THE ROLE OF ION SOURCES IN SYNTHESIS OF THE SUPER-HEAVY

ELEMENTSA.EFREMOV, S.BOGOMOLOV, V.MIRONOV

Flerov Laboratory of Nuclear Reactions

Joint Institute for Nuclear Research

Artificial elements

SHE synthesis: Motivation

Understanding of boundaries of the material world is one the fundamental

problems of natural science

Liquid drop model of nucleus:

nucleus is a charged liquid drop with a huge density of ~1015 g∙cm-3

The stability of the atomic nucleus is defined by the fission barrier

The fission barrier will rapidly decrease with increasing Z → spontaneous fission.

The stability limit of heavy nuclei – Z ~ 100 (fission barrier = 0)

From 238U (Z=92) to 256Fm (Z=100), the spontaneous fission half-life changed

from 1016 years to 2.5 h (by almost a factor of 1020).

SHE synthesis: Motivation

The macroscopic-microscopic (MM) nuclear model:

Nuclear matter in not amorphous. It has the inner structure,

which has an effect on nucleus deformation.

Nuclei with the “magic” numbers of protons 2, 8, 20, 28, 50 and 82

and neutrons 2, 8, 20, 28, 50, 82 and 126 have considerably higher nucleon binding energies.

Doubly magical stable nuclei with closed proton and neutron shells (4He, 16O, 40,48Ca and 208Pb)

have a spherical shape and the highest nucleon binding energy.

Iven stronger effect can be found

For A ~ 300, Z=114 and N=184,

The “island” of the superheavy elements.

Synthesis: Why 48Ca ?

1. Synthesis of transuranium elements in high-flux nuclear reactors through successive neutron capture

followed by beta decay allows producing nuclei with masses ≤ 257 (Z=100).

2. Accelerated ions (12C, 18O or 22Ne) were used to interact with the long-lived and neutron-rich heavy isotopes.

Very high excitation energy (37 -45 MeV) of compound nuclei. Transition of the nuclei to the ground state

can occur through emission of four or five neutrons.

Because of the strong competition with fission, only 10-6 ÷10-8 of the primary nuclei can `survive‘

in each step of neutron emission.

Nevertheless, few isotopes of the elements of the second hundred with Z=102 - 106 were synthesizedfor the first time.

Synthesis: Why 48Ca ?

3. Cold fusion

Target: double magic nuclei 208Pb or 209Bi

Projectile beam: ions with A≥40

Excitation energy of about 12÷15 MeV

The key advantage of cold fusion:

the compound nucleus should emit

only one neutron for “cooling”.

Elements with Z=107-112 have been synthesized

at GSI via cold fusion reactions using

ions such as 54Cr, 58Fe, 64Ni, 70Zn as a projectile beam

Limitations: The production cross sections decreases

exponentially with the increase of Z !

RIKEN: synthesis of element 113 were performed

already on the verge of experimental sensitivity70Zn+209Bi

Beam intensity 2 pµA,

totally 553 days of irradiation for 3 events

The cross section ~ 0.02 pb

Synthesis: Why 48Ca ?

4. Reactions with 48Ca

Targets: neutron reached isotopes

of Z = 94 – 98

Projectile beam: 48Ca

Cross sections measured for the fusion reactions:

the cold-fusion reactions (left panel) and hot fusion (right pane)

Hot fusion, but

244Pu + 48Ca → Z=114, N=178208Pb+ 76Ge → Z=114, N=170

The increase in the fission barrier with

approaching N=184 should considerably

enhance the `survivability' and increase

the production cross sections of SHE

A beam of 48Ca7+with an intensity of approximately 0.3pμA was obtained for the first time

in 1974 at the heavy-ion cyclotron U300 using an internal PIG ion source.

Further, the calcium ions from PIG ion source were accelerated at the cyclotron U400

Consumption of the working substance was in the range 4 ÷15 mg/h

The discovery of new transuranic elements and the role of the electron cyclotronresonance ion sourcesR. GellerReview of Scientific Instruments 70, 4737 (1999)

Kr

This Figure clearly shows the advantages of ECRIS over other types

of ion sources. These advantages can be listed as following:

• Highly charged ions obtained with gases as well as with solid material.

• Relatively intense ion beams (I~10 pµA) in order to reach

the required dose in a reasonable measuring time.

