determination of lifetimesof excited states in neutron ......determination of lifetimes of excited...

Vol. 50 (2019) Acta Physica Polonica B No 3

DETERMINATION OF LIFETIMESOF EXCITED STATES IN NEUTRON-RICH20O ISOTOPE FROM EXPERIMENT WITH

THE AGATA+PARIS+VAMOS SETUP∗

M. Ciemałaa, S. Zilianib,c, F. Crespib,c, S. Leonib,c, B. Fornala

A. Maja, P. Bednarczyka, G. Benzonic, A. Braccob,c, C. Boianoc

S. Bottonib,c, S. Brambillab, M. Bastd, M. Beckersd

T. Braunrothd, F. Camerab,c, N. Cieplicka-Oryńczaka

E. Clémente, O. Dorvauxf , S. Ertürkg, G. De Francee

A. Goldkuhled, J. Grębosza, M.N. Harakehh, Ł.W. Iskraa

B. Jacquote, A. Karpovi, M. Kicińska-Habiorj, Y. Kime

M. Kmiecika, A. Lemassone, H. Lie, I. Mateak, K. Mazureka

C. Michelagnolil, B. Millonc, C. Müller-Gatermannd

P. Napiorkowskim, V. Nanaln, M. Matejska-Mindam

M. Rejmunde, B. Sowickia, Ch. Schmittf , M. Stanoiuo, I. Stefank

B. Wasilewskaa, M. Zielińskap, M. Zięblińskia

aInstitute of Nuclear Physics Polish Academy of Sciences, 31-342 Kraków, PolandbUniversità degli Studi di Milano, Via Celoria 16, 20133, Italy

cINFN, sezione di Milano, ItalydIKP Cologne, Cologne, Germany

eGANIL, CEA/DSAM and CNRS/IN2P3, CAEN Cedex 05, FrancefCNRS/IN2P3, IPHC UMR 7178, F-67037 Strasbourg, France

gNigde University, TurkeyhKVI — Center for Advanced Radiation Technology, Groningen, The Netherlands

iJINR, Dubna, RussiajFaculty of Physics, Warsaw University, Warszawa, Poland

kIPN Orsay Laboratory, Orsay, FrancelInstitut Laue-Langevin (ILL), Grenoble, FrancemHeavy Ion Laboratory, University of Warsaw

Pasteura 5A, 02-093 Warszawa, PolandnDepartment of Nuclear and Atomic Physics

Tata Institute of Fundamental Research, Mumbai 400005, IndiaoIFIN-HH, Bucharest, Romania

pIRFU, CEA/DRF, Centre CEA de Saclay, 91191 Gif-sur-Yvette Cedex, France

(Received January 23, 2019)

∗ Presented at the Zakopane Conference on Nuclear Physics “Extremes of the NuclearLandscape”, Zakopane, Poland, August 26–September 2, 2018.

(615)

616 M. Ciemała et al.

An experiment aiming at measuring lifetimes of the second 2+ excitedstates in neutron-rich C and O isotopes was recently performed at theGANIL laboratory, in France. In these nuclei, recent ab initio calculationsprovide a detailed description of the nuclear excited states, predicting astrong sensitivity of the electromagnetic transition probabilities to the de-tails of the nucleon–nucleon interactions, especially in connection with therole played by the three-body (NNN) forces. Thus, the measurement of2+ states lifetimes in this region will be a stringent test of the importanceof including the NNN forces in describing the electromagnetic propertiesof selected nuclear states. This contribution reports on the status of theexperimental analysis for the case of lifetimes in 20O.

DOI:10.5506/APhysPolB.50.615

1. Introduction

Aim of the measurement was to determine the lifetimes (in the rangeof hundreds of femtoseconds) of the excited states in neutron-rich C andO isotopes, in particular in 16C and 20O. For these nuclei, ab initio cal-culations predict a strong sensitivity of selected electromagnetic transitionprobabilities to the details of the nucleon–nucleon interactions, especially tothe three-body term. Strong sensitivity is expected, in particular, in thecase of the second excited 2+ state, in each nucleus of interest. For exam-ple, the calculated value of the 2+2 state lifetime in O

20 is equal to 320 fs(with NN interactions), compared to 200 fs, obtained including NN+NNNinteractions [1, 2].

