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A simple mass separator for radioactive isotopes
Nuclear
34
A SIMPLE MASS SEPARATOR
Instruments
FOR RADIOACTIVE
and Methods
in Physics Research B26 (1987) 34-36 North-Holland, Amsterdam
ISOTOPES
S.B. KARMOHAPATRO Saha Insitute of Nuclear Physics, I ,lAF 3idha~na~a~, ~~l~utta-~~b4.
A mass separator
intended
for the separation
of radioactive
1. Introduction A mass separator has been constructed at the Saha Institute of Nuclear Physics for the purpose of separating radioactive isotopes for studies in nuclear physics. Its design and operational features are described in the sections below. The separator is a modification of a vfi instrument [l] with the ion source outside the magnetic field, since the pole gap is inadequate to accommodate the ion source. However, this modification also avoids overheating of the poles and the effect of the magnetic field on the ion source performance. To utilize the original pole tips of the ?rfi magnet without physical change, the collector was located inside the magnet gap at a sector angle of about 210 O. The performance of the separator shows that it can be used for off-line separation of short-lived isotopes produced by ( LY,xn) reactions at 60 MeV using the variable-energy cyclotron of the Bhabha Atomic Research Centre adjoining the Saha Institute Campus at Bidhannagar in Calcutta.
isotopes
is described.
Intitial
operational
parameters
are presented.
used with variable temperature, measured by a W/Rh 5% and W/Rh 20% thermocouple. To maintain the electron density at the axis of the hollow-cathode ion source, an external axial field of up to 300 G is applied to compensate for the field of the filament. Without this compensation, the ion current is about 30% lower than with it. The mass analyzer is an i~omogeneous two-directional focussing magnet with a radius of curvature for the mean ion path of R, = 0.381 m and a deflection angle of 210 O. The value of the field index, n, for the central radius is 0.500 + 0.001, with a variation up to + 0.012 at the edges; these values were determined by a method described in ref. [l]. The gfi analyzer magnet developed at this laboratory [1,4] was modified to locate the ion source outside the magnetic sector. (In using the n& magnet, which has two-directional focussing, both
Hollow cathode ion source
2. Mass separator The mass separator consists of a Hollow-Cathode Ion Source (HCIS), an accelerating voltage power supply (O-10 kV), a magnetic mass analyser. a vacuum system, and either a foil collector or a Faraday cup with beam indicator and integrator (figs. 1 and 2). The hollow-cathode ion source is similar to that described by Sidenius 121. Its operational characteristics have been reported earlier [3]. The ion source is constructed with a stainless steel cylindrical anode and a molybdenum cathode with a tungsten spiral filament. The ion exit aperture is a 0.6”mm opening in the upper lid of the cathode. Ions are extracted from the ion source by using a conical electrode at ground potential, and focussed with an Einzel lens having a variable positive potential. The ion source can be used with either solid or gaseous samples. For solids, an oven is 0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
India
B.V.
De
-4 9a
Diffusion
Fig. 1. Schematic
diagram
Pump
Fore Pump
of the mass separator.
S. B. Kurmohupatro
Fig. 2. Photograph
/ Simple mass sepurutor for radioactic~e isotopes
of the mass separator, showing: (1) hollow-cathode yoke, (5) shielding arrangement,
the ion source and the collector would need to be placed inside the magnetic field.) The modified magnet has annular pole pieces of 0.43-m outer radius and 0.33-m inner radius, with a pole gap of 0.05 m at the mean radius. The pole pieces are fitted with annular guard rings to a 0.47-m radius, tapered at an angle of 7”, in order to compensate the effect of fringe fields at the mean ion optical path radius. The magnet is powered by a supply regulated to 1 part in 104. To accommodate the HCIS without overheating the poles, the exit aperture of the ion source is placed at a distance of I, = 0.92 R, from the true boundary of the magnet. Optimum focussing is obtained with the collector placed at a deflection angle of 210”. This is in approximate agreement with the value of 206 o prediced by Glavish [5] and Livingood [6] for a field index of n = 0.5. The ion path between the ion source exit and the magnet boundary is magnetically shielded; the shielding did not introduce any detectabe change to the field index of the main analysing field. The vacuum chamber is shown in fig. 1 and is described by Chakraborty et al. [3,7]. The operating pressure is - 10m5 Torr. The mass-analysed ion beam is detected by a Faraday cup connected to a current indicator and integrator to measure the ion current and, when required, the ion dose. Collection of ions is made by use of foils mounted on a rigid target holder made of 0.0125-m thick brass. 3. Operational characteristics We have measured the resolving separator using the mass spectrum
power of the mass of neon, especially
ion source. (2) analyser (6) focusing electrode.
chamber,
35
(3) magnetic
pole pieces, (4)
the “Ne and 22Ne lines. We observe a value of 342, compared to the value deduced from Livingood [6] of 416 from the relation RI’. = R,F/4W,,
(1)
where We is the radial width of the object and F is the resolution function. The resolution function, F, is related to the radial magnification M, for an inhomogeneous magnetic field by F= 2(1 -l/M,).
