Report
First in-beam test with a
symmetric, 36-fold segmented AGATA detector
Th. Steinhardt, J. Eberth, D. Weißhaar, O. Thelen, H. Hess
Institut für Kernphysik, Universität zu Köln
Abstract
An in-beam test of the symmetric, 36-fold segmented, encapsulated AGATA-HPGe-detector
mounted in the Cologne single cryostat was performed at the Van-De-Graaff tandem
accelerator of the university of Cologne. Both, the detector placement close to the target
(3.4cm) at 90° with respect to the beam direction and the inverse transfer reaction d (37Cl, n)
38
Ar at 70MeV resulting in a large v/c of 5.6% were chosen in order to obtain a maximum
Doppler shift for the γ-quanta emitted in flight. In this experimental setup the Doppler
broadening of the γ-lines in the energy spectrum is dominated by the opening angle of the
detector. The position sensitivity of the AGATA detector was used to localize the first
interaction point of a γ in the HPGe-crystal. With this information the energy resolution of the
γ-lines was improved. From the resulting energy resolution the effective opening angle ΔΘ of
the detector was calculated and translated in a position resolution of better than 5mm for the
AGATA detector.
Experiment
In the inverse transfer reaction 37Cl on a deuterated Titanium target at a beam energy of
70MeV the nuclei 38Cl and 38Ar were produced. The 2+-level of 38Ar was populated
transfering a proton and via βdecay of 38Cl, respectively. The
ground state transition with a γenergy of 2167keV occurs in
flight for the first population
mechanism. This yields a
Doppler shift and therefore a
Doppler broadening of the γ-line
in the energy spectrum caused by
the opening angle of the detector.
In contrast, the population via βdecay of 38Cl happens in rest and
results in a sharp γ-line which
shows the intrinsic energy
resolution of the detector.
The detector itself was placed
under 90° with respect to the
of the experimental setup and the (partial) level
beam direction in a distance of Figure 1: sketch
scheme of 38Ar
3.4cm to the deuterated Titanium
target. The target thickness was
200μg/cm2. A monitor HPGe-detector under 90° and in a distance of 180cm to the target was
used to measure the contribution of the reaction kinematics and of the scattering to the energy
resolution. The detector events were processed with the digital spectroscopy electronics DGF-
4C, Rev. E from XIA at a sampling rate of 40MHz. For each event triggered by the core
electrode the digitized detector signals of 1μs length (40 sample points) and the γ-energies of
all 37 channels (36 segments + Core) were written to hard disk drives. During 6 days of beam
time 1.2·108 events were registered.
Analysis
Three types of spectra were generated. First the core energies were sorted in a single spectrum
which means treating the detector as a non-segmented one. In the second type, a set of spectra
with core energies gated on the different segment hit patterns were produced. These spectra
were corrected individually for the Doppler shift and added to one spectrum at the end. In a
last type the pulse shapes of the detector signals were analysed applying the MINIBALLalgorithms. In this approach the radial information of the first interaction point is extracted
from the signal of the core electrode by determining the charge collection time of the electron
component (steepest-slope algorithm). Mirror charges induced in the (four) neighbouring
segments yield an azimuthal position and a depth information of the interaction within the hit
segment (asymmetry algorithm). Also here a set of gated core energy spectra was generated,
corrected individually and added up to a final spectrum.
Generally the first interaction is identified by the main interaction concept which means that
the interaction with the highest energy deposition is assigned as first interaction. It should be
mentioned that no events were rejected during the analysis procedure.
Results
The resulting energy spectra of the core electrode for the ground state transition 2 +→0+
(2167keV) of 38Ar are shown in figure 2. The black spectrum with a full width half maximum
of 86keV shows the γ-line obtained without any position information. One can clearly see the
Figure 2: Spectra of the core for the Doppler broadened 2167keV transition of 38Ar using no position sensitive
infor-mation (black), hit pattern of segments (red) and applying pulse-shape analysis with MINIBALL
algorithms (blue).
sharp 2167keV-line of the ground state transition populated via the β-decay of 38Cl on top of
the Doppler broadened peak. This γ-line shows the intrinsic resolution of 3keV of the detector
at this energy. In the red spectrum the FWHM of 33keV indicates an improvement in energy
resolution by a factor of 3 using the hit pattern of the segments. Another improvement by a
factor of 3 is gained by applying the pulse-shape analysis with -MINIBALL-algorithms. The
γ-line in the corresponding blue spectrum has a FWHM of 11.5keV. Furthermore, the
improvement in energy resolution due to pulse-shape analysis enables the identification of the
transition 3822keV→1617keV (2204keV) of 38Cl in the spectrum which is not visible in the
two other spectra (The knee in the black spectrum originates from 226Ra from natural
background. Its intensity is spread over the 86keV energy interval for the red and blue
spectrum as the intensity of the stopped 2167keV component.).
The measured energy resolution ΔEmeasured is composed of the intrinsic energy resolution of
the spectroscopic channel ΔEintr. , the contribution of the kinematics ΔEkinematics and the
effective opening angle of the detector ΔEΔΘ
E
measured

E
2

 Eint r.  Ekinematics .
2
2
The value of ΔEintr. is 3keV for the AGATA detector at Eγ=2167keV. The kinematical
component ΔEkinematics = 5.9keV is measured with the monitor detector. These values yield for
the measured energy resolution of 11.5keV, achieved via the pulse-shape analysis, an opening
angle component ΔEΔΘ=9.5keV. Applying the approximation for Doppler broadening under
90°
v
E     E
c
the uncertainity of the observation angle turns out to be ΔΘ=4.5°. Hereby Eγ represents the γenergy, v the velocity of the γ-emitting nuclei (5.6%c), c the speed of light and ΔEΔΘ the
contribution of the opening angle to the resolution.
The effective opening angle of ΔΘ=4.5° can be translated in a position resolution l via the
geometric relation l = d · ΔΘ with d describing the distance of the detector to the target. While
the depth of 9cm of the HPGe-crystal is large in respect to the target-detector distance of
3.4cm one has to choose an effective distance. Most of the events (>67%) had their main
interaction point in one of the first three front slices of the detector (depth 3.6cm). Hence an
effective distance d not greater than 6cm is a good first approximation. This results in a
position resolution of better than 5mm.
An analysis of the energy resolution obtained for each slice where the effective distance d can
be given quite accurately shows for the two front slices a position resolution of even better
than 4mm and for the third slice of 4.8mm.
The results of this analysis will be discussed in more detail in an upcoming paper.