First results of a high precision mass measurement program for very short-lived
nuclides 


C. Toader*, G. Audi, G. Conreur, H. Doubre, S. Henry, M. Jacotin, J.F. Képinski,
G. Le Scornet, D. Lunney, C. Monsanglant, M. de Saint Simon, C. Thibault 
CSNSM-IN2P3-CNRS, Bat 108, F-91405 Orsay-campus

C. Borcea, M. Duma
INPE, P.O. Box MG-6, RO-76900 Bucharest-Magurele

G.Lebee
CERN, CH-1211 Geneva 23

H-J. Kluge
GSI, Planckstrasse 1, D-64291 Darmstadt

G. Bollen
Univ. Munich, Coulombwall 1, D-85748 Garching
	   
and the ISOLDE collaboration
CERN, CH-1211 Geneva 23



Recent commissioning of the new experimental program MISTRAL (Mass Measurements
at ISolde using a Transmission and Radiofrequency spectrometer on Line) brought
mass measurements of high precision performed on sodium, magnesium, aluminum,
potassium, calcium and titanium isotopes. This spectrometer is using a technique
of radiofrequency excitation of ion trajectories in a homogeneous magnetic
field. The mass is obtained by the comparison of the cyclotron frequency of the
unknown mass to that of a reference mass. This measurement procedure is well
suited for direct mass measurements of very short-lived nuclides, the shortest
half-life of an isotope measured with MISTRAL was 30.5 ms (28Na).

Though this first period of data taking was dedicated to the exploration of
MISTRAL performances it has allowed us to reduce the uncertainty in the mass of
the most exotic isotopes of sodium by almost one order of magnitude. Even for
the weakly produced 30Na (T1/2=48ms), the accuracy was still 8*10^-7.

Verifying binding energies and minimizing their uncertainties in this region of
the nuclear chart is important to clarify the long standing problem of the
strength of the N=20 shell closure. After an introduction concerning the
principles of MISTRAL we will explain the handling of the systematic errors and
we will discuss the results obtained for sodium isotopes. 

* Now at GSI-Darmstadt