The experiment was performed in the GANIL laboratory, with a com-bined detector setup including the segmented HPGe detectors of the AGATAtracking array [3, 4], the VAMOS spectrometer [5] and the PARIS scintil-lator-based calorimeter [6]. In order to cover the lifetime range of interest(below 500 fs), the technique involving the γ-ray lineshape analysis (DSAM— Doppler-Shift Attenuation Method) was used.

2. Experimental details

The nuclei of interest were populated in the deep inelastic processes in-duced by an 18Obeam at 126MeV (7.0MeV/u) on a 181Ta target, 6.64mg/cm2thick (4µm). The beam energy at the center of the target was ∼ 116MeV,i.e. 50% above the Coulomb barrier, and the projectile-like products hadv/c ∼ 10%. The experimental setup contained 31 AGATA crystals placedat backward angles with respect to the entrance of the VAMOS spectrome-ter (from ∼ 120◦ to ∼ 175◦) and two PARIS clusters (one with LaBr3:NaI-and the second with CeBr:NaI-type phoswiches) with two additional largevolume LaBr3 detectors at ∼ 90◦ (Fig. 1).

Determination of Lifetimes of Excited States in Neutron-rich 20O Isotope . . . 617

Fig. 1. Photograph of the experimental setup. The letters A and B mark the PARISclusters containing LaBr3:NaI and CeBr:NaI phoswiches, respectively. Above (andbelow) the reaction chamber, there are placed two large volume (3.5′′ × 8′′) LaBr3detectors. On the right-hand side photo, there are visible the AGATA modules.

The VAMOS spectrometer was placed at the maximum (45◦) angle withrespect to the beam axis. Its focal plane contains:

— four drift chambers, which give information on xf , yf position for thereconstruction of the θf , φf angles;

— segmented (six columns, four rows) ionization chambers measuring theion energy loss ∆E;

— plastic scintillator at the end of the focal plane, which gives: triggersignal, particle energy E and time with respect to the radio frequency(RF).

Moreover, two additional pairs of drift chambers were put at the entrance ofVAMOS (20 cm from the target) for a better determination of the ions en-trance θ, φ, which was done to improve the γ-ray Doppler correction [7]. Theγ decay of the identified reaction products was measured with the AGATAarray and the PARIS detectors. Using the velocity vector ~v reconstructed inVAMOS, a Doppler correction was applied to the γ-ray spectra. Calibrationand fine tuning of the AGATA detectors is presented in [8].


3. Data analysis

In order to apply the DSAM method in the present experiment, it is veryimportant to precisely determine ~v of the ions which emitted γ rays. Forthis purpose, all of the VAMOS focal plane detectors were calibrated andtuned. These are the main adjustments that have been performed:

— gain matching for the drift chamber wires;

— energy calibration of the ionization chambers and check of their sta-bility during the time of the experiment;

— time and energy calibration of the plastic detector.

Then, one can obtain ions velocity v by

v = D/T ,

where D is the ion path length from the target to the plastic detector andT is the time of flight calculated as a difference between the RF signal ofthe cyclotron and the plastic detector time-signals (time between followingbeam pulses was equal to 102 ns).

To be sure that the ions time-of-flight value is correct, one has to checkif the masses obtained using this time are reconstructed in the proper way.Unfortunately, the masses are calculated using (M/Q)×Q, which is corre-lated to both time and energy. Therefore, one has to simultaneously checktime and energy calibrations. Firstly, (M/Q) is calculated

M/Q =Bρ

3.105γβ,

where Bρ is reconstructed using the dedicated library (libVAMOS) from theθf , φf values, Bρref (which was equal to 0.75Tm for this experiment) andthe VAMOS response function.

Then

M0 =Etot

931.5016(γ − 1),

Q = M0/(M/Q) ,

where Etot is total energy (sum of ∆E measured by ionization chambers andE from plastic detector) and M0 is mass extracted from total energy.

Finally, the reconstructed mass Mr is

Mr = M/Q×Qint ,

where Qint is the nearest integer value of Q.


