Journal of Nuclear and Radiochemical Sciences,Vol. 3, No. 1, pp. 213–215, 2002

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Towards Super Heavy Nuclei Spectroscopy with a Gamma Ray Tracking Detector
A. Korichi∗
Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, Campus Universitaire d’Orsay, CNRSIN2P3, 91405 Orsay cedex, France
Received: December 23, 2001; In Final Form: May 7, 2002
In this contribution the emphasis will be on the capabilities of the new generation of Ge arrays based on a newconcept of detection using Gamma Ray Tracking and recent advances in crystal segmentation technology which
will enable in-beam spectroscopy towards super heavy elements.

1. Physics Motivations
The existence of super heavy nuclei is a striking manifestation of shell effects. Since the heaviest nuclei are weakly boundin their ground state, it is important to determine the limiting
spin and excitation energy that they can sustain. Their rotational
properties constitute an excellent laboratory to test HFB meanfield theory at extreme masses, far away from the ones where theeffective forces have been adjusted. Their observation at highspin gives additional information on the fission barrier whichgoverns their survival probability. Another important question
to address concerns the influence of the pairing correlations on
the rotational properties (pairing is crucial in reproducing thedynamical moment of inertia (2) ).
A beautiful example of γ-ray spectroscopy performed onheavy elements, came out from an experiment1, 2 in which theexcited states (up to spin 18h̄) of 254 No have been investigated(the heaviest nucleus studied in γ-ray spectroscopy so far). The
observed rotational band, as shown by M. Leino in this conference, firmly established that the nucleus is deformed and constitutes an important confirmation of the predictions. Furthermore,
recent calculations3 using a pairing force adjusted on SD bands
in the mass ∼150 region surprisingly reproduce the (2) of theobserved ground-state rotational band in 254 No. In addition, inorder to gain some understanding about the single-particle shellstructure, the emphasis will be on the investigation of the properties of odd-A heavy elements. However, to extend the study ofthese nuclei towards odd-mass nuclei and/or heavier masses isexperimentally difficult because they are created with extremely
low cross sections (few nanobarn) and with a very high fission
background. These problems can be overcome by combining
a very efficient γ-ray detection and an appropriate residual nucleus identification. On the other hand, because of the very highe− conversion probability at low energies and large Z, it willbe crucial to measure conversion e− in coincidence with γ raysfrom these states.
All these important features have been discussed in several
sessions of this conference by different speakers as can be seenin the final proceedings.

is surrounded by BGO shields, which suppress the Comptonescaped γ rays and therefore provide a significant improvementin the spectrum quality.
Even though these large arrays still enable new interestingstudies of physics at the extreme and will allow experiments atvery low cross sections as expected for super heavy nuclei orwith the radioactive beam facilities in the next few years, theirperformance is close to the ultimate limit that can be obtainedwith escape-suppressed devices.
A totally new concept is required in order to further increase
the efficiency and granularity. To make the next major advance,
one would replace the BGO shields by active Ge to build ahighly segmented Ge shell.
2.2. Design and Performances of the Future Array:
AGATA. The aim of the TMR (Training and Mobility for Researchers, launched in 1997 and coordinated by R. Lieder) program is to design a 4π Ge array based on a new concept of detection using Gamma Ray Tracking and recent advances in crystal
segmentation technology. This concept is based on the ability tolocate, within few mm, each interaction point in the Ge detector
and consequently track the scattering sequence of an incident γray. The tracking method will consist of the reconstruction of
the full γ-ray energy by combining the appropriate interaction
points. As a result, this will provide a significant gain in the efficiency because the Compton shields (which limit the Ge solidangle) will not be necessary and will be replaced by active Gedetectors to give for the first time a real 4π Ge ball. GEANT
simulations have already shown the capabilities of the tracking
concept for a new generation of 4πγ Ge array. However a highClover detectors
BGO shieldStandard GeGe

W collimators

2. Why Do We Need a New Generation of γ Arrays?
2.1. Current Arrays and their Limitations. During thelast two decades the development and improvement of very efficient 4π Ge detectors have provided an enormous amount of information on the structure of the atomic nucleus. The state of theart with respect to 4πγ arrays is represented by EUROBALL4 in
Europe and GAMMASPHERE5 in the USA. Figure 1 illustrates
the view of EUROBALL built from 239 Ge crystals. It consistsof 15 Clusters detectors (each composed of 7 encapsulated Gedetectors), 26 Clovers (4 Ge detectors in one cryostat) and 30
standard Ge detectors. Each type of the EUROBALL detectors∗Corresponding author. E-mail: korichi@csnsm.in2p3.fr.
FAX: +33-1-6915.7741.

