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Wien filter for cooled low-energy radioactive ion beams
Nuclear Instruments and Methods in Physics Research A 481 (2002) 718–730
Wien filter for cooled low-energy radioactive ion beams S. Nummelaa,*, P. Dendoovena, P. Heikkinena, J. Huikaria, A. Nieminena, . o. a,b A. Jokinena,b, S. Rinta-Antilaa, V. Rubchenyaa, J. Ayst a
Department of Physics, University of Jyvaskyl a, a, . . P.O. Box 35, Jyvaskyl . . Finland b CERN, CH-1211 Geneva, Switzerland Received 5 June 2001
Abstract A Wien filter for cooled radioactive ion beams has been designed at Ion Guide Isotope Separator On Line technique (IGISOL). The purpose of such device is to eliminate doubly charged ions from the mass separated singly charged ions, based on q ¼ þ2-q ¼ þ1 charge exchange process in an ion cooler. The performance of the Wien filter has been tested off-line with a discharge ion source as well as on-line with a radioactive beam. The electron capture process of cooled q ¼ 2þ ions has been investigated in a radiofrequency quadrupole ion cooler with varying partial pressures of nitrogen. Also, the superasymmetric fission production yields of 68oAo78 nuclei have been deduced. r 2002 Published by Elsevier Science B.V. PACS: 25.85.Ge; 28.60.+s; 82.30.Fi Keywords: Wien filter; Radioactive ion beams
1. Introduction The study of exotic nuclei is usually dealing with low production yields of nuclei of interest. This limitation requires a constant development of efficient production methods and detection techniques that are free from background radiation and contaminants. A standard method related to the production of exotic nuclei is mass separation, which together with more sophisticated methods, such as ion trapping, enables to pick a special weak channel out of numerous *Corresponding author. E-mail address: [email protected].fi (S. Nummela).
stronger channels in a nuclear reaction. One example of first on-line mass separators is the Ion Guide Isotope Separator On Line technique (IGISOL) [1] which has been successfully applied in the discovery of many new isotopes [2]. The IGISOL technique is based on thermalization of reaction products in a helium gas and the transportation of the resulting ions with the helium flow and by an electric field through a skimmer system. The reaction products are accelerated and finally mass separated according to their mass over charge ratio. This method is very fast, enabling the study of a large range of rare short-lived (sub-ms) isotopes in a chemically insensitive way.
0168-9002/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 3 6 2 - 6
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From a nuclear structure point of view, one of the nuclear regions under a great deal of interest is the neighbourhood of the doubly magic nucleus 78 Ni. It has been proposed that the superasymmetric fission mode would provide an access to produce nuclei in the region in question. In order to investigate this matter, yields of fission products of 68oAo78 have been measured at IGISOL [3]. A severe drawback so far, concerning measurements at IGISOL, has been the presence of doubly charged ions exiting from the ion guide. While the reaction products are stopped in helium, most of them are extracted from the ion guide as q ¼ 1þ ions but a small fraction remains as q ¼ 2 þ : Thus, with a given q=m ratio two masses, m ¼ A or m ¼ 2A; are transported through the separator corresponding to q ¼ 1þ and q ¼ 2þ ions, respectively. If the production rate for 2A is much higher than for nuclei with mass A, the q ¼ 2þ ions dominate the mass separated beam and may severely harm the spectroscopic study of the weakly produced exotic nuclei separated as q ¼ 1þ ions. This is the case in superasymmetric fission where the cross sections for AE70 nuclei are 103 times smaller than for AE140; resulting in a beam which is dominated by the latter. To overcome the problem of the doubly charged ions we have investigated the possibility to lower the charge state of the q ¼ 2þ ions into q ¼ 1þ and further separate the ions owing to m ¼ A and m ¼ 2A with a velocity filter. The electron capture and cooling would take place in an ion cooler [4], in which the ions interact with helium atoms and carefully chosen impurity molecules. After the second acceleration stage the different masses with equal charge, q ¼ 1þ; will have different velocities, so that the second mass separation is possible with a Wien filter. The goal was to design a compact and an easy-to-use device. The principle of the Wien velocity filter is well known since the beginning of 1900s and it has been adapted to numerous applications both in scientific research and commercial use. In this paper we will first describe the design and characteristics of the Wien filter. Then, the results of the ion cooler measurements will be presented as well as the superasymmetric fission yields for 68oAo78 nuclei.
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2. Wien filter design 2.1. Principle A Wien filter is a device which separates ions according to their velocity. The filter employs mutually perpendicular electric and magnetic fields, E and B; which set opposite forces on charged particles passing through the filter. When the magnetic force and electric force are equal, i.e. qvB ¼ qE
ð1Þ
ions with velocity v ¼ E=B pass the filter undeflected. In the case of a cooled and reaccelerated (acceleration voltage U) mass-separated radioactive beam at IGISOL the velocity of ions, with mass m and charge q; is given as rffiffiffiffiffiffiffiffiffi 2qU v¼ ð2Þ m and therefore the separation of different velocities with equal charge correspond to separation of masses. The dispersion between masses m and (m Dm) in the Wien filter can be written as laEDm ð3Þ D¼ 4Um where l is the drift distance from the centre of the filter to the slit (302 mm), a the length of the filter (50.5 mm), E the electric field in V/m, U and Dm=m corresponds to the mass dispersion. In our case the magnetic field B is constant and the electric field follows the relation E ¼ vB: Hence, it is useful to write the dispersion as rffiffiffiffiffiffiffiffiffiffiffi laDm 2q D¼ B ð4Þ 4m mU where q is the charge and m the mass of the ion with zero deflection in the filter and B the magnetic field in T. The dispersion increases linearly with the magnetic field and inversely with the square root of the acceleration voltage. The design of the IGISOL Wien filter started with the main requirement that the singly charged ions of mass 70oAo80 should be separated efficiently from their originally doubly charged counterparts of mass 140oAo160: In other words, the latter, after changing charge state from
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2+-1+, should be sufficiently deflected in the Wien filter in order to be stopped and removed by a following slit. The design was already restricted by some existing boundary conditions. First, the reacceleration voltage of the cooler is set as 40 kV, defining the velocity of ions. Secondly, the size of the beam exiting the cooler is approximately 2 mm as FWHM, defining the minimum amount of dispersion for adequate mass separation. Thirdly, the dimensions of the beam line after the cooler leave space of only about 40 cm for the Wien filter and the drift distance before the slit. Therefore, the proper amount of dispersion is to be established only by the strengths of the electric and magnetic fields and the effective length of the Wien filter. Keeping in mind these constraints, the design of the Wien filter was carried out by calculating the magnetic and electric fields with a 2d-code Poisson Superfish [5]. A schematic view of the IGISOL beam line and the location of the Wien filter is given in Fig. 1.
magnets available, with remanence of Br ¼ 1:12 T and coercivity of B HC ¼ 850 kA/m. The blocks would be placed one upon the other, so that the minimum magnet gap, h; is defined by the space required by the beam and the electrodes. On the other hand, the magnets should be close enough in order to create a magnetic field sufficiently strong for adequate beam deflection. In order to maximise the field, the return flux will be guided in an iron yoke, schematically shown in Fig. 2. Next we shall give a simple formula for the magnetic field in the middle of the magnet. For homogeneously magnetised magnets we can assume a surface current density M around the magnet, M being also the magnetisation. Hence, by replacing the magnet by a surface current and assuming very high permeability iron (H ¼ 0), we can write Ampe" re’s law as I ~d~ ð2d þ hÞH ¼ H l ¼ NI ¼ 2dM ð5Þ
2.2. Magnetic field
H¼
In order to keep the operation of the Wien filter as simple as possible, we considered a choice of permanent magnets for introducing the magnetic field. We had 2 2 0.5 inch pieces of Neorem
where h and d correspond to the gap between the magnets and the magnet thickness, respectively. In permanent magnets the magnetisation M depends weakly upon the magnetic field strength H: For
2d M 2d þ h
Fig. 1. Schematic view of IGISOL beam line used in this work.
