Institut Interuniversitaire des Sciences Nucléaires - IISN
Université catholique de Louvain - UCL

Editorial

The two contributions contained in the present issue of the CRC Newsletter are representative of two directions of research presently pursued at CRC: fundamental nuclear physics on one side, and applied physics on the other side. The contribution of Gerda Neyens and Katrien Vyvey from IKS-Leuven deals with the measurement of quadrupole moments for isomeric states, a technique developped by this group since more than a decade; the contribution of Karl Gill from CERN reports on ra

diation damage measurements induced by high-energy neutrons in optoelectronic components to be used in the CMS detector at CERN.

The present Board of CRC intends to provide both directions of research with high-quality beams and installations. Regarding the latter item, the refurbishment of igloo-R, where most of the nuclear astrophysics experiments are performed, was completed and the same kind of work is underway in the central line of ig

Inside this issue:

News from the editor

In its meeting of February 6, 2001, the CRC Board has decided that the beam time scheduled at the Planning meetings will cover a semester. Planning meetings, attended by spokesperson and/or contact persons of running experiments, organize the beam time schedule after the PAC meetings. The next PAC

meeting will be held on June 18, 2001 and will examine proposals for the period August to December 2001. Accordingly, the next Planning meeting will be held on June 27, 2001 and will therefore cover the period August to December 2001. We are confident that this new organization will allow a more suitable

Charge deformation of high-spin isomers as a test to nuclear structure

Since 1987 a super-conducting magnet, reaching a field up to 4.4 Tesla, is placed on-line to the CYCLONE110 cyclotron at the CRC. With this magnet, in-beam experiments have been performed to study the nuclear quadrupole moment of a wide variety of high-spin isomeric states produced in fusion-evaporation reactions. The technique that was used to measure the quadrupole moment has been developed by a research group of the IKS (University of Leuven) in the mid eighties. This method, called the Level Mixing Spectroscopy (LEMS) method, allows to measure the static quadrupole moment of high-spin isomers with lifetimes ranging from about 50 ns up to 100 ms. The technique is especially suited for the study of isomers with very high spins and lifetimes higher than about 50 microseconde, since no other technique is available for the study of the moments of these isomers.

The quadrupole moment is a parameter that gives a measure for the asymmetric charge distribution in nuclei and is thus related to the nuclear deformation. In near-spherical nuclei one could expect that the charge distribution is mainly determined by the valence proton distribution, since neutrons do not have a charge. However, due to the nucleon-nucleon or neutron-core residual interaction, it is

possible that the neutrons polarize the spherical core and thus induce an additional deformation of the nucleus. Over the last decade the Leuven group has measured several quadrupole moments of high-spin isomers in the trans-lead region left of the doubly magic ²⁰⁸Pb (Z³82, N£126). Special attention has been paid to investigate how the breaking of the ²⁰⁸Pb-core (by exciting neutrons across the N=126 or protons across the Z=82 shell gaps) influences the nuclear deformation. Such broken-core nuclear states occur as excited states in nuclei, and some of them are isomeric (spin-traps, intruder isomers). It has been found that breaking the neutron core does not induce a substantial deformation. The quadrupole moments of neutron core-excited states in the Fr-isotopes (with spins up to 65/2 hbar) are well reproduced assuming a core-polarization similar to that for a normal isomer with a similar number of neutron holes in the N=126 shell. Last year the first measurement on a proton core-excited state (11^- intruder isomer in ¹⁹⁶Pb) has been performed. The result reveals a substantial increase of the nuclear deformation by breaking the proton core. A future measurement of the quadrupole moment for a similar normal isomer in the Po-isotopes will have to confirm this.

Another type of isomers occurs in highly deformed nuclei, such as in the mass A~180 region where both protons and neutrons are far from magic numbers (Z~72 and N~108). Excited states in these nuclei are obtained either by rotation of the deformed charge state or by breaking a pair of nucleons (multi-quasi particle states). These quasi particle (qp) states are isomeric when their decay requires a change in the orientation of the nuclear spin with respect to the symmetry axis of deformation (K-

Radiation Resistant Optical Links for Readout of the CMS Silicon Tracker

Research has started recently at the Cyclotron Research Centre, on project PH-165, to measure the effects of radiation damage from neutrons in optoelectronic components. These components are intended for use in the fibre-optic readout system of the silicon tracking detectors in the Compact Muon Solenoid (CMS) Experiment, which will operate in the Large Hadron Collider at CERN. The role of the CMS Tracker is to find and measure with great precision the interesting charged particle tracks hidden amongst the traces from about one thousand other spiralling particle tracks, produced when bunches of 7 TeV protons collide at the centre of CMS.