(2)
The efficiency of the separator has been measured by separating radioactive 61Cu and 64Cu, made by bombarding nickel foil with 3.8-MeV protons from the Institute Cyclotron, and separating the reaction products. The 61Cu was deposited on a copper foil and the 660-keV y rays were detected using a Ge(Li) spectrometer. From the ratios of the activities of the collected isotopes and their presence in the bombarded charge material, the efficiency for separating copper is estimated to be 5% [S]. This efficiency is adequate for carrying out radioactive isotope separation on a small scale. However, for further improvement in the separation efficiency, it is necessary to study the effects of ion scattering during operation of the instrument [7]. 4. Conclusions The present mass separator is small in size and is convenient to use in separating masses up to about 200 amu. The operating pressure has recently been improved to about 1O-6 Torr with the addition of a second vacuum pump (Diffstak, Edwards). It is exI. GENERAL SEPARATOR TECHNOLOGY
36
petted that this improvement higher efficiency of separation contamination, but examination mance is yet to be done. The future efforts at this using the separator for off-line nuclides produced by (0~. xn) Energy Cyclotron at Calcutta.
S.B. Karmohapatro / Simple mass separator
will allow us to obtain a with a reduced scattering of the improved perforsytem will still involve separation of short-lived reactions at the Variable
References [l] S.B. Karmahapatro, Ind. J. Phys. 34 (1960) 407. [2] G. Sidenius, Proc. Int. Conf. on Electromagnetic
Isotope
for radioactioe isotopes
Separators and Techniques of their Applications, Marburg. (1970), eds., H. Wagner and W. Walcher, p. 423. S. Kundu and S.B. Karmohapatro, Ind. J. [31 T. Chakraborty, Phys. 55B (1981) 131. Ind. J. Phys. 33 (1959) 139. (41 S.B. Karmohapatro. I51 H.F. Glavish, Nucl. Instr. and Meth. 189 (1981) 43. WI J.J. Livingood, The Optics of Dipole Magnets (Academic Press. New York, 1969). S. Kundu, and S.B. Karmohapatro, Nucl. [71 T. Chakraborty, Instr. and Meth. 199 (1982) 531. and S.B. Karmohapatro, Proc. PI A.K. Das, D.K. Mukhejee Nucl. Phys. Solid State Phys. Symp., Calcutta, 18B (1975) 313.
34
A SIMPLE MASS SEPARATOR
Instruments
FOR RADIOACTIVE
and Methods
in Physics Research B26 (1987) 34-36 North-Holland, Amsterdam
ISOTOPES
S.B. KARMOHAPATRO Saha Insitute of Nuclear Physics, I ,lAF 3idha~na~a~, ~~l~utta-~~b4.
A mass separator
intended
for the separation
of radioactive
1. Introduction A mass separator has been constructed at the Saha Institute of Nuclear Physics for the purpose of separating radioactive isotopes for studies in nuclear physics. Its design and operational features are described in the sections below. The separator is a modification of a vfi instrument [l] with the ion source outside the magnetic field, since the pole gap is inadequate to accommodate the ion source. However, this modification also avoids overheating of the poles and the effect of the magnetic field on the ion source performance. To utilize the original pole tips of the ?rfi magnet without physical change, the collector was located inside the magnet gap at a sector angle of about 210 O. The performance of the separator shows that it can be used for off-line separation of short-lived isotopes produced by ( LY,xn) reactions at 60 MeV using the variable-energy cyclotron of the Bhabha Atomic Research Centre adjoining the Saha Institute Campus at Bidhannagar in Calcutta.
isotopes
is described.
Intitial
operational
parameters
are presented.
used with variable temperature, measured by a W/Rh 5% and W/Rh 20% thermocouple. To maintain the electron density at the axis of the hollow-cathode ion source, an external axial field of up to 300 G is applied to compensate for the field of the filament. Without this compensation, the ion current is about 30% lower than with it. The mass analyzer is an i~omogeneous two-directional focussing magnet with a radius of curvature for the mean ion path of R, = 0.381 m and a deflection angle of 210 O. The value of the field index, n, for the central radius is 0.500 + 0.001, with a variation up to + 0.012 at the edges; these values were determined by a method described in ref. [l]. The gfi analyzer magnet developed at this laboratory [1,4] was modified to locate the ion source outside the magnetic sector. (In using the n& magnet, which has two-directional focussing, both
Hollow cathode ion source
2. Mass separator The mass separator consists of a Hollow-Cathode Ion Source (HCIS), an accelerating voltage power supply (O-10 kV), a magnetic mass analyser. a vacuum system, and either a foil collector or a Faraday cup with beam indicator and integrator (figs. 1 and 2). The hollow-cathode ion source is similar to that described by Sidenius 121. Its operational characteristics have been reported earlier [3]. The ion source is constructed with a stainless steel cylindrical anode and a molybdenum cathode with a tungsten spiral filament. The ion exit aperture is a 0.6”mm opening in the upper lid of the cathode. Ions are extracted from the ion source by using a conical electrode at ground potential, and focussed with an Einzel lens having a variable positive potential. The ion source can be used with either solid or gaseous samples. For solids, an oven is 0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
India
B.V.
