Experiment for synthesis of neutrondeficient

Transcription

1 Experiment for synthesis of neutrondeficient isotopes Hua Bin Yang ( 杨华彬 ) Institute of modern Physics, CAS Tutored by Professor Zai Guo Gan ( 甘再国 )

2 Outline Estimation Experimental purposes ; Selection of reaction channel ; Production cross section estimation; Magnetic rigidity estimation; Estimation of decay properties of 211 ; Experimental details Results and discussions Alpha spectrum; Average Charge State of 210,211 Ac Energy-position-time correlation method; Identification of 212 ; Improved -decay data of 212 Summary and outlook

3 Estimation 1.Experimental purposes J.Kurcewicz et al. reported the first observation of the new isotopes 208 Th and 211. The statistics and the resolution of the ion identification procedure at this setting were not sufficient to claim unambiguous observation of these isotopes. Heredia et al. identified 208 Th as reported in the 2010 paper The new isotope 208 Th. An -decay energy of 8044 (30) kev and a half-life of 1.7( ) ms were deduced for 208 Th. J.Kurcewicz et al., Nucl. Phys. A 767, 1(2006). J.A.Heredia et al., Eur. Phys. J A 46, 337(2010).

4 Mitsuoka et al., reported identification of 212. A MeV 35 Cl beam from the JAERI tandem accelerator bombarded at a target of 182 W forming 212 in (5n) fusion-evaporation reaction. Only three decay chains have been reported up to now! 212 E =8.270(30) MeV T 1/2 =5.1( ) ms S.Mitsuoka et al., Phys. Rev. C 55,1555(1997). Investigate the alpha-decay properties of isotope 211. Confirmation of the alpha-decay properties and giving improved decay data of 212.

5 2. Selection of reaction channel Summary of the produced in the first reported experiment. Isotope Reaction Cross section Laboratory Year Reference W ( 35 Cl, 5n) nb JAERI 1997 [1] Er ( 51 V, 8n) nb GSI 1995 [2] Er ( 51 V, 7n) nb GSI 1995 [2] Ta ( 40 Ar, 6n) 215 GSI 1979 [3] Os ( 31 P, 4n) 216 Dubna 1972 [4] 190 Os ( 31 P, 5n) Au ( 24 Mg, 5n) Tl ( 20 Ne, 6n) 217 Berkeley 1968 [5] 208 Pb ( 20 Ne, 1p8n) Ta ( 40 Ar, 3n) 218 GSI 1979 [3] 40 Ca+ 175 Lu 215 * n; 40 Ca+ 175 Lu 215 * n; [1] S.Mitsuoka et al., Phys. Rev. C 55,1555(1997). [2] V.Ninov, et al., Z. Phys. A 351,125(1995). [3] K.-H.Schmidt, et al., Nucl.Phys.A318,253(1979). [4] G.Y.Sung-Ching-Yang, et al., Sov. J. Nuclear Phys.14, 725(1972). [5] K.Valli, et al., Phys.Rev.176, 1377(1968).

6 3. Production cross section estimation E0=204 MeV/E2=193 MeV 40 Ca+ 175 Lu E*=49 MeV Calculation with HIVAP code Calculation with LISE++ Cross Section (σ): 0.1 nb (10-34 cm 2 ) Target Thickness (Ns): 500 g/cm 2 of 175 Lu ( atoms/cm 2 ) Beam intensity (I, 40 Ca): 0.1 pa ( ions/s) Total efficiency (ε): 0.2 Y= ε σ Ns I = / s 1 event / 13h

7 4. Magnetic rigidity estimation The magnetic rigidity is related to the ionic charge as follows: ( B) 0 A ( v/ v ) [T m]. q ave v c 2 (1 ) 1 1 where The average equilibrium charge depends on the Z number, electron shell, velocity, type of gas and gas pressure and is estimated by empirical formulas. For this experiment ( 211 ): E mc v m s q / ; ave ( ) ; B =( ) T m. 2

8 4. Estimation of decay properties of 211 Data come from NNDC Formula Form.1 Form.2 Form.3 T 1/2 ( 211 ) 0.98 ms 1.2 ms 1.5 ms A.rkhomenko et al., Acta Phys.Pol.B36,3095(2005). Yibin Qian et al., Phys.Rev.C83,044317(2011). Q ( 211 )8.54 MeV E ( 211 )8.38 MeV Ren Yuejiao et al., Nucl Sci and Tech 24,050518(2013). T 1/2 =4.1 ms

9 Experimental details Beam: 40 Ca 12+ Energy: 5.1 MeV/u ~ 204 MeV Typical intensity: ions/s Effective irradiation time: 24 h Delivered by the SFC of HIRFL in Lanzhou Target: nat Lu 175 Lu(<97.4%), 176 Lu(<2.6%) and impurities thickness:40g/cm 2 (C)+500g/cm 2 (Lu) Energy degrader: 2 m Al foil Beam energy at the center of target: 193 MeV Experimental setup: Gas-filled recoil separator SHANS He pressure:0.8 mbar Detection system : MWPC + Si-box (no Veto) Data acquisition system:camac Dead time: 200 s

10 Gas filled recoil separator SHANS (Spectrometer for Heavy Atoms and Nuclear Structure) Detection system Fixed target The gas-filled separators use the different magnetic rigidities of the recoils and projectiles traveling through a low-pressure (about 0.8 mbar) gas-filled volume in a magnetic dipole field. A( v/ v ) A( v/ v ) A ( B) [T m] /3 1/3 qave Z ( v/ v0 ) Z Configuration Trajectory length Acceptance Central trajectory radius Bending angle of D Max. magnetic rigidity Qv-D-Qv-Qh 6.5 m 280mmmrad (h) 450mmmrad (v) 1.8 m Tm

