Historical overview of main activities
(See also PNPI HEPD Reports "Main Scientific Activities 1971-1996" and "Main Scientific Activities 1997-2001".)
1. Study of short-lived nuclei far from the beta-stability line.
Identification of new nuclides.
Detailed spectroscopic investigations of nuclides in exotic regions by using detectors of different type allowed to
identify new nuclides at the IRIS facility for the first time. The list of the identified new isotopes is presented in the review "The
IRIS facility and nuclear-spectroscopic investigations of nuclides far from the beta-stability region." in PNPI report of the High
Energy Physics Division "Main Scientific Activities 1971-1996").
Atomic mass determination by means of positron spectra endpoint energies.
An unknown atomic mass value can be determined if the mass of the daughter nuclide and the mass difference between
parent and daughter nuclides are known. The mass differences between parent and daughter nuclides can be obtained by means
of beta-spectrum endpoint energy measurement making use of a high resolution beta-spectrometer. Positron emitters in the region
of rare-earth elements were studied at the IRIS facility. For the first time the determination of positron spectrum endpoint
energies for a large range of isotopes was performed with a good precision.
Identification of the drip line of the proton unstable nuclides.
The nuclide mass determination, which was done using the measured endpoint energies of positron spectra, allowed to construct
the mass-surface for nuclides located extremely far from the beta-stability line. The precise measurements of alpha-spectra in the
rare earth region at the IRIS facility have led to observation of some unknown alpha-emitters. It allowed to join the fragments of
long alpha-decay chains and to obtain the masses of all nuclides in these chains using the known mass values of nuclides located
at the bottom of every chain. The obtained mass values were used to identify a fragment of the drip line for the proton unstable
nuclei.
(For more detailed review see article "The IRIS facility and nuclear-spectroscopic investigations of nuclides far from the
beta-stability region." in PNPI report of the High Energy Physics Division "Main Scientific Activities 1971-1996").
2. Beta-decay study of nuclei using a gamma-ray total absorption spectrometer.
In the nuclei near a beta stability line the main part of the Gamow-Teller (GT+ and GT-)-strength comes to the energy
region above a QEC-value, and only low energy tails of strength distributions are accessible for the beta decay. In a contrast, for
the strongly neutron deficient nuclei, the GT+ strength was expected on the basis of shell-model calculation to be within the QEC
window which makes the study of the beta decay of exotic nuclei particularly interesting. However, in studying the beta decay of
heavy nuclei there is a difficulty of performing the complete high-resolution experiment due to the high level density and
correspondingly the very fragmented -ray intensity. As a high efficiency alternative to the Ge detectors, a NaI detector of a large
volume was suggested. The idea was to measure the total energy of gamma radiation following the beta decay with a radioactive
source situated in the center of the large detector in order to provide the solid angle close to 4. Such a “Total Absorption
Gamma ray Spectrometer” (TAGS) was put in operation at the IRIS facility of PNPI in 1980 and had revealed the resonant
behavior of the GT+ strength for the decay of odd and odd-odd nuclei in the rare earth region. The further experiments in the rareearth region have given an evidence for a smooth isotopic dependence of the resonance position that remained even while
crossing a neutron magic gap N=82.
The investigation of GT strength function was continued at the total absorption spectrometer installed at the on-line
mass-separator GSI Darmstadt. The experiments were concentrated on the nuclei near the doubly magic 100Sn. Because the beta
decay in this region is dominated by only transformation g9/2 → g7/2, the strength function was expected to have a simple
structure accessible for the theoretical analysis. The first data have been obtained for the chain of silver isotopes. A most intrigue
result is very strong suppression of the total GT+ strength that exceeds the value expected from the comparison with the
suppression of GT- resonance excited in the reaction on the stable nuclei. The resonance excitation energies and widths have been
successfully reproduced in the frame of an advanced shell-model approach using a realistic nucleon-nucleon interaction, but the
model can not explain the experimental value of the suppression of GT+ strength. To explain this features, the further study of the
beta decay near 100Sn are carried out.
(For more detail review see article A.A. Bykov et al., Bull. Russian Academy of Science, Series of Physics, 1980, V. 44, P.918
and ” Beta Decay of 98Ag: Evidence for the Gamow-Teller Resonance near 100 Sn”,
Z. Hu, L. Batist et al., Phys. Rev. C 62
064315 (2000).