ð6Þ
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gives a fairly good approximation for the gap field for a dipole magnet constructed as in Fig. 2. The calculated vertical transverse magnetic field, perpendicular to the beam, is shown in Fig. 3 (left), both with an iron yoke (solid line) and without the iron yoke (dotted line). With a proper yoke definition, also the field in the longitudinal direction can be calculated. The calculated shape of the longitudinal magnetic field parallel to the beam direction is shown on the right hand side of Fig. 3 as a solid line. Fig. 2. Schematic drawing of the Wien filter magnet. The permanent magnets are connected with an iron yoke. For the available neomagnets h ¼ 14:6 mm and d ¼ 12:7 mm.
neomagnets, the magnetisation curve close to remanence goes approximately as M ¼ Mr þ 0:05HM
ð7Þ
where the remanence is given as m0 Mr ¼ Br and HM is the magnetic field strength inside the permanent magnet. Since in the case of permanent magnets the closed integral in Ampe" re’s law results in zero, HM is negative and in our simple geometry (assuming H inside the iron yoke to be zero) it gives 2dHM ¼ hH
ð8Þ
h ð9Þ HM ¼ H: 2d Therefore, we can write 2d h H¼ ðMr þ 0:05HM Þ ¼ Mr 0:05 H ð10Þ 2d þ H 2d 1 2d h 1 þ 0:05 H¼ Mr ð11Þ 2d þ h ð2d þ hÞ or B¼
1 2d h 1 þ 0:05 Br : 2d þ h 2d þ h
ð12Þ
With the dimensions of the present neomagnets and the iron yoke, and neglecting the correction term from the magnetisation curve, the magnetic field inside the gap of the Wien filter results in 0.71 T. A numerical calculation with the Poisson code, with realistic iron permeability, gives 0.663 T in the middle of the magnet gap. Eq. (12) therefore
2.3. Electric field Due to a permanent magnetic field in the Wien filter, the parameter to be tuned is the electric field. With the calculated magnetic field of B ¼ 0:66 T, the operating region for undeflected ions of A ¼ 70 and A ¼ 80 gives a corresponding electric field of 2.19 and 2.05 kV/cm, respectively, which is evidently applicable in the vacuum of 106 mbar. A sufficiently large volume of homogeneous electric field needs to be created between the magnetic poles of the filter. Two common ways of making a homogenous electric field between grounded magnet poles are U-shaped electrodes (see Fig. 4) or by evenly spaced strips between the electrode plate edges. We chose to use a configuration with nine strips. The strips are connected to each other in series with resistors. The 1 mm strip width and 2 mm spacing is enough to produce a homogenous field over a 20 10 mm2 cross-section, which is large enough for a beam of 2 mm width. The strips were realised by using a circuit board, on which 1 mm wide copper strips were introduced. The effect of the strips on the electric field shape is demonstrated in Fig. 5, where the calculated equipotential lines are shown both for the configuration with 9 strips (left) and without strips (right). The distance of the electrodes in both cases is 25 mm, with an applied 5.2 kV voltage difference (72.6 kV in the electrodes). The electric field in vertical transverse direction is plotted on the left in Fig. 6, so that the solid line corresponds to the configuration with 9 strips, and the dotted line without strips. As it has been demonstrated, the principle of the Wien filter lies on the ratio E=B: Therefore, the
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Fig. 3. Calculated magnetic field perpendicular (solid line on the left) and parallel (solid line on the right) to the beam direction with a magnet gap of 14.6 mm, including a yoke. The dotted line corresponds to a calculation without a yoke and the circles correspond to measured magnetic field values.
Fig. 4. Electric field (equipotential lines) with U-shaped electrodes. Only 14 of the geometry is shown due to symmetries.
same ratio should hold along the total effective field length. In our case, this condition is carried out by shaping the electric field along the beam direction to correspond to the shape of the magnetic field. The longitudinal extent and the shape of the electric field are controlled by
the length of the electrodes and by a grounded slit outside the gap in both ends. Therefore, a slit, with a 10 mm aperture at 5 mm distance from the edges of the magnets, was included in the simulations where the electrode length was set to 40 mm. The resulting electric field shaped along the beam direction is shown on the right side of Fig. 6 as a solid line. Correspondingly, the magnetic field is shown as dashed line. The maxima of the electric and magnetic fields are set in the figure at an equal level, thus demonstrating the constant ratio E=B over the whole extent of the fields, which assures proper functioning of the filter. As a comparison, the electric field without the slit is shown as a dotted line. A sketch of the Wien filter design is shown in Fig. 7. The filter is mounted inside a 100 mm beam tube and is positioned by adjustable screws.
3. Characteristics The performance of the Wien filter was first tested off-line. Before installing the device in the
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Fig. 5. Electric field (equipotential lines) for the Wien filter with strips (left) and without field shaping strips (right). Also the grounded magnet surface is shown. Only 14 of the geometry is shown due to symmetries.
Fig. 6. Left: Calculated electric field in vertical transverse direction with 9 strips (solid line) and without strips (dotted line). Right: Electric field in longitudinal direction with a slit (solid line) and without a slit (dash line). As a comparison, the magnetic field is shown as dotted line.
beam line the magnetic field was measured directly in the direction of the beam and compared with the calculated simulations. The measured magnetic field is plotted in Fig. 3 as circles, and demonstrates that the simulation with Poisson Superfish mimics the real situation extremely well.
Off-line tests were performed by using a discharge ion source, which is placed inside the ion guide, replacing the target used in on-line experiments. A selection of stable beams, such as 176–180 Hf, can be produced by choosing a suitable discharge electrode material, which are used for
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Fig. 8. Intensity of 180Hf ion beam measured by a Faraday cup after the Wien filter as a function of electrode voltage difference.
Fig. 7. Technical drawing of the Wien filter.
various purposes in testing the separator and beam line conditions. Since the measured magnetic field of the Wien filter was very close to the calculated one, we tested off-line how well the calculated electrode voltage for a specific undeflected mass reproduces the actual voltage used. For this we used a beam of 180Hf ions from the separator. The electrodes were connected to a high voltage supply, which gave two opposite signed outputs. The electrode voltage difference was scanned from 3400 to 4000 V and the outcoming beam current was measured by a Faraday cup after the Wien filter. The slit was kept at 2 mm aperture in order to avoid the beam hitting an XY-deflector before the Faraday cup and causing the deflected beam to scatter on the Faraday cup. The measured current as a function of the electrode voltage is shown in Fig. 8. The maximum of the current corresponds to electrode voltage difference of 3650 V whereas a second maximum appears 250 V higher. The calculated value for an undeflected mass of 180 gives 3420 V. The reason for the difference between the calculated and measured value is that the beam from the quadrupole deflector, after the cooler, is entering the Wien filter slightly off the symmetry axis. Therefore, a slight increase of the electrode voltage
is needed to produce the same deflection as was anticipated by the calculations. The asymmetric shape of the current curve can be explained by an asymmetric shape of the beam exiting the quadrupole deflector after the cooler. A more precise measurement of the transverse profile of the outcoming beam from the Wien filter was performed with 180Hf ions. The beam profile was obtained by measuring the outcoming beam current from a single slit at 35 cm distance which was scanned through in horizontal direction. The measured beam current was differentiated which gave the actual beam current profile. This resulted in a FWHM of 2.1 mm for the beam. The shape of the beam is shown in Fig. 9. The profile is slightly asymmetric, which was already observed by scanning the voltage. If the beam profile is fitted with two Gaussian peaks, the distance of the peak maxima is 1.5 mm. This distance corresponds to the effect of about 250 V difference in the electrode voltage difference, which was already observed in Fig. 8. A beam width of 2 mm means that the Wien filter can separate masses which are deflected more than 2 mm at the position of the slit. In the case of AE70 nuclei this corresponds to 7 m=q units. Therefore, the Wien filter can be used to separate the doubly charged ions from the single charged.