De
-4 9a
Diffusion
Fig. 1. Schematic
diagram
Pump
Fore Pump
of the mass separator.
S. B. Kurmohupatro
Fig. 2. Photograph
/ Simple mass sepurutor for radioactic~e isotopes
of the mass separator, showing: (1) hollow-cathode yoke, (5) shielding arrangement,
the ion source and the collector would need to be placed inside the magnetic field.) The modified magnet has annular pole pieces of 0.43-m outer radius and 0.33-m inner radius, with a pole gap of 0.05 m at the mean radius. The pole pieces are fitted with annular guard rings to a 0.47-m radius, tapered at an angle of 7”, in order to compensate the effect of fringe fields at the mean ion optical path radius. The magnet is powered by a supply regulated to 1 part in 104. To accommodate the HCIS without overheating the poles, the exit aperture of the ion source is placed at a distance of I, = 0.92 R, from the true boundary of the magnet. Optimum focussing is obtained with the collector placed at a deflection angle of 210”. This is in approximate agreement with the value of 206 o prediced by Glavish [5] and Livingood [6] for a field index of n = 0.5. The ion path between the ion source exit and the magnet boundary is magnetically shielded; the shielding did not introduce any detectabe change to the field index of the main analysing field. The vacuum chamber is shown in fig. 1 and is described by Chakraborty et al. [3,7]. The operating pressure is - 10m5 Torr. The mass-analysed ion beam is detected by a Faraday cup connected to a current indicator and integrator to measure the ion current and, when required, the ion dose. Collection of ions is made by use of foils mounted on a rigid target holder made of 0.0125-m thick brass. 3. Operational characteristics We have measured the resolving separator using the mass spectrum
power of the mass of neon, especially
ion source. (2) analyser (6) focusing electrode.
chamber,
35
(3) magnetic
pole pieces, (4)
the “Ne and 22Ne lines. We observe a value of 342, compared to the value deduced from Livingood [6] of 416 from the relation RI’. = R,F/4W,,
(1)
where We is the radial width of the object and F is the resolution function. The resolution function, F, is related to the radial magnification M, for an inhomogeneous magnetic field by F= 2(1 -l/M,).
(2)
The efficiency of the separator has been measured by separating radioactive 61Cu and 64Cu, made by bombarding nickel foil with 3.8-MeV protons from the Institute Cyclotron, and separating the reaction products. The 61Cu was deposited on a copper foil and the 660-keV y rays were detected using a Ge(Li) spectrometer. From the ratios of the activities of the collected isotopes and their presence in the bombarded charge material, the efficiency for separating copper is estimated to be 5% [S]. This efficiency is adequate for carrying out radioactive isotope separation on a small scale. However, for further improvement in the separation efficiency, it is necessary to study the effects of ion scattering during operation of the instrument [7]. 4. Conclusions The present mass separator is small in size and is convenient to use in separating masses up to about 200 amu. The operating pressure has recently been improved to about 1O-6 Torr with the addition of a second vacuum pump (Diffstak, Edwards). It is exI. GENERAL SEPARATOR TECHNOLOGY
36
petted that this improvement higher efficiency of separation contamination, but examination mance is yet to be done. The future efforts at this using the separator for off-line nuclides produced by (0~. xn) Energy Cyclotron at Calcutta.
S.B. Karmohapatro / Simple mass separator
will allow us to obtain a with a reduced scattering of the improved perforsytem will still involve separation of short-lived reactions at the Variable
References [l] S.B. Karmahapatro, Ind. J. Phys. 34 (1960) 407. [2] G. Sidenius, Proc. Int. Conf. on Electromagnetic
Isotope
for radioactioe isotopes
Separators and Techniques of their Applications, Marburg. (1970), eds., H. Wagner and W. Walcher, p. 423. S. Kundu and S.B. Karmohapatro, Ind. J. [31 T. Chakraborty, Phys. 55B (1981) 131. Ind. J. Phys. 33 (1959) 139. (41 S.B. Karmohapatro. I51 H.F. Glavish, Nucl. Instr. and Meth. 189 (1981) 43. WI J.J. Livingood, The Optics of Dipole Magnets (Academic Press. New York, 1969). S. Kundu, and S.B. Karmohapatro, Nucl. [71 T. Chakraborty, Instr. and Meth. 199 (1982) 531. and S.B. Karmohapatro, Proc. PI A.K. Das, D.K. Mukhejee Nucl. Phys. Solid State Phys. Symp., Calcutta, 18B (1975) 313.