3. Investigation of spins, electromagnetic moments and charge radii of radioactive nuclei by laser
spectroscopy.
One of the most important problems in nuclear physics is a correct description of the nuclear ground states. To solve
this problem, one needs experimental information on the ground states characteristics. The significance of this information rises
greatly if one can trace the changes of these characteristics with the changes of the nucleon numbers N and Z within sufficiently
wide limits far from the stability line. The task of the nuclear theory is twofold: firstly the general trends of the observed isotopic
and isobaric dependencies have to be described and secondly it is of importance to explain the sudden changes of some nuclear
parameters near some peculiar points on the chart of nuclides. The data on the nuclei near nucleon drip lines are of special
interest since particularly for these nuclei the validity of up-to-date theoretical approaches could be questioned first of all.
The laser spectroscopy plays the leading role in the creation of the two-dimensional picture of the ground state
properties. It enables one to determine spins I, magnetic dipole moments, electric quadrupole moments Q s and also variations of
the mean square charge radii.
In 1979 the construction of a high-sensitive laser installation for hyperfine structure and isotopic shifts studies was
started at the IRIS facility. The first experimental results - isotopic changes of charge radii for long-lived Eu isotopes - were
obtained in 1983. In summary, laser spectroscopic studies of more than a hundred nuclides have been carried out at IRIS. Spins,
electromagnetic moments and isotopic changes of mean square charge radii for these nuclei have been determined. Theoretical
analysis of experimental data has shown the significance of the obtained results for the understanding of nuclear structure.
As it has been shown during the latest two decades in the trend of ISOL facilities the idea of a refractory high
temperature cavity use to construct a high efficient selective laser ion source which was firstly proposed and realized at the IRIS
facility turned to be very fruitful for efficient and selective ionization of many elements of the periodic table. Furthermore, as it
has been demonstrated during the latest experiments, the developed LIS construction was used practically without any
modification for laser spectroscopic investigations of a very large region of exotic nuclei.
(For more detailed review see article " Investigation of spins, electromagnetic moments and charge radii of radioactive nuclei
by laser spectroscopy." in PNPI report of the High Energy Physics Division "Main Scientific Activities 1971-1996" and
“Measurements of charge radii and electromagnetic moments of nuclei far from stability by photoionization spectroscopy in
a laser ion source.” in PNPI report of the High Energy Physics Division "Main Scientific Activities 1997-2001").
4. Ion source-target unit.
The ion source-target unit is the main part of an ISOL system. Therefore the development and manufacture of high
efficiency ion sources and targets was one of the most important tasks at the IRIS facility. The ion-source target system should
satisfied to many requirements, the major ones of which are the fast release time and the acceptable for experiments yields of
radioactive nuclides. The different types of targets and ion sources were developed enabling to produce radioactive isotopes of
the periodic table of elements.
Among the different types of targets developed at the IRIS it is worth noting the high temperature (up to 2300oC)
targets on base of carbon-metal composite formed by pyrolises of diphtalocyanine of corresponding metal . As a metal there
were used elements in a wide range of the Periodic Table, such as Ti, Zr, Nb, Mo, some REE, Th, U, Cm and others. Due to high
porosity these targets revealed excellent release properties, what allowed to obtain and investigate a set of new nuclides near the
edge of as proton as well as neutron stability. The peculiarity of this material consists in forming of micro-cavities in carbon
matrix during the pyrolises process. Heterogeneous atoms, which happen to be put inside these cavities, can not be escaped out.
For example, the full evaporation of Xe out of this matrix is going only by 2100oC. This experimental fact was the main base for
ISTC proposal (№2391) on preparation of carbon matrixes for the long-living isotope transmutation.
(For more detailed review see article " The IRIS facility and nuclear-spectroscopic investigations of nuclides far from the
beta-stability region." in PNPI report of the High Energy Physics Division "Main Scientific Activities 1971-1996" and NIM,
B70 (1992) ,69).
