Fission-product spectrometer

Reactor hall, beam tube H9


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

400 for target size 0.8 x 7.5 cm2
1500 for target size 0.16 x 4 cm2

Magnet characteristics

angle of deflection


radius of deflection

4 m

Electric condenser field characteristics

angle of deflection


radius of deflection

5.6 m

Focussing magnet characteristics

angle of deflection


focussing edge


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

Instrument description

Lohengrin is a recoil mass spectrometer for unslowed fission products [1]. It allows the study of mass, kinetic energy and charge distribution in the products of thermal-neutron-induced nuclear fission at very high resolution. The beam intensity at the separator also makes it possible to detect 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 magnetic and electric sector fields 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/qvalue 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 distribution of the fission products is about 14 MeV, a 100 cm stretch along the parabola is illuminated by one type of mass at the exit position of  the spectrometer.

By a suitable choice of 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. 

Reverse Energy Dispersion (RED) dipole magnet can be used to increase particle density up to a factor of seven and 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, the fission product mass and kinetic energy Y(A,E ) produced during 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 items of equipment can be installed next to Lohengrin's exit slit: ionization chambers, surface barrier detectors, Si and Ge detectors, plastic scintillators and long counters for delayed neutrons. There is also a fast tape transport system and gas jet available for users.