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electron from the neutral gas atom or molecule. No universal theory exists which would describe the charge exchange process for qp4þ ions in gases but numerous experimental data exist concentrating on restricted parameters and elements. A survey of the literature concerning experimental data and some specified theories is given in Ref. [8]. In general, the charge transfer reaction between an ion R with charge state qþ and a neutral helium atom can be written as Rqþ þ He-Rðq1Þþ þ Heþ þ DE;
Fig. 9. Current profile of the movable slit after the cooler.
180
Hf beam measured on a
4. Results 4.1. Charge exchange tests with the cooler An essential role related to the function of the Wien filter is played by the ion cooler and the charge exchange processes taking place in the buffer gas of the cooler. Therefore, we examined the q ¼ 2 þ -q ¼ 1þ charge exchange process inside the cooler both with mass separated reaction products and stable ions. Originally, the cooler has been designed to improve the ion optical properties of the IGISOL beam. This is necessary for collinear laser spectroscopy [6] as well for efficient injection into a Penning trap [7] soon to be constructed in conjunction with the IGISOL beam line. In addition, the cooler can be used as a buncher, in which ions can be stored for a definite time and released in short bunches. However, as the idea of the Wien filter at IGISOL was created, the use of the cooler was extended to operate as a charge exchange device. The cooler is a radiofrequency quadrupole in which ions are cooled down to thermal energies in collisions with the buffer gas. The cooler lies at a 40 kV platform, so that when the ions enter the cooler they are decelerated, cooled and eventually accelerated with a narrower energy spread. In specific conditions, depending on the buffer gas and the velocity of the ions, charge exchange processes take place in which an ion captures an
ð13Þ
where the energy defect, DE; is the difference in ionisation potentials of R and He. It has been shown that the cross section for electron capture below energies of 25 keV/u is in most cases almost independent on the kinetic energy of the ion. In addition, electron loss measurements have shown that the charge state q ¼ 1þ is dominating in helium in this velocity region, just what can be observed for the reaction products stopped in helium in the IGISOL ion guide. However, for the remaining small fraction of the q ¼ 2þ there exists no explicit theory how the electron capture processes happen when the ions are cooled in helium gas. There are studies concerning the reactions for doubly charged ions at near thermal energies in different gases [9] and the results can be partly adapted to our case where we have ions of Eo1 eV inside the cooler. These studies show how the electron capture rate coefficient of a doubly charged ion reaches its maximum at specific internuclear separation RX, where RX ¼ 14:4=DE ( As a conclusion, only the exoergicity in units of A. (DE > 0), and not the nature of the ion, determines the rate of the electron capture reaction. Also, high reactivities are observed nearly always when the neutral reactants are in molecular form. As the first ionisation potential of helium (24.48 eV) is higher than the second ionisation potential of most of the elements (i.e. DEo0), the electron capture of a q ¼ 2þ ion in helium seems improbable. However, if a small amount of impurity molecules are mixed in helium, so that the first ionisation potential of the impurity molecule is between the first and the second ionisation potential of the doubly charged ion, the electron capture becomes possible. The choice of such an impurity molecule
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is a delicate issue since the created ions may induce some more complicated effects in the charge exchange process. The survival of q ¼ 1þ and q ¼ 2þ ions in the cooler was studied on-line with radioactive nuclei, produced in a 30 MeV proton induced fission. The mass separated radioactive ions were cooled, deflected with the Wien filter and collected on a movable tape. The yield of ions was determined by observing beta delayed g rays, detected with a Ge detector. The goal was to study how nitrogen (N2) impurity molecules affect the electron capture process of chemically dissimilar radioactive ions. In the first measurement the yield of 112Rh ions, as a function of nitrogen partial pressure, was measured for three different cases. First, q ¼ 1þ ions were mass separated and passed through the Wien filter. Secondly, 112Rh ions corresponding to q ¼ 2þ charge state were mass separated and selected through the Wien filter with a voltage twice as high compared to the q ¼ 1þ ions. Thirdly, the Wien filter voltage was adjusted for the q ¼ 1þ ions which had lowered their charge from 2+ to 1+ in the cooler. Thus, the survival of 2+ and 1+ ions could be studied independently. Small amounts of nitrogen gas (N2) were mixed in the He buffer gas so that the total pressure was kept as 4.0 mbar. The pressure was measured from the gas injection line, so that the pressure inside the cooler is approximately 20 times less than the measured value, based on gas flow calculations. The results are shown in Fig. 10 where the yield maxima in each case are normalised as 1. It can be seen that the intensity of the 112Rh ions, undergoing no charge exchange processes, decreases with increasing N2 concentration. However, the probability of charge exchange from q ¼ 2þ into q ¼ 1þ ions reaches a maximum at about 3% nitrogen concentration. The behaviour was similar for lower total buffer gas pressures. A similar measurement, testing the influence of nitrogen partial pressure on the charge exchange process, was done for 72Cu (q ¼ 1þ) and the corresponding doubly charged counterparts 144Ba and 144La (q ¼ 2þ). The total pressure of He was kept at 4.3 mbar. The yields of these ions were measured simultaneously, since they are extracted with identical separator magnet and Wien filter
Fig. 10. Yields of 112Rh as a function of nitrogen partial pressure in the cooler for three different situations of charge transfer: q ¼ 1 þ -q ¼ 1þ (squares), q ¼ 2 þ -q ¼ 2þ (circles) and q ¼ 2 þ -q ¼ 1þ (triangles).
settings. The results are presented in Fig. 11 as an intensity ratio of 72Cu/144Ba and 72Cu/144La in order to highlight the effect of a charge transfer and the possible loss of 2+ ions. The fraction of 72 Cu is increasing in the very low partial pressures but seems to saturate very quickly. These results will be discussed later based on ionisation potential arguments.
Fig. 11. Yield ratios of 72Cu/144Ba and 72Cu/144La measured after the Wien filter as a function of nitrogen partial pressure in the cooler.
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4.2. Fission yields The yields of highly asymmetric fission products of 68oAo78 were measured for 30 MeV proton induced fission of 238U at IGISOL [10] with a proton-beam current of 4 mA. The mass separated reaction products were collected onto a tape, which was moved periodically. The pulsed tape movement ensured that the most exotic short-lived nuclei were observed efficiently whereas the activity originating from longer lived daughter nuclei was reduced. The tape movement was adjusted for each mass according to the half-lives in question. In order to measure the yield also for the longer lived daughter nuclei at saturation, a second measurement without tape movement was performed. The collection point was surrounded by a cylindrical plastic 2 mm thick detector with beta detection efficiency of 70%. Three Ge detectors (Nordball-type) in close geometry around the beta detector were used to detect grays in coincidence with betas. In order to prevent a large fraction of the b’s entering the Ge detectors, a 15 mm Plexiglass absorber was introduced between the beta detector and the Ge detectors. The efficiency of the ion guide was checked before each change of mass by observing the yield of 112Rh, which stayed at a constant level
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of about 4000 ions/s. As an example, the g-ray spectrum for A ¼ 71 is shown in Fig. 12. It is evident that the doubly charged ions of A ¼ 142 are dominating the spectrum and that the charge exchange process, q ¼ 2 þ -q ¼ 1þ; does not take place very much for 142Ba and 142Cs ions in the present ion cooler conditions. The cumulative production yields were obtained from the measured radioactivity of the mass separated nuclei by detecting g-rays resulting from the b decay of the nucleus of interest. For most cases, the g branching ratios were obtained from literature [11]. In addition, for most of the Ni isotopes and 71,73,74Cu isotopes, branchings were obtained from [12,13] and [14,15], respectively. The mass distributions for Co, Ni, Cu, Zn and Ga were finally extracted from the cumulative production yields. The independent isotopic yields for 68oAo78 are given in Table 1 in units atoms/s.