Fibre-optic readout provides the only practical means of acquiring the 8-bit analogue data from the 10 million channels in the silicon microstrip Tracker with a trigger rate up to 100 kHz. Links to the many, widely distributed, detector modules are possible using components that are very compact, have low-mass, and require relatively little power. These are important factors that apply to all of the materials used in the tracker, influencing both the physical tracker layout and its ultimate performance.

The analogue optical link readout system consists of 50000 fibre channels, each transmitting 8-bit analogue signals at 40 M samples/s by direct modulation of 1310 nm lasers at the front-end. Data are multiplexed from two readout chips each bonded to 128 silicon strips. The main analogue link requirements are: 48 dB peak-signal/noise, <2% non-linearity, and 100 MHz bandwidth. In comparison with the performance achieved in analogue distribution networks, such as cable TV, our requirements are modest. However, these speci

fications must be met at a very low cost, given the large number of channels. We have therefore had to avoid custom development of the optical link components and, wherever possible, commercial off-the-shelf (COTS) components have been selected.

Another key issue is that components must survive the extreme conditions inside the CMS Tracker, namely the harsh radiation environment. Each 7+7 TeV proton-proton collision generates ~100 charged particles, mainly pions with an energy ~100 MeV, plus a similar number of high energy photons from decaying neutral pions. Over a ten year operating lifetime components must withstand a total dose of up to ~100 kGy and the displacement damage due to the passage of  ~ 10¹⁴ particles/cm². 

Previous research in the Nuclear and Space applications communities, which has been highlighted in RADECS, SPIE and NSREC conferences, has established that commercial optoelectronic and fibre-optic devices can be very sensitive to radiation damage.

In lasers for example, which are considered to be the most sensitive element in the CMS Tracker optical links, a variety of radiation damage effects are possible. Degradation occurs due to the introduction of non-radiative recombination sites in the semiconductor band-gap, generated when defects form through displacement damage in and around the optical cavity. The main practical effects of this damage are an increase in the threshold current required to produce the laser action, in addition to a decrease in the output light efficiency.

Only by measuring these radiation damage effects, in the actual de

vices being considered for use in the Tracker, can we prepare for the radiation damage that will occur during the final application. Replacing failed components, once installed in the Tracker, will be practically impossible as opening and closing CMS for any maintenance of the Tracker elements requires ~1 year.

Using various calibrated radiation sources, such as the T2 source at CRC, we have measured the damage induced by a certain amount of radiation in candidate lasers. It has therefore been possible to build-in an extra degree of safety in the optical link system by providing compensating mechanisms against the radiation effects. In the analogue optical transmitter for example, the laser driver includes an adjustable laser bias-current output, as well as an adjustable gain, which allows tracking and compensation of the threshold and efficiency degradation in lasers over the lifetime of the CMS Tracker.

In summary all of the devices being considered for use in the optical links must be qualified to ensure that their radiation resistance and long-term reliability are sufficient. Extensive validation testing has already started in the development phase of optical links, and will now continue through to the final production. This is the objective of project PH-165 at CRC.

Karl Gill
CMS Experiment
EP Division, CERN

http://cms-tk-opto.web.cern.ch

PAC - The CRC Program Advisory Committee

The experiments proposed at the CRC have to be approved by a Users Selection Panel or Program Advisory Committee (PAC), which at present includes 9 scientific experts, of whom 4 are non Belgians, 4 are Belgians external to the facility, and 1 is a member of the Nuclear Physics Department (FYNU) at the UCL.

The present members of the CRC PAC are:

• M. Arnould (Université Libre de Bruxelles - Belgium)
  
• J. Aystö (University of Jyväskylä - Finland)
  
• J. Cugnon (Université de Liège - Belgium)
  
• K. Dietrich (Technische Universität München - Germany)
  
• Y. El Masri (Université catholique de Louvain-Belgium)
  
• M.N. Harakeh (Rijksuniversiteit Gröningen - The Netherlands)
  
• W. Kutschera (Universität Wien- Austria)
  
• P. Van Duppen (Katholieke Universiteit Leuven - Belgium) - Chairman
  
• C. Wagemans (Universiteit Gent - Belgium).
  