• Robust and reliable ion injector systems capable of working

for days, weeks, and even months nonstop.

The production of an intense beam of 48Ca ions required solving a number of technical problems:

Calcium has no gaseous compounds

The enriched isotope 48Ca is extremely expensive

Minimum consumption!

ECR ion sources ionize gases with very high efficiency (≥ 70%)

For metals the ionization efficiency is smaller by around one order of magnitude

Assuming the ionization efficiency in the range of 5% ÷10%,

we had estimated the consumption of Ca required for production of about 100 eµA of Ca5+

as 1.5÷3 mg/h.

The result did not look so promising for long-term experiments

CAPRICE-type ion source

UHF

Gas

The evaporated metal is condensed at the water-cooled chamber wall.

To change this balance: a hot screen to evaporate of metal atoms condensed on it.

The temperature of the screen should be close to that for the oven.

This screen also should not affect the correct functioning of the ion source.

Irradiation of 48Ca + 242Pu

(3 March–4 May 1999).

The ionization efficiency is of about 70%,

the same level as for gases;

An average ion current of Ca+5 ~ 1 pµA;

An average consumption of Ca ~ 0.4 mg/h;

Of about 25% of Ca was collected and

regenerated from the discharge chamber wall

after the ion source running.

long lifetime micro oven (~ 1 year)

one crucible → ion beam at a physical target during one week

one gram of 48Ca → 2500 h of the target irradiation

During the last 20 years:

more than 1000 crucibles

more than 70 g of metal 48Ca

were used to produce 48Ca ion beam.

More than 50 superheavy nuclei from Z =104 to Z=118,

have been synthesized in hot-fusion reactions between 48Ca beams

and radioactive actinide targets

Число синтезированных ядер

Элемент 118 4

Элемент 117 20

Элемент 116 26

Элемент 115 37

Элемент 114 43

Элемент 113 2

Элемент 112 8

Yu.Ts. Oganessian 1, F.Sh. Abdullin 1, C. Alexander 2, P.D. Bailey 3, D.E. Benker 3, M.E. Bennett 4, J. Binder 2, S.L. Bogomolov 1, R.A. Boll 2, N. T. Brewer 2, G.V. Buklanov 1, S.N. Dmitriev 1,

J. Ezold 2, K. Felker 2, B.N. Gikal 1, J.M. Gostic 3, R.K. Grzywacz 2,5, G.G. Gulbekian 1, J.H. Hamilton 6, R.A. Henderson 3, S. Iliev 1, R.I. Il'kaev 7, M.G. Itkis 1, O.V. Ivanov 1, Ye.A. Karelin 8,

J.M. Kenneally 3, J.H. Landrum 3, C.A. Laue 3, Yu.V. Lobanov 1, R.W. Lougheed 3, A.N. Mezentsev 1, K. Miernik 2, D. Miller 5, K.J. Moody 3, S.L. Nelson 3, J.B. Patin 3, A.N. Polyakov 1,

C.E. Porter 2, A.V. Ramayya 6, F.D. Riley 2, J.B. Roberto 2, M.A. Ryabinin 8, K.P. Rykaczewski 2, R.N. Sagaidak 1, D.A. Shaughnessy 3, I.V. Shirokovsky 1, M.V. Shumeiko 1, M.A. Stoyer 3,

N.J. Stoyer 3, V.G. Subbotin 1, K. Subotic 1, R. Sudowe 4, A.M. Sukhov 1, A.N. Tatarinov 9, R. Taylor 2, Yu.S. Tsyganov 1, V.K. Utyonkov 1, S.P. Vesnovskii 7, A.A. Voinov 1, G.K. Vostokin 1,

J.F. Wild 3, P.A. Wilk 3, V.I. Zagrebaev 1

1 Joint Institute for Nuclear Research, RU-141980 Dubna, Russian Federation2 Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 3 Lawrence Livermore National Laboratory, Livermore, California 94551, USA 4 University of Nevada Las Vegas, Las Vegas, Nevada 89154, USA 5 Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA 6 Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA 7 Russian Federal Nuclear Center, All-Russian Research Institute of Experimental Physics, 607190 Sarov, Russian Federation8 Research Institute of Atomic Reactors, RU-433510 Dimitrovgrad, Russian Federation9 State Enterprise Electrohimpribor, RU-624200 Lesnoy, Russian Federation