To choose ion’s atomic number Z, one has to select the correspondingregion in the ∆E–Etot matrix (Fig. 2, with gate on the Z = 8 region). Usingthe technique described above, the plot of Q versus Mr can be obtained.Such a plot for Z = 8 is shown in Fig. 3 — here, two oxygen ions chargestates are visible, with Q = 8 and 7.

Fig. 2. ∆E–Etot plot with region corresponding to Z = 8 selection (part of data).

Fig. 3. Plot of ions’ charge Q versus their mass A reconstructed using the VAMOSfocal plane detectors (part of data). (For better readability, the formula Mr =M/Q× 8 was used, so masses for Q = 7 are shifted.)

By checking product intensity as a function of mass for Z = 5 to 9, verygood agreement with the expected mass distribution was observed: thismeans that the time and energy calibration of the VAMOS focal plane aredone properly. Figure 4 shows the populations intensity of the ions identifiedin this experiment.


Fig. 4. Graph presenting the population of detected and identified ions during theexperiment.

Check of the stability of the reconstructed masses values (Mr) in thecourse of measurement was also performed. The upper panel of Fig. 5 showsthe evolution of the reconstructed 18O ion mass with respect to the time ofthe experiment. A drift is visible with the same pattern as drift of the PARIStime-signal with respect to RF (middle panel of Fig. 5). In consequence, byapplying a correction to the ion T extracted from PARIS time vs. RF, onecan obtain a better stability of the reconstructed masses (bottom panel ofFig. 5).

For PARIS detectors, the energy calibration was done with the use ofstandard 60Co, 137Cs, 152Eu γ-ray sources as well as AmBe–Fe for calibra-tion points in the high-energy region (up to 7MeV). Gating on the 20Oions and the proper PARIS time with respect to RF signal, the Doppler-corrected spectrum in Fig. 6 was obtained, with 2+2 → 2

+1 (line of interest)

and 4+ → g.s. transitions visible. PARIS data will be used for measuringthe γ-ray angular distributions, providing a reference point for the anglearound 90◦.

Finally, in Fig. 7, the Doppler-corrected AGATA γ-ray spectrum gatedon 20O is presented: the transition of interest, 2+2 → 2

+1 2396 keV, is clearly

displayed. The potential of the present measurement in accessing the lifetimeof the transition of interest is shown in Fig. 8. Here, the relative differencesbetween the measured and the tabulated γ-ray energy for 2+1 → 0+ and2+2 → 2

+1 transitions in

20O, detected at three emission angles (120◦–140◦,140◦–160◦ and 160◦–180◦), are presented. For the 2+1 → g.s. line, the averagedifference is 0, since its emission occurs outside the target, while for the2+2 → 2

+1 line, a deviation from tabulated energy, increasing with increase

of the angle, is visible. The same trend is observed for the 18O transitionwith known lifetime of 27 fs [8, Fig. 4], and other short (below ∼ 300 fs)transitions in the O region (for example, for the 7/2+ level in 19O withlifetime of 92 fs).


Fig. 5. Upper panel: reconstructed 18O mass drift during the experimental time.Middle panel: PARIS detectors time-signal with respect to the RF, with the samedrift pattern. Bottom panel: reconstructed 18O mass with the RF drift correctedby using the PARIS timing information.


1800 1900 2000 2100 2200 2300 2400 2500

E [keV]

200

300

400

500

600

700

800

900

1000

1100

1200

coun

ts /1

0 ke

V

20O PARIS

2396 keV+ → 22

+2

1894 keV 4+ → 2+

Fig. 6. PARIS Doppler-corrected γ-ray spectrum for 20O ions.

Fig. 7. 20O Doppler-corrected γ-ray energy spectrum measured with AGATA(inset – 2+2 → 2

+1 transition).

To extract the value of the lifetime from the experimental data, simula-tions with variable lifetime and γ-ray energy are needed. Varying the γ-rayenergy is necessary because of the lack of precise determination of its valuefor the 20O 2+2 → 2

+1 transition in literature [9, 10].

The simulation process is divided into five stages:

— Event generator:The beam is passing through the target decreasing its energy.Multi-nucleon transfer is taking place with linear probability in thetarget.