11 3 Position Sensitive Silicon Non-position sensitive silicon detectors (Side Silicon Detector-SSD); Active area mm 2 ; Each PSSD was divided into 16 vertical strips; Energy resolution was 60 kev and position resolution 1.5 mm (for 5-8 MeV particle); Si Box detector Detection efficiency 80%; PSSD SSD Energy calibration Position calibration Energy calibration High-energy calibration Low-energy calibration (Internal calibration) High-energy calibration Low-energy calibration High-energy calibration Low-energy calibration The Internal energy calibration is based on the well-known - emitters : 210,211 Ac, 209,210 Ra, 206,207 Fr, 208,209 F r, 205,206 Rn which are produced in the same experiment.

12 MWPC detector Made by the Research Group of Gas Detector in IMP. The detector (MWPC) signals are used to distinguish implantation from radioactive decays of previously implanted nuclei. Working parameters of the MWPC Active size mm 2 Anode bias +400 V Cathode bias -100 V Anode-cathode distance 7 mm Wire step 20 m Be(Cu) 2 mm (anode) 1mm (cathode) Isobutane pressure range mbar Helium pressure 0.8 mbar in the separator Entrance window 0.5 m Mylar Window-PSSD 150 mm detector distance Size of the mm 2 entrance window Time resolution 15 ns (FWHM) (alpha source) Detection efficiency >95% (alpha source)

13 Results and discussions 1. Alpha spectrum Run028 The pulses generated by reaction products or scattered projectiles are suppressed by an anticoincidence condition between the TOF detector and the silicon detector. The spectrum measured in the reaction 40 Ca+ 175 Lu at beam energy of 193 MeV.

14 2. Average Charge State of 210,211 Ac The position distribution of the decay of 210,211 Ac obtained with the implantation detectors at 0.8 mbar pressure of He and the 1.72 Tm magnetic rigidity. The image is projected onto the area of mm 2. x mm ( B) 1.72 Tm 0 ( B) 1.69 Tm q ave ion 7.07 ( B ) ( B ) (1 x/100 D) q A( v/ v ) / ( B) ave ion 0 Where D=9.1 mm/1% change in (B) 0 ion

15 3. Energy-position-time correlation measurement (1)The must be located in the same physical position in space, (2) have energies closed to those expected for the nuclides of interest, (3) and follow each other in manner consistent with the expected lifetime distribution. Unknown parent decays Known daughter decays Mother and daughter particle energies for correlated chains of the type ER-1-2. Maximum search times were 0.1 s for ER-1 pair and 10 s for 1-2 pair. Vertical position window <2 mm This procedure does not necessarily lead to the correct identification of the members in the chain!

16 4. Method of determining half-lives and cross sections Good statistics f t ae be ( r) t r t ( ). is the decay constant, r is the average counting rate of acceptable recoil nuclei in the detector. M.E. Leino et al., Phy.Rev.C 24,2370(1981) A.Chatillon et al., Eur.Phys.J.A 30, 397(2006) Poor statistics f x p e x 10 ( ) 010. n t ( t ) / n ; T ln2t m m i 1/2 m i1 ln 2 tm ln 2 t m The error limits:, 1+ z / n 1- z/ n K.-H.Schmidt et al., Z.Phys.A316,19(1984) x

17 N Nobs N dose s eff chain N obs is the number of decay chains observed; N dose is the total dose of projectile ; N s is the areal density of target atoms; eff is the transmission efficiency of the separator; chain is the reaction product detection efficiency; N dose ions; N s /cm 2 ; eff =14%; Cross Section (nb) 0.20 nb nb 40 Ca beam energy (MeV) The cross sections were calculated with the HIVAP code (BARFAC= 0.63).

18 Summary of part of the product produced in the 40 Ca+ 175 Lu reaction.

19 5. Identification of 212 Run 26 The probability of random correlation: < The method used for calculating this probability can be found in Folden C M. PHD thesis, USA: University of California,(2004)

20 6.Improved -decay data of 212 Taking into account the three decay chains identified in previous work and the one found in this work, we have arrived at the improved values 8.250(20) MeV and 5.1( ) ms for the -particle energy and half-life of 212. E =8.250(20) Me V T 1/2 =5.1( ) ms E =8.270(30) MeV T 1/2 =5.1( ) ms This result has been reported in Huabin Yang et al., J.Phys.G 41,105104(2014).

21 Summary and outlook The goals of this work and some important estimations before this experiment were presented. A brief introduction of the gas-filled recoil separator SHANS and the detection system. Based on the energy-position-time correlation measurement, Ac, 212 and 211 Th were identified. Previously reported decay properties of the ground state in 212 were confirmed and improved values were obtained. As the very short beam time, we did this experiment again in September this year. And the experimental data is under processing. CAMACVME ; 1 MWPC2 MWPC; CLOVER detector; Longer beam time.

22 中国科学院近代物理研究所超重核研究组全体人员 Thank you for your attention!

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24 t m T 1/2 1 ( ) ms 4 ln 2 t 5.1 ms ln 2 (7.375) ( T1/2) u 10.2 ms 11/ 4 ln 2 (7.375) ( T1/2) l 3.4 ms 11/ 4 T (5.1 ) ms /2 1.7 m MeV MeV i 2 2 i i E MeV E 1 9 / 4 E i 17 kev i 212 E =8.270(30) MeV T 1/2 =5.1( ) ms 212 E =8.250(20) MeV T 1/2 =5.1( ) ms