Cluster detectors
Figure 1. Section view of EUROBALL built from 239 Ge crystals.
It consists of 15 Clusters detectors at backward angles, 26 Clovers at90◦ , and 30 standard Ge detectors at forward angles. Each detector issurrounded by BGO shield.

c 2002 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 6/30/2002

214 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

KorichiFigure 2. The final AGATA configuration with encapsulated crystals grouped triple-clusters detectors is shown in the left side of the panel. The
right panel shows the detector module for AGATA. The 36-fold segmented Ge is marked by (1), preamplifier (2), frame-support (3), digital pulse
processing electronics (4), fiber-optics read-out (5), LN2-dewar (6), and target position marked by (7).
TABLE 1: The tracking array performances illustrated in terms of comparison between EUROBALL and AGATA. The efficiencies andpeak-to-total quantities are given for low and high γ-ray multiplicities.

Ge weight / kg

Detectors [Crystals]

233
225
210

—70 [190]
239

Ideal sphere
AGATAEUROBALLεω [P/T] / %Mγ = 165 [85]40 [65]
9.4 [50]εω [P/T] / %Mγ = 30
36 [60]25 [50]6.5 [37]

TABLE 2: Comparison between GAMMASPHERE and AGATA in terms of expected counts for a 200 keV line in double coincidence andfor two different reactions which populate the indicated nuclei. The fusion cross sections are also indicated and taken from Reference 9.

ReactionPb(48 Ca, 2n)254 No
208
Pb(50 Ti, 2n)256 Rf

208

Z = 102Z = 104

Cross section
3 µb≈10 nbdegree of confidence in the reconstruction of the γ-ray path can
only be obtained with highly segmented detectors which results
in a better localisation of the γ-ray position. It has been demonstrated that a position resolution of ≤5 mm, which is needed for
γ-ray tracking, can be achieved by pulse shape analysis of thesegment signals using digital electronics.
This new type of spectrometer requires the optimisation ofseveral parameters:• the full-energy or photopeak efficiency must be maximised,• a very good peak-to-toal ratio (P/T),• an excellent angular resolution in order to reduce the Doppler
broadening,• a high counting rate capabilities,• a suitable free inner space available for additional detectors
inside the Ge ball.
These features can be obtained with the new generation of spectrometers such as AGATA (Advance GAmma Tracking Array).6
Indeed, this new generation of γ-ray calorimeters, such as
AGATA in Europe or GRETA7 in the USA, will provide a dramatic gain in efficiency above any presently existing array suchas EUROBALL or GAMMASPHERE. An important gain in
the resolving power will also be achieved since the angle atwhich the γ ray is emitted will be much better defined allowing the Doppler broadening to be greatly reduced, especially for
nuclei with large v/c. Indeed, to quantify the performance of a
4π array, the concept of resolving power is usually introduced;
this corresponds to the ability to isolate a given sequence of γrays from a complex spectrum.
This future detector will be a powerful tool to be used withhigh intensity stable beams as well as for the next generation ofradioactive beam accelerators such as SPIRAL2 or EURISOL
in Europe or the RI Beam Factory in RIKEN. It will clearly
open up new possibilities concerning rare phenomena at highspin (or high multiplicity) and high excitation energy as well asvery far from stability.
In view of its modularity and symmetry, the selected configu-GAMASPHERE
250.2AGATA
1000
10ration for AGATA corresponds to a Ge ball with 70 cryostats
composed of triple 36-segmented clusters. The final geometry will consist of 180 irregular hexagons and 10 pentagons asshown in Figure 2. The total number of segments in the array istherefore 6780.
2.3. Performances of AGATA. The tracking performances
are illustrated in terms of a comparison between EUROBALL
and AGATA arrays. The simulations are performed using theGEANT package for different γ multiplicities as can been seenin Table 1. The obtained results (efficiency, Peak/Total) aretaken from Reference 8 and references therein.
Table 1 clearly illustrates the capabilities of the new generation of Ge arrays which enable the in-beam spectroscopy ofheavy elements. However, for the study of heavier element(Z = 104 and beyond) would not be possible with existing arrays
as the production cross section drops dramatically.
For example, the production yield of 256 Rf in the 50 Ti + 208 Pb
reaction is only about few nanobarn! and one would expect 10counts in the peak at 200 keV γ ray with AGATA instead of lessthan 1 count with EUROBALL in 10 days run. Table 2 shows
a comparison between GAMMASPHERE (9% efficiency) andAGATA in terms of expected counts for γ-ray spectroscopy.
Again, γ-ray spectroscopy studies become possible with acombination of high granularity and ultra-high counting rate capabilities. Only a Gamma Ray Tracking spectrometer is able
to satisfy these different and sometimes conflicting needs. With
such new devices a gain of 50 to 100 for the sensitivity is expected compared to EUROBALL.
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