Dubna

Dimitrovgrad

Oak Ridge

Knoxville

NashvilleLivermore

Reproducibility of results

Elements 114 – 118:

flerovium, moscovium, livermorium, tennessine and oganesson

JINR-LLNL-ORNL-RIAR-SEE-VNIIEF-VU-UT collaboration

Heaviest target: 249Cf → Zmax= 118 Direct synthesis of elements with Z>118 in fusion reactions

requires to use heavier projectiles than calcium nuclei (50Ti, 54Cr, 58Fe, 64Ni)

Attempts to synthesize elements 119 and 120 undertaken at different laboratories failed

What is beyond element 118?

Predicted cross-sections

for production of isotopes

of elements 119 and 120

are 10–100 times lower

than those for the production

of elements 114 and 115 with 48Ca

Further progress in research is possible only with the development of new experimental facilities

providing a substantial increase in the efficiency of experiments:

“Factory of superheavy elements”.

Ncn = σR ×Ntg ×ηsep ×D

To compensate the cross-section drop we should increase nearly 100-fold the production rate.

The creation of this facility are associated with:

development of new separators and detection modules;

development of targets with high thermal and radiation stability;

construction of a new powerful accelerator of stable and long-lived isotopes

with the intensity up to 10 pµA and the energy up to 8 MeV/n.

Sufficient increase of overall experiment efficiency is needed!

Dubna Gas-Filled Recoil Separator (DGFRS-II)

Simulated yields of Fl isotopes

as functions of the PuO2 target thickness

for the DGFRS-I, II

ION Z Beam Intensity from ECR Efficiency of

acceleration

Expected beam intensities of

(4÷8) MeV/n ions

on targets

ppseA pps

20Ne 3 150 3.1014 50% 1.5·1014

40Ar 7 300 3.1014 50% 1.5·1014

48Са 7/9 160 1.3.1014 50% 6.2·1013

50Ti 8/9 80 6.2.1013 50% 3.1.1013

54Cr 9 125 8.1013 50% 4.1013

58Fe 9/10 125 8.1013 50% 4·1013

64Ni 10/11 125 8.1013 50% 4·1013

70Zn 11/12 100 5.1013 50% 2,5.1013

136Хе 22/23 150 4.1013 50% 2·1013

Expected beam parameters of the DC-280

Main RF-

resonator

Magnet

yoke

HV platform

(Umax=70 kV)

Electrostatic

deflector

(Bender)

Separating

magnet

DECRIS-PM

ECR-source

(Umax=20 kV)

Magnet coils

Ion beam

extraction

Flat –top

resonator

Vacuum

pump

Accelerating tube

Configuration of the DC-280

Polyharmonic

buncher

DECRIS-PM

14 GHz

The total weight of the permanent magnets

is around 550 kg

Ion DECRIS-PM LAPECR2 ECR-4M

Ar8+ 920 460 600

Ar9+ 500 355 450

Ar11+ 210 166 200

Ar12+ 150 62 100

Xe20+ 75 85

Xe26+ 50 40 25 (Xe25+)

50Ti

The MIVOC method seems to be extremely promising in terms of the beam intensity,

stability, reliability, and material consumption.

(CH3)5C5Ti(CH3)3

The compound’s major drawback:

its sensitivity to air, moisture, temperature, and light.

The synthesis of the compound is rather complicated, especially when using enriched titanium.

Collaboration between IPHC (Strasbourg, France) and FLNR JINR.

Two step chemistry: 50TiCl4 (enrichment 92.57% ) → C5(CH3)5TiCl3 → C5(CH3)5Ti(CH3)3

The major challenge was long time transportation of samples from IPHC to JINR

which caused the compound destruction.

Therefore, we decided to perform the final step of synthesis at the FLNR chemistry

laboratory.

The selection of the best method to feed solids into ECR ion sources strongly depends

on specific properties of materials:

Ca, Mg (oven + hot screen technique), Ti and Fe ions (MIVOC method).

DECRIS-PM

Q+ 5+ 7+ 8+ 9+ 10+ 11+ 12+

24Mg 450 140 40 15

40Ca 220 158 58

50Ti 90 72 60 23

56Fe 85 80 55

Ion yields of solids from DECRIS-PM

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