5. Development of high temperature targets and ion sources for on-line radioactive isotope production.
At the IRIS facility high temperature targets for effective production of a large set of short lived nuclides have been
worked out and used for on-line experiments. The use of a tungsten foil as a target container material allows to maintain the
working temperature for refractory foil and refractory metal carbide targets up to (2800–3000)°C and up to 2500°C for uranium
carbide targets. Tests of Ta, Ta-W, TaC, NbC and UC target materials in a tungsten container in the temperature interval of
(2400–3000)°C demonstrated a high reliability of used targets during long (of about 100 hours) on-line runs. A large set of the
neutron deficient and the neutron rich nuclides were produced making use the developed target. From a high temperature
refractory target with a tantalum foil as a target material the isotopes of practically all rare-earth elements were produced. That
target was used for laser spectroscopic investigations of chains of Yb and Tm neutron-deficient isotopes. In collaboration with
LNL, Italy and GANIL, France a number of off-line and on-line tests on the R&D of uranium carbide target of a high density
(11g/cm3) have been carried out. A large variety of nuclides from Na to Fr have been produced making use the high density
uranium carbide target coupled with a high temperature electron beam ion source. Presently the experiments have been continued
on the delay times measurement and production yields of neutron rich isotopes of Rb, Cs, In, Ag, and Sn.
(For more detailed review see the article “Development of high temperature targets and ion sources for on-line radioactive
isotope production.” in PNPI report of the High Energy Physics Division "Main Scientific Activities 1997-2001").
6. Fine-mesh photodetectors for CMS endcap electromagnetic calorimeter.
PNPI in collaboration with the Research Institute “Electron” (RIE, St.Petersburg) performed R&D studies of various
kinds of VPT produced at RIE. These studies resulted in the construction of a VPT (FEU-188) with parameters optimized for
application in the CMS Endcap ECAL.
The quantum efficiency of the photocathode is around 20%. A large fraction of the photoelectrons emitted by the
photocathode passes through the anode mesh and impacts on the dynode where secondary electrons are produced. The secondary
electrons are attracted to the anode mesh where a substantial fraction is captured. The resulted gain of the VPT proved to be
G=10-12. The gain is weakly sensitive to variations of the anode voltage (dG/G=0.1% per 1 Volt variation). The other
advantages are low sensitivity to ionizing particles, small input capacity and fast signal (1ns).
It was very important to minimize dependence of the VPT gain on magnetic field. In FEU-188 this was achieved owing
to the reduced inter-electrode space and very small anode mesh cell.
A sample of 500 VPTs was produced at RIE and it has been demonstrated that all the VPT parameters are well
reproducible. The radiation hardness of VPTs was studied in gamma irradiation tests. It was shown that the signal amplitude
degraded only by 7% after the irradiation dose of 20 kGy (maximum dose for 10 LHC years), this degradation being determined
mostly by some deterioration of the light transmittance of the faceplate.
Further tests of the VPT FEU-188 were performed at CERN with a full-size ECAL prototype (5•5 PbWO4 crystal). The
energy resolution obtained in the beam tests was: σ/E=6.5%
parameters.
(For more details see NIM, A442 (2000) 146-149.)
E  122Mev/ E  0.29%
which is closed to the designed
7. Collaboration of PNPI with GSI on mass measurements at the storage ring
During last decade the novel methods of mass measurements were invented at the GSI (Darmstadt). The first one belongs to
Schottky Mass Spectrometry (SMS) which deals with frequency measurement of cooled circulating highly charged ions (or even
“bare” nuclides) in the storage ring, the second concerns to the TOF measurements of ions in the storage ring operating in the
isochronous mode (IMS). The radioactive nuclides are produced via fragmentation or fission of relativistic heavy ions (Bi, U)
and after passing in-flight the fragment separator they are stored in the ring.
The first measurements with Bi-beam and SMS were devoted to the direct mass measurements of neutron-deficient nuclides
which are at the ends of alpha-decay chains. The combined knowledge of measured masses with the alpha-spectroscopy data
allowed determining the masses for a vast region of very exotic short-lived nuclides which gave a new physics insight:

the proton drip line for nuclides of heavy elements from Tb to U has been identified;

the two-proton drip line has been identified for the region from Pt to Bi;

the washing out of shell closer effect has been observed for neutron-deficient lead nuclides;

the increase of energy gap for both neutron and proton pairing towards the proton drip-line has been observed systematically
for the region of heavy nuclides of elements from Yb to Po;

a test of different current mash models showed that there is no mass formula which can pretend to predict mass values for
neutron-deficient side of the Chart of Nuclides better than 400 keV.