5. Discussion 5.1. Charge transfer The results of the charge transfer measurements are now examined by using simple ionisation potential considerations of helium and N2, and
Fig. 12. The b gated g-ray spectrum recorded at mass A ¼ 71: The strongest lines are due to the doubly charged ions.
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Table 1 Independent fission product yields in atoms/s for elements 27oZo31 at IGISOL. The yields are measured for the proton (Ep ¼ 30 MeV) induced fission of 238U, with a proton beam current of Ip ¼ 4 mA A
Co
Ni
Cu
68 70 71 72 74 76 78
0.3 (1) o0.5
o0.9 1.7 (4) 0.7 (1) 0.43 (15)
o0.2 3.1 (8) 1.8 (4) 9.5 (9) 3.1 (2) 0.45 (4)
Zn
Ga
10.9 (10) 35 (3) 4.3 (2)
22 (4) 33 (6) 45 (3)
the radioactive nuclei in question. In Table 2 the first and second ionisation potentials are given for the studied elements. It is evident from Table 2 that none of the studied radioactive elements can capture an electron from neutral helium in their q ¼ 2þ states due to endoergicity of the reaction. However, rhodium is the only one of the studied doubly charged ions which has a higher second ionisation potential than the first ionisation potential of N2, thus being able to capture an electron from a nitrogen molecule. This explains the behaviour of the 112Rh(q ¼ 2þ) ions in the cooler, which have a maximum cross-section in electron capture with a certain N2 partial pressure (see Fig. 10). However, in the case of doubly charged 144Ba and 144La such a maximum in electron capture cross section is not observed and the ratio between q ¼ 2þ and q ¼ 1þ stays nearly constant with increasing nitrogen partial pressure (see Fig. 11). These results give us confidence that with a proper choice of impurity molecules, related to simple ionisation potential considerations, we can control the electron capture process of doubly charged ions in the IGISOL beam. This requires more extensive experimental testing of impurities, keeping in mind the various
chemical effects that might arise when introducing impurities in the cooler buffer gas. 5.2. Fission yields Despite of not succeeding to eliminate the doubly charged ions from the radioactive beam of fission products in our present experiment, the obtained superasymmetric fission yields complement the results of the previous yield measurement for 71oAo82 nuclei performed at IGISOL [3]. While the previous measurements revealed the yield of only one Ni isotope, 71Ni, in the present measurement we were able to determine yields for four Ni isotopes, 68Ni, 70Ni, 71Ni and 72Ni. Also, for Cu isotopes three new yield values, 68Cu, 70Cu and 76Cu, were obtained. Furthermore, yields for two Co isotopes, 68Co and 70Co, are now reported. The results for Zn and Ga isotopes are in general agreement with the previous measurement. The experimental fission yields were analysed in the framework of a theoretical model, proposed in [17,18], which was later modified in [3]. The model for calculating mass and independent fission product yields after emission of neutrons from excited fission fragments is taking into account the influence of nuclear shells, the charge polarisation and odd-even effects in charge and mass distributions. The total mass distribution is a contribution from four different fission modes, one symmetric and three asymmetric. The symmetric fission mode is approximated by a single Gaussian distribution, while the three asymmetric fission modes are each described by a pair of Gaussian distributions. The previous study of fission yields at intermediate excitation energy concluded that the contribution of the most asymmetric component, so called superasymmetric fission mode, is significant near the nuclear shells Z ¼ 28 and N ¼ 50: The measured independent yields and theoretical isotopic yields are plotted in Fig. 13. The
Table 2 1st and 2nd ionisation potentials for several elements [16]
1st ionisation pot (eV) 2nd ionisation pot (eV)
He
N2
Cu
Ba
La
Rh
24.481 54.403
15.576
7.724 20.29
5.21 10.001
5.61 11.43
7.46 18.07
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Fig. 13. Experimental independent yields (points with error bars) and calculated isotopic distributions (dotted lines) of Co, Ni, Cu, Zn and Ga in 30 MeV proton induced fission of 238 U. The numbers above each element gives the scale factor mb/(at/s).
theoretical isotopic yields can be converted to cross-sectional units of mb by multiplying with scale factors, shown in brackets above each isotopic distribution. The model parameters were fitted according to data from yield measurements in Leuven [19] and in this work the theoretical isotopic distributions for each element have merely been multiplied with the scale factor in order to fit best with the present experimental data. The theoretical prediction for the average isotopic distribution of Ni lies at Aav ¼ 69:93 [20], while the present experimental value is around Aav ¼ 70: This is in a good agreement with the fission yield measurement performed in Leuven [12] with the IGLIS on-line method. As a comparison, in the previous yield measurement at IGISOL [3] the average distribution of Ni resulted in Aav ¼ 71:23: The obtained yields of superasymmetric fission products, together with the theory, can be used as an estimate for the production of doubly magic
Ni at IGISOL. The direct extrapolation of the 78Ni yield, based on the calculated isotopic distribution of Ni shown in Fig. 13, gives 7 106 ions/s (E0.6 ions/day). However, there are several factors which are expected to improve considerably the fission yields at IGISOL in the near future. First, with the present ion-guide configuration the fission yields do not increase linearly with increasing proton beam current, but more as the square root of the beam current. Along the improvements in the ion guide and in the transmission between the ion guide and the skimmer, we expect to obtain a system which allows a linear increase in fission yield with a linear increase in proton beam current. With the new H- ion source at the Accelerator laboratory of Jyv.askyl.a, one has reached a current of Ip ¼ 50 mA for 30 MeV protons. Secondly, the transmission of the ion cooler could be increased from 35% up to 100% by modifying the injection. Finally, in the present experiment the poor conditions in the ionguide helium gas resulted in about 90% loss in yield, compared to the normal conditions. Therefore, we expect an increase by a factor of 750, compared to the yields presented in this work, and give a future estimate for the yield of 78Ni as 450 ions/day.
6. Summary We have built a compact and low cost Wien filter based on permanent magnets. The effective length of the filter is 50.5 mm and the whole filter fits inside a 100 mm diameter and 100 mm long beam tube. The filter can be used to separate q ¼ 2þ and q ¼ 1þ ions of the IGISOL beam by creating proper buffer gas conditions in the cooler, enabling q ¼ 2 þ -q ¼ 1þ reaction to occur. Together with the Wien filter, the observation of 78 Ni, produced in superasymmetric fission at IGISOL, may become accessible in the near future.