After the reaction, the kinetic energy of the fragments is calculatedassuming the excitation of both fragments (tuned by experimentallymeasured ions velocity distribution).An excited level is let to decay with fixed lifetime τ , emitting γ raysisotropically, with a Doppler shift calculated using the current ionvelocity. If the emission occurs inside the target, the ion is processedto the end of target.γ-ray and ion energies and directions are written to the output file.

— AGATA simulation:The data produced in the previous step are used as input to theAGATA simulation package [11], which is being configured for theGANIL 31 AGATA crystals detector geometry. As output, the energydeposited in the AGATA crystals, together with the velocity vector ofevery γ-ray emitter, is written.

— AGATA tracking:AGATA simulated data are tracked (the same as experimental data)and Doppler corrected. The output is written as: γ-ray energy, detec-tor_ID, relative theta between γ-ray and ion and ion velocity.

— Adding detector response:Experimentally extracted energy resolutions for the AGATA crystalsand differences in counting rates are being included in the simulateddata.

— Comparison:Simulated data are compared to experimentally measured γ-ray spec-tra calculating χ2.

4. Conclusions and outlook

Gamma-ray energy shifts of the 2+2 → 2+1 transition in

20O, displayed as afunction of the angle between the γ-ray direction and the ion velocity, showedsensitivity of these shifts to the lifetime of the 2+2 excitation (Fig. 8) andindicated that it is below ∼ 300 fs. It has also been verified that the statisticscollected for the 2+2 → 2

+1 transition in

20O is sufficient to determine thelifetime of the second 2+ state by using the line-shape technique.

Simulations of the line-shape as a function of lifetime and transitionenergy have been developed. Finally, two-dimensional (lifetime and γ-rayenergy) minimization of χ2 constructed as comparison of experimental andsimulated 20O γ-ray lineshapes will be performed. This will lead to thedetermination of the lifetime value of the second 2+ state in 20O.


110 120 130 140 150 160 170 180−0.1

0

0.1

0.2

0.3

0.4

Theta [◦ ]

Eg(tabulated)

−E

g(m

easured)

Eg(tabulated)

[%] 1674 keV: 2+1 →g.s. t�= 10..5 ps

239 6 keV: 2+2→2

+1

Fig. 8. Relative differences between measured and tabulated energies as a functionof the emission angle for 2+1 → g.s. and 2

+2 → 2

+1

20O γ-rays.

We acknowledge the important technical contributions of J. Goupil andthe GANIL accelerator staff. The authors acknowledge support from the Eu-ropean Union’s Horizon 2020 through ENSAR2 grant agreement No. 654002,the National Science Centre, Poland (NCN) Contract No. 2013/08/M/ST2/00257, 2014/14/M/ST2/00738 and 2016/22/M/ST2/00269 and COPIN-IN2P3 collaboration.

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(2016).[8] S. Ziliani et al., Acta Phys. Pol. B 50, 625 (2019), this issue.[9] M. Wiedeking et al., Phys. Rev. Lett. 94, 132501 (2005).[10] M. Stanoiu et al., Phys. Rev. C 69, 034312 (2004).[11] M. Labiche et al., AGATA Geant4 Simulations for AGATA@GANIL.

http://dx.doi.org/10.1103/PhysRevLett.108.242501http://dx.doi.org/10.1088/0954-3899/40/5/055105http://dx.doi.org/10.1016/j.nima.2011.11.081http://dx.doi.org/10.1016/j.nima.2017.02.063http://dx.doi.org/10.1016/j.nima.2011.05.007https://www.actaphys.uj.edu.pl/findarticle?series=reg&vol=40&page=565http://dx.doi.org/10.1016/j.nima.2015.12.040http://dx.doi.org/10.1016/j.nima.2015.12.040http://dx.doi.org/10.5506/APhysPolB.50.625http://dx.doi.org/10.1103/PhysRevLett.94.132501http://dx.doi.org/10.1103/PhysRevC.69.034312

1 Introduction2 Experimental details3 Data analysis4 Conclusions and outlook

determination of lifetimesof excited states in neutron ......determination of lifetimes of excited...

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