Searching for collinear ternary decays using double - hit approach - - PowerPoint PPT Presentation

Searching for collinear ternary decays using “double-hit” approach

Zingisa Skedama, University of Fort Hare, SA Supervisor: Dr N.M.Jacobs Co-supervisors: Dr D.V.Kamanin S.M.Ilich A.O.Strekalovskiy

Background and Motivation

Nuclear fission, the main process generating nuclear energy, was discovered in 1938. This is a process where a heavy nucleus such as Uranium decays into two fragments of about the same mass – so called conventional binary fission.

Binary fission

Ternary fission

However fission for more than two fragments may occur. For example ternary fission – very interesting phenomenon from the scientific point of view. Fig.1 Schematic view of possible modes of ternary fission

Ternary fission

Fig.1.2 Schematic view of possible modes of ternary fission It was found that the fission barrier in the case of oblate configuration is higher then that in the prolate configuration. Thus prolate (collinear) configurations are more profitable energetically for ternary fission.

[Die73] Diehl H., Greiner W. Ternary fission in the liquid drop model, Phys. Lett. B. 45 (1973) 35. [Die74] Diehl H., Greiner W., Theory of ternary fission in the liquid drop model, Nucl. Phys. A. 229 (1974) 29.

Ternary fission

Fig.1.3 Schematic view of possible modes of ternary fission And fission process with three fragments of comparable mass with comparable relative angles (as a “star”) – so called “true ternary fission” - was not observed on the level

• f 10-8 per fission.

Ternary fission

The name “ternary fission“ is nowadays usually applied to process, where light nuclei (6He – 28Mg) are

• bserved mostly perpendicular to the

axis defined by the two heavy fragments.

Ternary fission: motivation

But “true ternary fission” was discovered in Flerov lab as collinear cluster decay and named Collinear Cluster Tripartition (CCT). However evidence for third fragment was obtained in the “missing-mass” approach. It means that only two of the three fragments was detected. Thus direct detection of all fragments in CCT is an actual scientific problem.

Collinear Cluster Tripartition (CCT)

Direct detection of all fragments in CCT

β ~ 1° L2 L1 Det1 Det2 The solution of this problem is not so simple. According to studies of CCT the relative angle between two fragments flying in the same direction determined by the scattering in the backing of the Cf-source and it is less than 1°. Due to that it is very complicated to design experiment where all particles will be directly detected. If β=1°, to register two fragments separately in standard silicon detectors L1 must be > 57 cm (because L2 > 5 mm). Big distances => small flux => long experiment

Direct detection of all fragments in CCT

β ~ 1° L1 Det1

So one way to solve the problem is double-hit registration mode of data acquisition system which will give ability to register two collinear fragments in one silicon detector.

Time axis

St MCP2 MCP1 PIN1 PIN2

252Сf

Experiment

The special setup named LIS was used for this purpose. Photo of the spectrometer

Experiment

Schematic view of LIS setup is shown below. It consists of two silicon PIN-diodes (red) and three Time-Detectors based on microchannel plates (yellow). Cf-source fixed on Start-Time detector in the center. All detectors connected to preamplifiers inside the vacuum chamber.

Experiment

Data acquisition system consists of CAEN flash-ADC attached to PC.

CAEN flash-ADC, 5 GHz sampling rate 200 ps per point

Flash-ADC gives possibility to operate with shape of each

• signal. Preamplifiers with sharpest shape of signal was used.

Experiment

So the typical picture of signals in one event looks like this: Time axis Amplitude axis PIN1-signal Start TimeDetector2 TimeDetector1 PIN2

Data processing: results

And double hit events was found ! Time axis Amplitude axis TimeDetector1 DoubleHit ! PIN1-signal Pile-up

Data processing: results

The shape of signal for double-hit event in silicon PIN-diode was modeled according to data from previous experiments. This pile-up shape is presented in this picture:

Threshold level for detecting of the pile-up FF approximately dTOF~ 30ns at A1/A2=10 (i.e. if energy of third fragment is 10 times less then the other).

The possibility of separation of signals in PIN-diode need to be checked using conservation laws.

Conclusion

• 1. Approximately sixty double-hit events were detected by

the timing MCP based detectors using fast flash-ADC.

• 2. Parameters of the events observed agree with the

predictions based on data from previous experiments treated as the results of ternary decays (CCT).

• 3. The number of found events gives probability of the CCT

about 10-4 relative to the binary fission which has good agreement with predictions.

• 4. More detailed analysis of the double-hit events detected

still in progress. Verification of conservation laws can give information about mass of third fragment.

Thank you for your attention!

Zingisa Skedama, University of Fort Hare, SA