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[3] M. Huhta, P. Dendooven, A. Honkanen, A. Jokinen, G. Lhersonneau, M. Oinonen, H. Penttil.a, K. Per.aj.arvi, V.A. . o, . Phys. Lett. B 405 (1997) 230. Rubchenya, J. Ayst . o, . P. [4] A. Nieminen, J. Huikari, A. Jokinen, J. Ayst Campbell, E.C.A. Cochrane and the EXOTRAPS collaboration, Nucl. Instr. and Meth. A 469 (2001) 244. [5] J.H. Billen, L.M. Young, Poisson Superfish, LA-UR-961834, Los Alamos National Laboratory, 1998. [6] P. Campbell, et al., Hyperfine Interactions 127 (2000) 83. [7] A. Jokinen, et al., ENAM98, AIP Conference Proceedings, Vol. 455, p. 981. . [8] J. Arje, Thesis, Jyv.askyl.a, 1986. [9] W. Lindinger, Phys. Scripta. T3 (1983) 115. [10] P. Dendooven, et al., in: G. Fioni et al. (Eds.), Nuclear Fission and Fission-Product Spectroscopy, AIP CP447, New York 1998, p. 135. [11] R.B. Firestone, V.S. Shirley, Table of Isotopes, 8th Edition, Wiley, New York, 1996. [12] W.F. Mueller, B. Bruyneel, S. Franchoo, M. Huyse, J. Kurpeta, K. Kriglov, Y. Kudryantsev, N.V.S.V. Prasad, R. Raabe, I. Reusen, P. Van Duppen, J. Van Roosbroeck,
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L. Vermeeren, L. Weissman, Phys. Rev. C 61 (2000) 054308. W. Mueller, Private communication. S. Franchoo, M. Huyse, K. Kruglov, Y. Kudryavtsev, W.F. Mueller, R. Raabe, I. Reusen, P. Van Dupper, J. Van . Roosbroeck, L. Vermeeren, A. Wohr, Phys. Rev. Lett. 81 (1998) 3100. S. Franchoo, Private communication. R.C. Weast, (Ed.), Handbook of Chemistry and Physics, 55th Edition, CRC Press, Cleveland, 1974. E. Karttunen, M. Brenner, V.A. Rubchenya, S.A. Egorov, V.B. Funschtein, V.A. Jakovlev, Yu.A. Selitskiy, Nucl. Sci. Eng. 109 (1991) 35. P.P. Jauho, A. Jokinen, M. Leino, J.M. Parmonen, . o, . K. Eskola, V.A. Rubchenya, Phys. H. Penttil.a, J. Ayst Rev. C 49 (1994) 2036. K. Kruglov, B. Bruyneel, S. Dean, et al., Proceedings of 5th international Conference on Radioactive Nuclear Beams, RNB 2000, Divonne, France, 3–8 April, 2000, Nucl. Phys. A, in press. V. Rubchenya, Private communication.
Wien filter for cooled low-energy radioactive ion beams S. Nummelaa,*, P. Dendoovena, P. Heikkinena, J. Huikaria, A. Nieminena, . o. a,b A. Jokinena,b, S. Rinta-Antilaa, V. Rubchenyaa, J. Ayst a
Department of Physics, University of Jyvaskyl a, a, . . P.O. Box 35, Jyvaskyl . . Finland b CERN, CH-1211 Geneva, Switzerland Received 5 June 2001
Abstract A Wien filter for cooled radioactive ion beams has been designed at Ion Guide Isotope Separator On Line technique (IGISOL). The purpose of such device is to eliminate doubly charged ions from the mass separated singly charged ions, based on q ¼ þ2-q ¼ þ1 charge exchange process in an ion cooler. The performance of the Wien filter has been tested off-line with a discharge ion source as well as on-line with a radioactive beam. The electron capture process of cooled q ¼ 2þ ions has been investigated in a radiofrequency quadrupole ion cooler with varying partial pressures of nitrogen. Also, the superasymmetric fission production yields of 68oAo78 nuclei have been deduced. r 2002 Published by Elsevier Science B.V. PACS: 25.85.Ge; 28.60.+s; 82.30.Fi Keywords: Wien filter; Radioactive ion beams
1. Introduction The study of exotic nuclei is usually dealing with low production yields of nuclei of interest. This limitation requires a constant development of efficient production methods and detection techniques that are free from background radiation and contaminants. A standard method related to the production of exotic nuclei is mass separation, which together with more sophisticated methods, such as ion trapping, enables to pick a special weak channel out of numerous *Corresponding author. E-mail address: [email protected].fi (S. Nummela).
stronger channels in a nuclear reaction. One example of first on-line mass separators is the Ion Guide Isotope Separator On Line technique (IGISOL) [1] which has been successfully applied in the discovery of many new isotopes [2]. The IGISOL technique is based on thermalization of reaction products in a helium gas and the transportation of the resulting ions with the helium flow and by an electric field through a skimmer system. The reaction products are accelerated and finally mass separated according to their mass over charge ratio. This method is very fast, enabling the study of a large range of rare short-lived (sub-ms) isotopes in a chemically insensitive way.
0168-9002/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 3 6 2 - 6
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From a nuclear structure point of view, one of the nuclear regions under a great deal of interest is the neighbourhood of the doubly magic nucleus 78 Ni. It has been proposed that the superasymmetric fission mode would provide an access to produce nuclei in the region in question. In order to investigate this matter, yields of fission products of 68oAo78 have been measured at IGISOL [3]. A severe drawback so far, concerning measurements at IGISOL, has been the presence of doubly charged ions exiting from the ion guide. While the reaction products are stopped in helium, most of them are extracted from the ion guide as q ¼ 1þ ions but a small fraction remains as q ¼ 2 þ : Thus, with a given q=m ratio two masses, m ¼ A or m ¼ 2A; are transported through the separator corresponding to q ¼ 1þ and q ¼ 2þ ions, respectively. If the production rate for 2A is much higher than for nuclei with mass A, the q ¼ 2þ ions dominate the mass separated beam and may severely harm the spectroscopic study of the weakly produced exotic nuclei separated as q ¼ 1þ ions. This is the case in superasymmetric fission where the cross sections for AE70 nuclei are 103 times smaller than for AE140; resulting in a beam which is dominated by the latter. To overcome the problem of the doubly charged ions we have investigated the possibility to lower the charge state of the q ¼ 2þ ions into q ¼ 1þ and further separate the ions owing to m ¼ A and m ¼ 2A with a velocity filter. The electron capture and cooling would take place in an ion cooler [4], in which the ions interact with helium atoms and carefully chosen impurity molecules. After the second acceleration stage the different masses with equal charge, q ¼ 1þ; will have different velocities, so that the second mass separation is possible with a Wien filter. The goal was to design a compact and an easy-to-use device. The principle of the Wien velocity filter is well known since the beginning of 1900s and it has been adapted to numerous applications both in scientific research and commercial use. In this paper we will first describe the design and characteristics of the Wien filter. Then, the results of the ion cooler measurements will be presented as well as the superasymmetric fission yields for 68oAo78 nuclei.
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2. Wien filter design 2.1. Principle A Wien filter is a device which separates ions according to their velocity. The filter employs mutually perpendicular electric and magnetic fields, E and B; which set opposite forces on charged particles passing through the filter. When the magnetic force and electric force are equal, i.e. qvB ¼ qE
ð1Þ
ions with velocity v ¼ E=B pass the filter undeflected. In the case of a cooled and reaccelerated (acceleration voltage U) mass-separated radioactive beam at IGISOL the velocity of ions, with mass m and charge q; is given as rffiffiffiffiffiffiffiffiffi 2qU v¼ ð2Þ m and therefore the separation of different velocities with equal charge correspond to separation of masses. The dispersion between masses m and (m Dm) in the Wien filter can be written as laEDm ð3Þ D¼ 4Um where l is the drift distance from the centre of the filter to the slit (302 mm), a the length of the filter (50.5 mm), E the electric field in V/m, U and Dm=m corresponds to the mass dispersion. In our case the magnetic field B is constant and the electric field follows the relation E ¼ vB: Hence, it is useful to write the dispersion as rffiffiffiffiffiffiffiffiffiffiffi laDm 2q D¼ B ð4Þ 4m mU where q is the charge and m the mass of the ion with zero deflection in the filter and B the magnetic field in T. The dispersion increases linearly with the magnetic field and inversely with the square root of the acceleration voltage. The design of the IGISOL Wien filter started with the main requirement that the singly charged ions of mass 70oAo80 should be separated efficiently from their originally doubly charged counterparts of mass 140oAo160: In other words, the latter, after changing charge state from
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2+-1+, should be sufficiently deflected in the Wien filter in order to be stopped and removed by a following slit. The design was already restricted by some existing boundary conditions. First, the reacceleration voltage of the cooler is set as 40 kV, defining the velocity of ions. Secondly, the size of the beam exiting the cooler is approximately 2 mm as FWHM, defining the minimum amount of dispersion for adequate mass separation. Thirdly, the dimensions of the beam line after the cooler leave space of only about 40 cm for the Wien filter and the drift distance before the slit. Therefore, the proper amount of dispersion is to be established only by the strengths of the electric and magnetic fields and the effective length of the Wien filter. Keeping in mind these constraints, the design of the Wien filter was carried out by calculating the magnetic and electric fields with a 2d-code Poisson Superfish [5]. A schematic view of the IGISOL beam line and the location of the Wien filter is given in Fig. 1.
