Observation of the shape isomer states in fission fragments from fission of low excited actinides

The results of two methodically different experiments dedicated to the effect of a break-up of fission fragment while it passing through a solid-state foil are discussed. The hypothesis is confirmed that some part of fission fragments which were born in the shape isomer states undergo a break-up due to inelastic scattering in the foil.


Introduction
The phenomenon of the delayed fission of actinides was discovered in FLNR (JINR) in 1961.The effect was explained by the population of the shape isomer states in the second well of the double humped fission barrier and subsequent spontaneous fission of the mother nucleus via these states.The nuclei with such specific decay channel were called fission isomers.The shape isomerism is not unique peculiarity of the actinide nuclei.Calculations of the potential energy surface (PES) of the lighter nuclei show local potential minima in the multidimensional deformation space which can give rise to shape isomer states [1].Deexcitation by the emittance of the kx-radiation of the low lying (below the top of the fission barrier) shape isomer states in fission fragments (FFs) was reported in Ref. [2].In our previous experiments [3,4], we observed for the first time the break-up of the FF while it passes through the solid-state foil that is delayed in regards to the moment of conventional binary fission.We hypothesized that this effect is caused by the inelastic scattering of the FF in the shape isomer state.New arguments in favor of this hypothesis are presented below.

Experiment and results
The experiment Ex1 was performed at the LIS (Light Ions Spectrometer) spectrometer in FLNR (JINR) [5].The layout of the setup is shown in figure 1(a).LIS is a double-armed time-of-flight spectrometer which includes three microchannel based timing detectors TD1, TD2, TD3 and a PIN diode.PIN diode provided information for estimation of both FF's energy and time-of-flight.Copper degrader-foil D of 4.11 microns thick was placed in the TD2 detector.The data acquisition system consisted of the fast digitizer CAEN DT5742 that allows measuring the signal's value at the input with an interval of 0.2 ns, The experiment Ex2 was performed using VEGA (Velocity-Energy Guide based Array) setup [6] at the MT-25 microtron (FLNR, JINR) with the beam energy Ee = 22 MeV.The scheme of the spectrometer is shown in figure 2(a).The FFs from the (ɣ, f) reaction in the target (1) were captured by the electrostatic guide system (EGS) consisting of a tube (2) and the central wire (3).The FF's energy E and velocity V required for the calculation of the FF's mass were measured at the time-of-flight spectrometer, consisting of the microchannel-plates based timing detector (4) and the mosaic of four PIN diodes (5).The data acquisition system was similar to that used in Ex1.Our approach to the data processing is presented in Ref. [7].
Presence of the EGS is a principal difference between VEGA setup and other time-of-flight spectrometers used in our experiments earlier.Some fraction of the ions emitted from the target can be involved in the spiral-like movement along the guide axis.According to Ref. [8], the collection efficiency of the guide is proportional to the applied voltage and inversely proportional to fragment energy.Only minor part of the ions already caught in the guide will be lost along the flight pass.Thus, the EGS allows it to increase a counting rate at the detector placed several meters away from the target.The mass-velocity distribution of the heavy FFs from the 235 U(ɣ, f) reaction for the FF multiplicity m = 2 is shown in figure 2(b).The FF multiplicity m = 2 means that two fragments with the masses M1 and M2 were detected in two different PIN diodes (figure 2(a)) in coincidence.By definition, the FF masses of each event are indexed in such a way that M1 > M2.The comparison of the heavy FFs mass spectra for the FF multiplicity m = 2 (in black) and m = 1 (in gray) is presented in figure 2(c).The ratio of the yields Y(m = 2)/Y(m = 1) for all detected events is about 10 -2 .The spectra differ substantially.This is a strong indication of the nonrandom character of the coincidence between the fragments in the fission events with m = 2.The missing mass in each fission event is defined as follows: Mmiss = (235-M1-M2) u.Experimental information about the events with M1 in the range of M1 = (127-129) u and V1 = (0.92-1.07) cm/ns (i.e., for the well resolved vertical line in figure 2(b)) is summarized in figure 2(d).The "initial" mass of the heavy FF before it passes through the foil of the timing detector ( 4) is defined by the expression MH_in= M1 + M2.In order to compare the results of Ex1 and Ex2 in the same mass ranges of the heavy FFs, the values of MH_in are limited to the right by a mass of 180 u in figure 2(d).

Discussion
The main result of Ex1 is a clear demonstration of the break-up of the heavy FF while it passes through the metal foil.As can be inferred from figure 1(b), the FF in the wide range of initial Mtt masses turns into the magic nucleus, mainly 128 Sn, which manifest as the most pronounced peak in the Mte spectrum in figure 1(c).The missing mass linked to this transformation lies in the range of Mmiss = (8÷30) u.The energy of the FF from conventional binary fission is less than the Coulomb barrier for the nuclear

Figure 1 .
Figure 1.Lay-out of Ex1 experiment (a).Time-of-flight spectrometer LIS includes three timing detectors TD1, TD2, TD3, PIN diode 1.8x1.8cm 2 , 252 Cf(sf) source and degrader foil D inserted into TD2 detector.All three time-of-flight paths are equal in length, L = 15 cm.(b) -correlation distribution of the FF Mte and Mtt masses measured by TOF-E and TOF-TOF methods, respectively.Schematical view of the prescission configuration is shown in top insert.Position of the first rupture is marked by the solid vertical line the dashed line shows the position of the break-up.The hatched part of the mother nucleus will form the third missing fragment.Position of the magic isotopes on the Mte axis is shown in the right insert.White tilted line connects the dots where Mte= Mtt.(c) -projection of the distribution onto Mte axis.Masses of the magic nuclei are marked by arrows.

Figure 2 .
Figure 2. Lay-out of Ex2 experiment on studying 235 U(ɣ, f) reaction at the VEGA setup (a).The spectrometer includes a target of 235 U (1), electrostatic guide system (EGS) four meters long consisting of a tube (2) at zero potential and the central wire (3) at a potential -6 kV, timing detector (4) and a mosaic of four PIN diodes (5); (b) -mass-velocity distribution of heavy FFs for multiplicity m = 2, (c) -projection of this distribution onto M1 axis.The spectrum of heavy FFs for m = 1 is shown in gray.(d) -mass-correlation distribution of MH_in, M2, Mmiss under condition M1 ≈ 128 u.Schematical view of the prescission configuration is shown in top insert.Position of the first rupture is marked by the solid vertical line the dashed line shows the position of the break-up.The hatched part of the mother nucleus will form the third missing fragment.Experimental values of E1 and E2 energies are varied in the ranges shown in the bottom insert.