Photons of high energy (g quanta) can excite nuclear states. Since the photons transfer mainly spin 1 and, to a lesser extent, spin 2, low-spin states are excited. These states deexcite by emitting g transitions of discrete energies - according to the spacings between the excited states and lower lying states (see schematic figure). As this emission is induced by a photon of the appropriate (resonant) energy, the process is called "nuclear resonance fluorescence". The properties of the g transitions allow the quantum numbers of the excited states to be deduced:
The excited states with spin J = 1 are referred to as
"dipole excitations". These excitations may involve collective excitations
of the nucleus such as vibrations. The study of these excitations and the
discovery of so far unknown types of nuclear motion is important for the
understanding of many-body quantum systems, which play a role also in
other fields of modern physics, e.g. in solid-state physics.
In order to realise the reactions described above, an intense photon beam with
a continuous energy spectrum is needed. Such a beam is delivered by
bremsstrahlung, which is produced if an electron beam hits an appropriate
material ("radiator"). The electrons slow down in the material and lose their
energy either by ionisation or by emission of radiation. For our experiments,
we use the electron beam of the superconducting linear accelerator
at the FZD with a maximum electron energy of 20 MeV.
The bremsstrahlung facility at the ELBE accelerator is located in the centre of the floorplan between the first and second accelerator modules and in the experimental hall for nuclear physics.
The electron beam is lead out of the main beam line by a non-dispersive system
of two 22.5o dipole magnets and a quadrupole magnet
in between. A doublet of quadrupole magnets focuses the beam on to the
radiator (the blue-red yoke of one of them is visible at the right border of
the figure below. The following doublet of dipole magnets (steerers) will be
used to produce a beam of polarised photons (see below).
The radiator (within the chamber in front of the dipole magnet with the blue
yoke) converts only a small fraction of the electron current to radiation.
The main part passes through the dipole magnet, where it is deflected by
45o and is subsequently stopped in the
electron beam dump made of graphite. This beam dump is surrounded by the
stainless vacuum vessel within the yellow-painted iron shielding.
The radiator consists either of a thin niobium foil with a thickness between
2 and 12 mm. A water-cooled holder of copper with
six positions for radiators of 16 mm diameter is used (see figure below). This
allows us to switch between different radiators without breaking the vacuum.
The radiator is described in detail in the
Annual Report 2002, p. 30.
The collimator produces a beam of limited diameter from the spatial
distribution of the bremsstrahlung photons. In our case, the collimator is
placed in the wall of heavy concrete between the accelerator hall and our
experimental cave. The collimator starts at a distance of 972 mm behind the
radiator and has a length of 2600 mm (see figure). The drill-hole is conical
and has a diameter of 5 mm at the entrance and 24 mm at the exit. In order to
minimise the production of neutrons, the collimator has been made of pure
aluminium, which has a relatively high neutron-separation energy of
Sn = 13 MeV. Further details of the
collimator are described in the
Annual Report 2001, p. 37.
The beam hardener and shutter shown in the figure is placed in front of the
entrance of the collimator and consists of a cylindrical housing which
contains 3 cylinders of aluminium (Al), carbon (C) and tungsten alloy (W),
respectively (see figure). These cylinders can be moved vertically such that
one or none of them is located in front of the collimator.
The aluminium and carbon cylinders absorb mainly low-energy photons while they
influence the high-energy part of the spectrum only to a small extent. In this
way, they "harden" the photon spectrum. The tungsten cylinder is to shut the
collimator. This allows us to work in the experimental cave while the
accelerator is running for experiments in other caves. For further details see
Report 2001, p. 41.
In order to deduce the parity of the excited states (see top of page), it is
necessary to measure the linear polarisation of the
g transitions. The knowledge of the polarisation
enables a discrimination between electric and magnetic multipole radiation, in
our case mainly between electric dipole (E1) and magnetic dipole (M1)
transitions. For this purpose, we will irradiate the target with polarised
bremsstrahlung. The method used to produce polarised bremsstrahlung is shown
in the figure. It makes use of the fact that the electric vector of the
radiation goes tangentially around a cross section of the spatial distribution
of the photons (see figure). Using the steering magnets mentioned above and
described in the
Annual Report 2002, p. 31, the electron beam will be deflected from the normal
direction and then deflected back such that it hits the radiator in the centre
under a particular angle Qo. As a consequence, an off-axis portion of the spatial distribution of the
photons is cut out by the collimator. This off-axis portion of the
bremsstrahlung cone is dominated by a definite direction of the electric
vector, which means that the photon beam passing through the collimator is
partly polarised. The degree of polarisation depends on the angle
Qo and has a maximum at
Qo = moc2 / E, where
moc2 = 511 keV is the rest energy of the electron and E is the energy of the
During long-time measurements it is important to monitor the degree of
polarisation of the photons. This can be done via the photodisintegration of
the deuteron. Because of predominant E1 absorption in this process the protons
and neutrons are emitted preferentially in the direction of the electric field
vector of the polarised bremsstrahlung. The energies of the incident photons
correlate with the measured proton energies. The degree of polarisation can be
deduced from azimuthal asymmetries of the intensities of the protons.
The detector setup shown in the photograph below contains 4 high-purity
germanium (HPGe) detectors. Two of them (top and bottom) are positioned
perpendiculer to the photon-beam direction, while the two horizontal ones
can be moved between angles of 90o and
127o relative to the photon beam. In this way
the setup can be used either to measure azimuthal asymmetries of the
g-ray intensities in experiments with polarised
bremsstrahlung or to measure angular distributions of the scattered
All HPGe detectors are surrounded by escape-suppression shields.
These shields consist of 8 optically separated bismuth germanate (BGO)
scintillation detectors arranged in a cylindrical shell and each read out by
a photomultiplier tube. The BGO shields with the photomultipliers can be
seen in the above photograph. The BGO shields are to register
g rays which escape after Compton scattering or
pair creation in the HPGe detector and, thus, contribute to the background
in the HPGe spectra. The suppression of such events is realised by an
anticoincidence between HPGe detector and BGO shield. In order to prevent the
BGO detectors from direct hits by g rays emitted
from the target, the front side of the BGO shield is covered by a lead
collimator. This collimator and the front part of the BGO shield with the HPGe
detector inside are shown in the drawing below. See also the Annual Report 2002, p. 24 and 26.
The photon-beam dump is shown in the figure below. This beam dump was designed
such that the backscattering of photons towards the detectors is minimised.
The beam dump consists of an trapezoidal absorber block of polyethylene (PE)
with a front side of 80 cm X 80 cm and a length at the beam axis of 100 cm.
This inner absorber is surrounded by 0.5 mm thick sheets of cadmium and a lead
housing with a thickness of 20 cm at the front and 10 cm at the sides. For
details about the choice of the materials see the
Annual Report 2000, p. 40.