Proposals accepted by the PAC (12 January 2001)

Five new proposals were accepted by the PAC at its last meeting of 12 January 2001:
PH-175: The ⁷Be +p elastic scattering at low energies
Spokesperson: Carmen Angulo (UCLouvain) - 18 shifts allocated

PH-176: Spectroscopy of ¹⁹F by transfer reaction and application to nova g-ray emission
Spokesperson: Alain Coc (CSNSM-Orsay) - 21 shifts allocated

PH-177: Beta decay of neutron deficient nuclei below ¹⁰⁰Sn
Spokesperson: Magdalena Gorska (KULeuven) - 16 shifts allocated

PH-178: Test of the Adiabatic Resonance Crossing (ARC) technique for the production of  medical radioisotopes
Spokesperson: Isabelle Tilquin (UCLouvain) - 32 shifts allocated

PH-179: Characterization of a detector designed to measure the flux of high energy ma gnetospheric charged particles with high mass, angular and spectral resolution
Spokesperson: Mathias Cyamukungu (UCLouvain)  - 2 shifts allocated

Calendar of forthcoming deadlines

April 20, 2001


April 25, 2001

May 18, 2001


June 18, 2001

June 22, 2001


June 27, 2001

Deadline for updated beam time request for the period May 1 to July 19, 2001

Planning meeting at 14:00, for the period May 1 to July 19, 2001

Deadline for the submission of status reports and new proposals for the period August to December, 2001.

Meeting of the Program Advisory Committee.

Updated beam time requests for the period August to December, 2001.

Planning meeting at 14:00, for the period August to December, 2001.

Call for proposals of experiments (HPRI program)

Since October 1996, the Cyclotron Research Centre is a Large Scale Facility supported by the European Commission. The new HPRI contract  enables the CRC to reimburse the travel and living expenses of researchers coming to carry out experiments if:

• the Spokesperson of the experiment is a citizen of a Member State or an Associated State and
  
• the researchers are citizens of a Member State of the European Union or of an Associated State and
  
• they belong to an Institution of a Member or Associated State, and
  
• the experiment was approved by the PAC and acknowledged by the EC.
  

sure a hotel reservation, the spokespersons of experiments are requested to contact the CRC Secretary, Mme Carine Baras at least 1 month before the arrival. All hotel reservation must be done through the CRC secretary.

The European researchers eligible for support in the framework of the above-mentioned contract are invited to submit one-page application for financial support to the CRC User Support, Carmen Angulo (see below). This page should mention the approved experiment and include the names, nationalities, ages, affiliations, and positions of the applicants as well as an estimation of the financial needs. An example of application form can be found in the CRC web site:
www.cyc.ucl.ac.be/HPRI.html

Contact address:
C. Angulo
Centre de Recherches du Cyclotron
Chemin du Cyclotron 2
1348 Louvain-la-neuve, Belgium
Phone: + 32 10 473231/2998
Fax: + 32 10 45 2183
E-mail: angulo@cyc.ucl.ac.be

"To ensure a hotel reservation, the spokespersons of the experiments should contact the CRC secretary 1 month before the arrival.

All hotel reservation must be done through the CRC secretary".

News and Events at FYNU

In July 2001, Roger Versin (mechanical workshop) retired. Roger has worked during more than 10 years in close collaboration with the group of low-energy physics at the cyclotron laboratory. He produced many masterwork pieces for the radioactive beam experiments, in special those related with the LEDA chamber. In the more recent years, he worked on the setup of the recoil separator ARES. His long-term experience and advice were of invaluable assistance during the preparation and setting up of experiments at the CRC.

Dr. Gabriel Tabacaru has joined the FYNU department in February 2001. Gabriel graduated in physics at the University of Bucharest and has recently got his PhD degree at the

IPN-Orsay. He will mainly work at the ARES separator.


Visitors:

Prof. Valentin Corcalciuc - Institute of Atomic Physics, Bucharest, Romania, (October-December 2000).

Prof. Moshe Gai - University of Connecticut, Storrs, USA (November 2000).

Prof. Pierre Depommier -- University of Montreal, Canada (February 2001).

C.A.

Cyclotron Research Centre
Université catholique de Louvain
Chemin du Cyclotron, 2
1348 Louvain-la-Neuve
Belgium

Secretary: C. Baras
baras@cyc.ucl.ac.be

Phone: 32 10 47 2998
Fax:     32 10 45 2183