magnets available, with remanence of Br ¼ 1:12 T and coercivity of B HC ¼ 850 kA/m. The blocks would be placed one upon the other, so that the minimum magnet gap, h; is defined by the space required by the beam and the electrodes. On the other hand, the magnets should be close enough in order to create a magnetic field sufficiently strong for adequate beam deflection. In order to maximise the field, the return flux will be guided in an iron yoke, schematically shown in Fig. 2. Next we shall give a simple formula for the magnetic field in the middle of the magnet. For homogeneously magnetised magnets we can assume a surface current density M around the magnet, M being also the magnetisation. Hence, by replacing the magnet by a surface current and assuming very high permeability iron (H ¼ 0), we can write Ampe" re’s law as I ~d~ ð2d þ hÞH ¼ H l ¼ NI ¼ 2dM ð5Þ
2.2. Magnetic field
H¼
In order to keep the operation of the Wien filter as simple as possible, we considered a choice of permanent magnets for introducing the magnetic field. We had 2 2 0.5 inch pieces of Neorem
where h and d correspond to the gap between the magnets and the magnet thickness, respectively. In permanent magnets the magnetisation M depends weakly upon the magnetic field strength H: For
2d M 2d þ h
Fig. 1. Schematic view of IGISOL beam line used in this work.
ð6Þ
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gives a fairly good approximation for the gap field for a dipole magnet constructed as in Fig. 2. The calculated vertical transverse magnetic field, perpendicular to the beam, is shown in Fig. 3 (left), both with an iron yoke (solid line) and without the iron yoke (dotted line). With a proper yoke definition, also the field in the longitudinal direction can be calculated. The calculated shape of the longitudinal magnetic field parallel to the beam direction is shown on the right hand side of Fig. 3 as a solid line. Fig. 2. Schematic drawing of the Wien filter magnet. The permanent magnets are connected with an iron yoke. For the available neomagnets h ¼ 14:6 mm and d ¼ 12:7 mm.
neomagnets, the magnetisation curve close to remanence goes approximately as M ¼ Mr þ 0:05HM
ð7Þ
where the remanence is given as m0 Mr ¼ Br and HM is the magnetic field strength inside the permanent magnet. Since in the case of permanent magnets the closed integral in Ampe" re’s law results in zero, HM is negative and in our simple geometry (assuming H inside the iron yoke to be zero) it gives 2dHM ¼ hH
ð8Þ
h ð9Þ HM ¼ H: 2d Therefore, we can write 2d h H¼ ðMr þ 0:05HM Þ ¼ Mr 0:05 H ð10Þ 2d þ H 2d 1 2d h 1 þ 0:05 H¼ Mr ð11Þ 2d þ h ð2d þ hÞ or B¼
1 2d h 1 þ 0:05 Br : 2d þ h 2d þ h
ð12Þ
With the dimensions of the present neomagnets and the iron yoke, and neglecting the correction term from the magnetisation curve, the magnetic field inside the gap of the Wien filter results in 0.71 T. A numerical calculation with the Poisson code, with realistic iron permeability, gives 0.663 T in the middle of the magnet gap. Eq. (12) therefore
2.3. Electric field Due to a permanent magnetic field in the Wien filter, the parameter to be tuned is the electric field. With the calculated magnetic field of B ¼ 0:66 T, the operating region for undeflected ions of A ¼ 70 and A ¼ 80 gives a corresponding electric field of 2.19 and 2.05 kV/cm, respectively, which is evidently applicable in the vacuum of 106 mbar. A sufficiently large volume of homogeneous electric field needs to be created between the magnetic poles of the filter. Two common ways of making a homogenous electric field between grounded magnet poles are U-shaped electrodes (see Fig. 4) or by evenly spaced strips between the electrode plate edges. We chose to use a configuration with nine strips. The strips are connected to each other in series with resistors. The 1 mm strip width and 2 mm spacing is enough to produce a homogenous field over a 20 10 mm2 cross-section, which is large enough for a beam of 2 mm width. The strips were realised by using a circuit board, on which 1 mm wide copper strips were introduced. The effect of the strips on the electric field shape is demonstrated in Fig. 5, where the calculated equipotential lines are shown both for the configuration with 9 strips (left) and without strips (right). The distance of the electrodes in both cases is 25 mm, with an applied 5.2 kV voltage difference (72.6 kV in the electrodes). The electric field in vertical transverse direction is plotted on the left in Fig. 6, so that the solid line corresponds to the configuration with 9 strips, and the dotted line without strips. As it has been demonstrated, the principle of the Wien filter lies on the ratio E=B: Therefore, the
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Fig. 3. Calculated magnetic field perpendicular (solid line on the left) and parallel (solid line on the right) to the beam direction with a magnet gap of 14.6 mm, including a yoke. The dotted line corresponds to a calculation without a yoke and the circles correspond to measured magnetic field values.
Fig. 4. Electric field (equipotential lines) with U-shaped electrodes. Only 14 of the geometry is shown due to symmetries.
same ratio should hold along the total effective field length. In our case, this condition is carried out by shaping the electric field along the beam direction to correspond to the shape of the magnetic field. The longitudinal extent and the shape of the electric field are controlled by
the length of the electrodes and by a grounded slit outside the gap in both ends. Therefore, a slit, with a 10 mm aperture at 5 mm distance from the edges of the magnets, was included in the simulations where the electrode length was set to 40 mm. The resulting electric field shaped along the beam direction is shown on the right side of Fig. 6 as a solid line. Correspondingly, the magnetic field is shown as dashed line. The maxima of the electric and magnetic fields are set in the figure at an equal level, thus demonstrating the constant ratio E=B over the whole extent of the fields, which assures proper functioning of the filter. As a comparison, the electric field without the slit is shown as a dotted line. A sketch of the Wien filter design is shown in Fig. 7. The filter is mounted inside a 100 mm beam tube and is positioned by adjustable screws.
3. Characteristics The performance of the Wien filter was first tested off-line. Before installing the device in the
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Fig. 5. Electric field (equipotential lines) for the Wien filter with strips (left) and without field shaping strips (right). Also the grounded magnet surface is shown. Only 14 of the geometry is shown due to symmetries.
Fig. 6. Left: Calculated electric field in vertical transverse direction with 9 strips (solid line) and without strips (dotted line). Right: Electric field in longitudinal direction with a slit (solid line) and without a slit (dash line). As a comparison, the magnetic field is shown as dotted line.
beam line the magnetic field was measured directly in the direction of the beam and compared with the calculated simulations. The measured magnetic field is plotted in Fig. 3 as circles, and demonstrates that the simulation with Poisson Superfish mimics the real situation extremely well.
Off-line tests were performed by using a discharge ion source, which is placed inside the ion guide, replacing the target used in on-line experiments. A selection of stable beams, such as 176–180 Hf, can be produced by choosing a suitable discharge electrode material, which are used for
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Fig. 8. Intensity of 180Hf ion beam measured by a Faraday cup after the Wien filter as a function of electrode voltage difference.
