reactor hall, beam tube H9 | | Sources | 229Th, 233U, 235U, 237Np, 239Pu, 241Pu, 241Am, 245Cm, 249Cf, 251Cf | flux at target position | 5.3·1014 n cm-2 s-1 | horizontal aperture
| 2β ≤ 2.3° | vertical aperture | 2α ≤ 0.57° | solid angle W | ≤ 3.2 10-5 sr | total length of main path | 23 m | length of exit slit | 72 cm | mass dispersion for 1 % mass difference | 3.24 cm | energy dispersion for 1 % energy difference | 7.2 cm | mass resolution A/dA
(FWHM) | 400 for target size 0.8 x 7.5 cm2
1500 for target size 0.16 x 4 cm2 | magnet characteristics | angle of deflection | 45° | radius of deflection | 4 m | electric condensor field characteristics | angle of deflection | 35.35° | radius of deflection | 5.6 m | focussing magnet characteristics | angle of deflection | 65° | focussing edge | 35° | radius of deflection | 0.6 m | length of parabola to be focussed | 40 cm | distance magnet- focus | 1.05 m | size of image | < 1 x 6 cm2 | ionization chamber | energy resolution | E/ΔE > 100 | nuclear charge resolution | Z/ΔZ ≤ 36 |
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"LOHENGRIN" is a recoil mass spectrometer for unslowed fission products [1]. It allows us to study mass, kinetic energy and charge distribution for products from thermal neutron induced nuclear fission at very high resolution. The beam intensity at the separator allows also detection of γ-rays, conversion electrons, β-rays and delayed neutrons and coincidences between these particles.
Fission products originating from a target of a fissile isotope placed near the core of the reactor in a thermal neutron flux of 5.3 x 1014 n cm-2 s-1, are selected by a combination of a magnetic and an electric sector field whose deflections are perpendicular to each other. Both sector fields have focusing properties only in their plane of deflection. The combined action of the two fields separates ions with the same velocity into different parabolas according to their A/q value at the exit slit of the spectrometer where A and q are mass and ionic charge of the ions, respectively. The energy dispersion in the direction along each parabola amounts to 7.2 cm for 1% difference in energy. The mass dispersion perpendicular to each parabola amounts to 3.24 cm for 1% mass difference. As the width of the fission product distribution is about 14 MeV, 100 cm along the parabola are illuminated by one mass at the exit position of the spectrometer.
By a suitable choice of the field strengths, the particles of a chosen A/q value are deflected into the 72 cm exit slit. Depending on the target size and the collimator settings, mass resolving powers up to A/ΔA =1500 can be reached as standard, energy resolution values E/ΔE are between 100 and 1000.
A Reverse Energy Dispersion (RED) dipole magnet can be used to increase up to a factor of seven the particle density and to strongly reduce the background at a new focal position. This allows accurate studies of rare fission events in ternary, symmetric and far-asymmetric fission, and the determination of the decay characteristics of neutron-rich nuclei. The RED-magnet focuses a section of 40 cm along the Lohengrin parabola. This corresponds to an energy range of +/- 2.7 % about the central value.
The flight-path for the fission products is 23 m. The separation time is of the order of 2 µsec, so that fission products reach the detector before undergoing β-decay. Due to the characteristics of the separator, fission product mass and kinetic energy Y(A,E )produced in thermal neutron fission can be investigated.
The design and construction of ionization chambers allows us to additionally separate the fragments with respect to their nuclear charge, with a resolution Z/ΔZ = 36 up to Z = 42, and to reach a sensitivity of about 10-9 in the yield.
For time resolved measurements for nuclear isomers and in β-decay an electrostatic chopper is available, which imposes a time structure on the particle beam of 100 µsec to the second range.
Up to 105 fission products per second can be obtained behind the exit slit in the most abundant parabolas with a mass resolving power of A/ΔA = 250 (fw 1/10 m).
For spectroscopy work on neutron rich nuclei different equipment may be installed next to the exit slit of "Lohengrin" such as: ionization chambers, surface barrier detectors, Si and Ge detectors, plastic scintillators and long counters for delayed neutrons. Also a fast tape transport and a gas jet are available for users.
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