The further experimental runs have been devoted to mass measurements of neutron-rich nuclides produced in the fission of
relativistic uranium projectiles. As the half-lives of nuclides with unknown masses in the fission region are typically less than 5 s,
that is a lower limit for the SMS, the IMS was used. The latter allows measuring the masses for nuclides with half-lives as short
as 100 s. The main achievement of these measurements was that some of the nuclides which are at the expected path of the
astrophysical r-process have been identified and their masses have been determined.
The pioneering methods used at the GST allowed mapping the big mass surfaces unknown so far. The masses for about 350
nuclides have been determined with typical uncertainty 50 keV for SMS and 200-3 00 keV for TOF TM. (For more details see
Nucl.Phys. A677 (2000) 75-99).
8. Investigation of nuclides with mass number A~80 and Z~N at ISOLDE-CERN and IGISOL-JYFL
Topside of rp-process
The knowledge of the nuclear properties in the N~Z region of A~80 is important for nuclear astrophysics because the
process of rapid proton (rp) capture in stellar conditions is passing just through it. This process, which is responsible for energy
generation and galactic nucleosynthesis, proceeds during the star bursts at the Z=N line up to 56Ni. However, for higher Z-values
where proton capture rates become smaller the reaction path can be broader and shifted it to less neutron-deficient nuclides by
about one-two mass units towards the beta-stability line. This shift can be even more for steady state burning model (e.g., in Xpulsars).
The properties of nuclides, and mainly, the mass values and beta-decay half-lives are needed to perform the network rpprocess rate calculations. It is also necessary to have some information about spectroscopic data for isobaric nuclides in order to
calculate the abundance of nuclides in the cooling stage after burst. The discovery of new long-lived isomeric states at the rpprocess path is of a paramount importance because these states have to be included in the calculations too.
The role of 73Rb and 70Kr
The binding energy and the decay modes are critical for the rp-process. The negative proton binding energy of 73 have
to suppress the consiquentive proton capture over this nuclide, then the process can proceed from 72Kr via -decay or two proton
capture. The identification of proton instability of 73Rb means that the slit to produce heavier nuclides became much narrower.
Search for 80mY
The astrophysical rp-process in X-ray bursts is passing through the nuclide 80Y. In the stellar plasma 8 is expected to be
fully ionized. A “bare” half-life of this state is determined to be equal to 6.6s. This bare isomeric state is strongly populated in the
rp-process from 80Zr and has therefore to be taken into account in X-ray burst model calculations. Because of the similarity of the
-decay half-lives of “bare” ground and isomeric states we find a maximum reduction in the effective -decay lifetime of 80Y of
only 17%. Our results pave the way for a future investigation of the impact of the isomeric state on the “effective” 80Y proton
capture rate.
Mirror nuclides with weakly bound protons
New physical phenomena associated with nuclear structure and decays along the Z = N line are expected to arise due to
the equal filling of the proton and neutron states, as well as due to the increasing of the Coulomb repulsion in heavier nuclei..
Nuclei near and at the Z=N line provide an ideal laboratory to study fundamental symmetries in nuclei.
According to the conserved vector current (CVC) hypothesis, the matrix elements of the super- allowed Fermi transitions between the (J,T) = (0+,1) states should all be equal, independent of nuclear structure apart from small terms for
nucleus-dependent radiative correction R and for Coulomb correction C. Provided this is true, the experimental ft values that
include isospin mixing and radiative corrections, allow an accurate determination of the weak vector coupling constant G Up to
now, super- allowed (T=1) 0+  0+ transitions have been measured to a precision of 0.1 % or better in the decays of nine nuclei
between 10C and 54Co This low-energy nuclear physics result, together with the muon-decay data, gives presently the most
precise value for the up-down quark mixing matrix element Vud in the Cabibbo (CKM) matrix. The present nuclear beta-decay
result differs at the 98 % confidence level from the unitarity condition of the CKM matrix. The uncertainty is largely due to
theoretical corrections for R and C.
The Coulomb correction C increases with increasing Z and may reach even a level of 1-2% for isotopes beyond 54Co
The value C can be factored to the contribution due to isospin mixing IM and due to the deviation from unity of the overlap
between the proton and neutron radial wave functions RO The observation of feeding the non-analog
provides a test for the total Coulomb correction and helps to distinguish between different theoretical values.
0 2 state
transitions
The measurement of the -decay half-life and the observation of a nonanalog transition to the first exited state in 74Kr
have been carried out at ISOLDE (CERN).For more information see PNPI report “Main scientific activities 1997-2001”, p 195.