Fig. 7. Technical drawing of the Wien filter.
various purposes in testing the separator and beam line conditions. Since the measured magnetic field of the Wien filter was very close to the calculated one, we tested off-line how well the calculated electrode voltage for a specific undeflected mass reproduces the actual voltage used. For this we used a beam of 180Hf ions from the separator. The electrodes were connected to a high voltage supply, which gave two opposite signed outputs. The electrode voltage difference was scanned from 3400 to 4000 V and the outcoming beam current was measured by a Faraday cup after the Wien filter. The slit was kept at 2 mm aperture in order to avoid the beam hitting an XY-deflector before the Faraday cup and causing the deflected beam to scatter on the Faraday cup. The measured current as a function of the electrode voltage is shown in Fig. 8. The maximum of the current corresponds to electrode voltage difference of 3650 V whereas a second maximum appears 250 V higher. The calculated value for an undeflected mass of 180 gives 3420 V. The reason for the difference between the calculated and measured value is that the beam from the quadrupole deflector, after the cooler, is entering the Wien filter slightly off the symmetry axis. Therefore, a slight increase of the electrode voltage
is needed to produce the same deflection as was anticipated by the calculations. The asymmetric shape of the current curve can be explained by an asymmetric shape of the beam exiting the quadrupole deflector after the cooler. A more precise measurement of the transverse profile of the outcoming beam from the Wien filter was performed with 180Hf ions. The beam profile was obtained by measuring the outcoming beam current from a single slit at 35 cm distance which was scanned through in horizontal direction. The measured beam current was differentiated which gave the actual beam current profile. This resulted in a FWHM of 2.1 mm for the beam. The shape of the beam is shown in Fig. 9. The profile is slightly asymmetric, which was already observed by scanning the voltage. If the beam profile is fitted with two Gaussian peaks, the distance of the peak maxima is 1.5 mm. This distance corresponds to the effect of about 250 V difference in the electrode voltage difference, which was already observed in Fig. 8. A beam width of 2 mm means that the Wien filter can separate masses which are deflected more than 2 mm at the position of the slit. In the case of AE70 nuclei this corresponds to 7 m=q units. Therefore, the Wien filter can be used to separate the doubly charged ions from the single charged.
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electron from the neutral gas atom or molecule. No universal theory exists which would describe the charge exchange process for qp4þ ions in gases but numerous experimental data exist concentrating on restricted parameters and elements. A survey of the literature concerning experimental data and some specified theories is given in Ref. [8]. In general, the charge transfer reaction between an ion R with charge state qþ and a neutral helium atom can be written as Rqþ þ He-Rðq1Þþ þ Heþ þ DE;
Fig. 9. Current profile of the movable slit after the cooler.
180
Hf beam measured on a
4. Results 4.1. Charge exchange tests with the cooler An essential role related to the function of the Wien filter is played by the ion cooler and the charge exchange processes taking place in the buffer gas of the cooler. Therefore, we examined the q ¼ 2 þ -q ¼ 1þ charge exchange process inside the cooler both with mass separated reaction products and stable ions. Originally, the cooler has been designed to improve the ion optical properties of the IGISOL beam. This is necessary for collinear laser spectroscopy [6] as well for efficient injection into a Penning trap [7] soon to be constructed in conjunction with the IGISOL beam line. In addition, the cooler can be used as a buncher, in which ions can be stored for a definite time and released in short bunches. However, as the idea of the Wien filter at IGISOL was created, the use of the cooler was extended to operate as a charge exchange device. The cooler is a radiofrequency quadrupole in which ions are cooled down to thermal energies in collisions with the buffer gas. The cooler lies at a 40 kV platform, so that when the ions enter the cooler they are decelerated, cooled and eventually accelerated with a narrower energy spread. In specific conditions, depending on the buffer gas and the velocity of the ions, charge exchange processes take place in which an ion captures an
ð13Þ
where the energy defect, DE; is the difference in ionisation potentials of R and He. It has been shown that the cross section for electron capture below energies of 25 keV/u is in most cases almost independent on the kinetic energy of the ion. In addition, electron loss measurements have shown that the charge state q ¼ 1þ is dominating in helium in this velocity region, just what can be observed for the reaction products stopped in helium in the IGISOL ion guide. However, for the remaining small fraction of the q ¼ 2þ there exists no explicit theory how the electron capture processes happen when the ions are cooled in helium gas. There are studies concerning the reactions for doubly charged ions at near thermal energies in different gases [9] and the results can be partly adapted to our case where we have ions of Eo1 eV inside the cooler. These studies show how the electron capture rate coefficient of a doubly charged ion reaches its maximum at specific internuclear separation RX, where RX ¼ 14:4=DE ( As a conclusion, only the exoergicity in units of A. (DE > 0), and not the nature of the ion, determines the rate of the electron capture reaction. Also, high reactivities are observed nearly always when the neutral reactants are in molecular form. As the first ionisation potential of helium (24.48 eV) is higher than the second ionisation potential of most of the elements (i.e. DEo0), the electron capture of a q ¼ 2þ ion in helium seems improbable. However, if a small amount of impurity molecules are mixed in helium, so that the first ionisation potential of the impurity molecule is between the first and the second ionisation potential of the doubly charged ion, the electron capture becomes possible. The choice of such an impurity molecule
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is a delicate issue since the created ions may induce some more complicated effects in the charge exchange process. The survival of q ¼ 1þ and q ¼ 2þ ions in the cooler was studied on-line with radioactive nuclei, produced in a 30 MeV proton induced fission. The mass separated radioactive ions were cooled, deflected with the Wien filter and collected on a movable tape. The yield of ions was determined by observing beta delayed g rays, detected with a Ge detector. The goal was to study how nitrogen (N2) impurity molecules affect the electron capture process of chemically dissimilar radioactive ions. In the first measurement the yield of 112Rh ions, as a function of nitrogen partial pressure, was measured for three different cases. First, q ¼ 1þ ions were mass separated and passed through the Wien filter. Secondly, 112Rh ions corresponding to q ¼ 2þ charge state were mass separated and selected through the Wien filter with a voltage twice as high compared to the q ¼ 1þ ions. Thirdly, the Wien filter voltage was adjusted for the q ¼ 1þ ions which had lowered their charge from 2+ to 1+ in the cooler. Thus, the survival of 2+ and 1+ ions could be studied independently. Small amounts of nitrogen gas (N2) were mixed in the He buffer gas so that the total pressure was kept as 4.0 mbar. The pressure was measured from the gas injection line, so that the pressure inside the cooler is approximately 20 times less than the measured value, based on gas flow calculations. The results are shown in Fig. 10 where the yield maxima in each case are normalised as 1. It can be seen that the intensity of the 112Rh ions, undergoing no charge exchange processes, decreases with increasing N2 concentration. However, the probability of charge exchange from q ¼ 2þ into q ¼ 1þ ions reaches a maximum at about 3% nitrogen concentration. The behaviour was similar for lower total buffer gas pressures. A similar measurement, testing the influence of nitrogen partial pressure on the charge exchange process, was done for 72Cu (q ¼ 1þ) and the corresponding doubly charged counterparts 144Ba and 144La (q ¼ 2þ). The total pressure of He was kept at 4.3 mbar. The yields of these ions were measured simultaneously, since they are extracted with identical separator magnet and Wien filter
Fig. 10. Yields of 112Rh as a function of nitrogen partial pressure in the cooler for three different situations of charge transfer: q ¼ 1 þ -q ¼ 1þ (squares), q ¼ 2 þ -q ¼ 2þ (circles) and q ¼ 2 þ -q ¼ 1þ (triangles).
settings. The results are presented in Fig. 11 as an intensity ratio of 72Cu/144Ba and 72Cu/144La in order to highlight the effect of a charge transfer and the possible loss of 2+ ions. The fraction of 72 Cu is increasing in the very low partial pressures but seems to saturate very quickly. These results will be discussed later based on ionisation potential arguments.
Fig. 11. Yield ratios of 72Cu/144Ba and 72Cu/144La measured after the Wien filter as a function of nitrogen partial pressure in the cooler.
