s 7A17, 8A14 and 10A13.

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I~ Nagai et aL / Triple focussing electron spectrum selector

[10] H. Backe, L. Richter, R. Willwater, E. Kankeleit, E. Kuphal and Y. Nakayama, Z. Physik A285 (1978) 159. [11] M. Komma, Nucl. Instr. and Meth. 154 (1978) 271. [12] Y. Gono, U.S.A.E.C. Report ORO-4322-8 (1978); A. Cambi, T.F. Fazzini, A. Giannatiempo and P.R. Maurenzig, Nucl. Instr. and Meth. 103 (1972) 331. [13] H. Ejiri, Phys. Scripta (1981), Proc. Nobel Syrup. on High spin nuclei, Lund (1980). [14] Y. Nagai, T. Shibata, S. Nakayama and H. Ejiri, Nucl. Phys. A282 (1977) 29; S. Nakayama, T. Kishimoto. T. Itahashi, T. Shibata and H. Ejiri, J. Phys. Soc. Japan 45 (1978) 740; H. Suzuki, T. Itahashi, T. Shibata, Y. Nagai, T. Kishimoto and H. Ejiri, Nucl. Phys. A311 (1978) 477.

[15] H.A. Enge, Rev. Sci. Instr. 35 (1964) 278. [16] R.A. Alvarez, K.L. Brown, W.K.H. Panofsky and C.T. Rockhold, Rev. Sci. Instr. 31 (1960) 556. [17] J.J. Livingood, The optics of dipole magnets (Academic, New York and London, 1966) and references there in. [181 H.A. Enge, Deflecting magnets in focusing of charged particles, ed., A. Septier (Academic, New York, 1967) vol. 2, ch. 4.2. [19] H.A. Enge, private communication (1974); M. Fujiwara and S. Morinobu, private communication (1975). [20] M. Piiparinen, M. Kortelahti, A. Pakkanen, T. Komppa and R. Komu, Nucl. Phys. A342 (1980) 57.