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4.2. Fission yields The yields of highly asymmetric fission products of 68oAo78 were measured for 30 MeV proton induced fission of 238U at IGISOL [10] with a proton-beam current of 4 mA. The mass separated reaction products were collected onto a tape, which was moved periodically. The pulsed tape movement ensured that the most exotic short-lived nuclei were observed efficiently whereas the activity originating from longer lived daughter nuclei was reduced. The tape movement was adjusted for each mass according to the half-lives in question. In order to measure the yield also for the longer lived daughter nuclei at saturation, a second measurement without tape movement was performed. The collection point was surrounded by a cylindrical plastic 2 mm thick detector with beta detection efficiency of 70%. Three Ge detectors (Nordball-type) in close geometry around the beta detector were used to detect grays in coincidence with betas. In order to prevent a large fraction of the b’s entering the Ge detectors, a 15 mm Plexiglass absorber was introduced between the beta detector and the Ge detectors. The efficiency of the ion guide was checked before each change of mass by observing the yield of 112Rh, which stayed at a constant level
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of about 4000 ions/s. As an example, the g-ray spectrum for A ¼ 71 is shown in Fig. 12. It is evident that the doubly charged ions of A ¼ 142 are dominating the spectrum and that the charge exchange process, q ¼ 2 þ -q ¼ 1þ; does not take place very much for 142Ba and 142Cs ions in the present ion cooler conditions. The cumulative production yields were obtained from the measured radioactivity of the mass separated nuclei by detecting g-rays resulting from the b decay of the nucleus of interest. For most cases, the g branching ratios were obtained from literature [11]. In addition, for most of the Ni isotopes and 71,73,74Cu isotopes, branchings were obtained from [12,13] and [14,15], respectively. The mass distributions for Co, Ni, Cu, Zn and Ga were finally extracted from the cumulative production yields. The independent isotopic yields for 68oAo78 are given in Table 1 in units atoms/s.
5. Discussion 5.1. Charge transfer The results of the charge transfer measurements are now examined by using simple ionisation potential considerations of helium and N2, and
Fig. 12. The b gated g-ray spectrum recorded at mass A ¼ 71: The strongest lines are due to the doubly charged ions.
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Table 1 Independent fission product yields in atoms/s for elements 27oZo31 at IGISOL. The yields are measured for the proton (Ep ¼ 30 MeV) induced fission of 238U, with a proton beam current of Ip ¼ 4 mA A
Co
Ni
Cu
68 70 71 72 74 76 78
0.3 (1) o0.5
o0.9 1.7 (4) 0.7 (1) 0.43 (15)
o0.2 3.1 (8) 1.8 (4) 9.5 (9) 3.1 (2) 0.45 (4)
Zn
Ga
10.9 (10) 35 (3) 4.3 (2)
22 (4) 33 (6) 45 (3)
the radioactive nuclei in question. In Table 2 the first and second ionisation potentials are given for the studied elements. It is evident from Table 2 that none of the studied radioactive elements can capture an electron from neutral helium in their q ¼ 2þ states due to endoergicity of the reaction. However, rhodium is the only one of the studied doubly charged ions which has a higher second ionisation potential than the first ionisation potential of N2, thus being able to capture an electron from a nitrogen molecule. This explains the behaviour of the 112Rh(q ¼ 2þ) ions in the cooler, which have a maximum cross-section in electron capture with a certain N2 partial pressure (see Fig. 10). However, in the case of doubly charged 144Ba and 144La such a maximum in electron capture cross section is not observed and the ratio between q ¼ 2þ and q ¼ 1þ stays nearly constant with increasing nitrogen partial pressure (see Fig. 11). These results give us confidence that with a proper choice of impurity molecules, related to simple ionisation potential considerations, we can control the electron capture process of doubly charged ions in the IGISOL beam. This requires more extensive experimental testing of impurities, keeping in mind the various
chemical effects that might arise when introducing impurities in the cooler buffer gas. 5.2. Fission yields Despite of not succeeding to eliminate the doubly charged ions from the radioactive beam of fission products in our present experiment, the obtained superasymmetric fission yields complement the results of the previous yield measurement for 71oAo82 nuclei performed at IGISOL [3]. While the previous measurements revealed the yield of only one Ni isotope, 71Ni, in the present measurement we were able to determine yields for four Ni isotopes, 68Ni, 70Ni, 71Ni and 72Ni. Also, for Cu isotopes three new yield values, 68Cu, 70Cu and 76Cu, were obtained. Furthermore, yields for two Co isotopes, 68Co and 70Co, are now reported. The results for Zn and Ga isotopes are in general agreement with the previous measurement. The experimental fission yields were analysed in the framework of a theoretical model, proposed in [17,18], which was later modified in [3]. The model for calculating mass and independent fission product yields after emission of neutrons from excited fission fragments is taking into account the influence of nuclear shells, the charge polarisation and odd-even effects in charge and mass distributions. The total mass distribution is a contribution from four different fission modes, one symmetric and three asymmetric. The symmetric fission mode is approximated by a single Gaussian distribution, while the three asymmetric fission modes are each described by a pair of Gaussian distributions. The previous study of fission yields at intermediate excitation energy concluded that the contribution of the most asymmetric component, so called superasymmetric fission mode, is significant near the nuclear shells Z ¼ 28 and N ¼ 50: The measured independent yields and theoretical isotopic yields are plotted in Fig. 13. The
Table 2 1st and 2nd ionisation potentials for several elements [16]
1st ionisation pot (eV) 2nd ionisation pot (eV)
He
N2
Cu
Ba
La
Rh
24.481 54.403
15.576
7.724 20.29
5.21 10.001
5.61 11.43
7.46 18.07
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78
Fig. 13. Experimental independent yields (points with error bars) and calculated isotopic distributions (dotted lines) of Co, Ni, Cu, Zn and Ga in 30 MeV proton induced fission of 238 U. The numbers above each element gives the scale factor mb/(at/s).
theoretical isotopic yields can be converted to cross-sectional units of mb by multiplying with scale factors, shown in brackets above each isotopic distribution. The model parameters were fitted according to data from yield measurements in Leuven [19] and in this work the theoretical isotopic distributions for each element have merely been multiplied with the scale factor in order to fit best with the present experimental data. The theoretical prediction for the average isotopic distribution of Ni lies at Aav ¼ 69:93 [20], while the present experimental value is around Aav ¼ 70: This is in a good agreement with the fission yield measurement performed in Leuven [12] with the IGLIS on-line method. As a comparison, in the previous yield measurement at IGISOL [3] the average distribution of Ni resulted in Aav ¼ 71:23: The obtained yields of superasymmetric fission products, together with the theory, can be used as an estimate for the production of doubly magic
Ni at IGISOL. The direct extrapolation of the 78Ni yield, based on the calculated isotopic distribution of Ni shown in Fig. 13, gives 7 106 ions/s (E0.6 ions/day). However, there are several factors which are expected to improve considerably the fission yields at IGISOL in the near future. First, with the present ion-guide configuration the fission yields do not increase linearly with increasing proton beam current, but more as the square root of the beam current. Along the improvements in the ion guide and in the transmission between the ion guide and the skimmer, we expect to obtain a system which allows a linear increase in fission yield with a linear increase in proton beam current. With the new H- ion source at the Accelerator laboratory of Jyv.askyl.a, one has reached a current of Ip ¼ 50 mA for 30 MeV protons. Secondly, the transmission of the ion cooler could be increased from 35% up to 100% by modifying the injection. Finally, in the present experiment the poor conditions in the ionguide helium gas resulted in about 90% loss in yield, compared to the normal conditions. Therefore, we expect an increase by a factor of 750, compared to the yields presented in this work, and give a future estimate for the yield of 78Ni as 450 ions/day.
6. Summary We have built a compact and low cost Wien filter based on permanent magnets. The effective length of the filter is 50.5 mm and the whole filter fits inside a 100 mm diameter and 100 mm long beam tube. The filter can be used to separate q ¼ 2þ and q ¼ 1þ ions of the IGISOL beam by creating proper buffer gas conditions in the cooler, enabling q ¼ 2 þ -q ¼ 1þ reaction to occur. Together with the Wien filter, the observation of 78 Ni, produced in superasymmetric fission at IGISOL, may become accessible in the near future.
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