E.M. Kozulin

Flerov Laboratory of Nuclear Reactions, JINR, Dubna, Russia

Fission dynamics in the proton induced fission of actinide nuclei

at intermediate energies

13, 20, 40, 55+ 242Pu → 243Am

13,20,40,55

20, 40, 55

+ 232Th → 233Pa

+ 238U → 239Np

MeV

Elab

The aim of the present work has been investigation more completely the proton induced reactions p + 232Th, 238U, 242Pu using the complex correlation measurements of the fission fragment mass and total kinetic energy distributions and of the double differential neutron and γ-rayspectra;

to investigate of super asymmetric fission mode connected with influence of the nuclear shells N=50 and Z=28(78Ni fission mode)

P

small angular momentum transferred to the compound system;

the possibility to reach very low excitation energy of the compound system;

in contrast to heavy ion induced fission, there is no quasi-fission and inelastic processes in these reactions.

Fission reactions induced by light particles such as protons, neutrons etc. are interesting due to:

Target

232Th, 238U,242Pu

F1

F2

solid angle – 0.3 sr

angular resolution – 0.30

TOF-startdetector

BeamP

position sensitivestop detector

x, y, TOF

mass resolution – 2 a.m.u

TOF-startdetector

The measurements were carried out at the Accelerator Laboratory, University of Jyväskylä (Finland), using the setup that included:

the two-armed time-of-flight reaction products spectrometer CORSET built with the use of microchannel plates (MCP);

8 detector time-of-flight neutron spectrometer DEMON;

High Efficiency Neutron DEtection System (HENDES) facility to measure neutron multiplicity;

12 BaF2 detectors of γ-rays

CORSETthe two-armed time-of-flight reaction productspectrometer CORSET composed of microchannelplates•Time resolution δt 120-150 ps•Mass resolution δΜ/Μ 2 amu•Angular resolution δΘ, δϕ ±0.3°•Solid angle of each arm 360 msr•Range of measured angles:

in the reaction plane Θ from 20°to 160° ±20°out of plane ϕ ±7°

DEMON8 detector time-of-flight neutron spectrometerDEMON using scintillation modules•Time resolution δt 1.5 ns•Neutron energy resolution - 4% at 3 MeV•Intrinsic efficiency 50% at 3 MeV

HENDESTime resolution δt 120-150 psIntrinsic effeciency 34% to 22% at 1-10 MeVPosition resolution ±5cm

BaF2 detectors12 detectors scintillation γ-quanta multiplicityspectrometer

120140160180200220

T

KE

, MeV

242Pu(p,f)238U(p,f)

232Th(p,f)

01234

Yie

ld, %

140150160170180

,

MeV

60 80 100 120 140 1600

50100150200

σ2 T

KE, M

eV2

mass, u80 100 120 140 160

mass, u60 80 100 120 140 160

mass, u

AHy140 AHy140 AHy140

From top to bottom: two dimensional mass-TKE matrices, mass distribution of fission fragments, average TKE and square variance of TKE as a function of fission fragment mass for proton-induced fission of 232Th, 238U, 242Pu

220 240 26080

100

120

140

160

243Pu233Th

132Sn

AH

AL

Mea

n m

ass o

f fis

sion

frag

men

t gro

up

Mass number of fissioning nucleus

239U

– our experiments

J.P.Unik et al., in Proc. Symp. Physics and Chemistry of Fission, Vol.2, IAEA, Vienna, 1974, 20.

The experimental mass distributions are similar for all studied reactions. The most probable mass of the heavy fission fragment for all target nuclei studied is close to the AH=140 (asymmetric mode S2). The origin of this mode could be attributed to the deformed neutron-shell closure N≈88 in the heavy fragments.

120

140

160

180

200

220

TKE,

MeV

Ep=55 MeVEp=40 MeVEp=20 MeV

Ep=13 MeVp +232Th →233Pa

012345

Yie

ld, %

130

140

150

160

170

, M

eV

60 80 100 120 140 1600

50

100

150

σ2 TK

E, M

eV2

mass, u

80 100 120 140 160

mass, u

60 80 100 120 140 160

mass, u

60 80 100 120 140 160 180

mass, u

From top to bottom: two dimensional mass-TKE matrices, mass distribution of fission fragments, average TKE and square variance of TKE as a function of fission fragment mass at the proton energies 13, 20, 40 and 55 MeV

60 80 100 120 140 16010-4

10-3

10-2

10-1

100

0

1

2

3

4

5

6

Yie

ld, %

Mass, amu

Yie

ld, %

Ep=13 MeV Ep=20 MeV Ep=40 MeV Ep=55 MeV

232Th(p,f)

The mass distribution in linear and logarithmic scale for the proton-

induced fission of 232Th at the proton energies 13, 20, 40 and 55 MeV

70 80 90 100 110 12010-5

10-4

10-3

10-2

10-1

100

101

242Pu(p,f) Ep=13MeV 242Pu(p,f) Ep=20MeV 242Pu(p,f) Ep=55MeV 242mAm(nth,f)

Lohengrin data

mass, u

Yie

ld, %

A=78

Y=0.089% for the 242Pu(p,f) Ep=55 MeV

Y=0.035% for the 242Pu(p,f) Ep=13 MeV

Y=0.016% for the 242Am(nth, f)

60 80 100 120 140 160 18010-410-310-210-1100101

242Pu(p,f)NCN/ZCN=1.56

mass, u

Yie

ld, %

60 80 100 120 140 160 18010-410-310-210-1100

238U(p,f)NCN/ZCN=1.57

Yie

ld, %

60 80 100 120 140 160 18010-410-310-210-1100101

232Th(p,f)NCN/ZCN=1.56

Ep: 13 MeV 20 MeV 40 MeV 55 MeVY

ield

, %

A=78

Ion beams:

DEMON

CORSET

Gamma

10-410-310-210-1

10-410-310-210-1

10-410-310-210-1

10-410-310-210-1

10-410-310-210-1

10-410-310-210-1

2 4 6 8 10 12 1410-4

10-310-210-1

2 4 6 8 10 12 14

Θ=14.50

φ= 00

p(13MeV)+232Th

Θ=29.50

φ= 00

Θ=44.50

φ= 00

Θ=59.70

φ= 00

Θ=89.20

φ= 00

Θ=119.20

φ= 00

Θ=164.00

φ= 00 Θ=-14.50

φ= 00

Θ=-29.50

φ= 00

Θ=-44.50

φ= 00

Θ=-59.70

φ= 00Θ=-89.20

φ= 00

Θ=-119.20

φ= 00

d2N

/dEd

Ω, M

eV-1st

r-1

E, MeV

Θ=-164.00

φ= 00

E, MeV

10-510-310-1

10-510-310-1

10-510-310-1

10-510-310-1

10-510-310-1

10-510-310-1

0 10 20 30 4010-5

10-310-1

0 10 20 30 40

Θ=14.50

φ= 00

p(40MeV)+232ThΘ=29.50

φ= 00

Θ=44.50

φ= 00Θ=59.70

φ= 00

Θ=89.20

φ= 00Θ=119.20

φ= 00

Θ=164.00

φ= 00Θ=-14.50

φ= 00

Θ=-59.70

φ= 00

Θ=-29.50

φ= 00

Θ=-89.20

φ= 00

Θ=-44.50

φ= 00

Θ=-119.20

φ= 00

En, MeV

d2N

/dE

dΩ, M

eV-1st

r-1Θ=-164.00

φ= 00

En, MeV

Double differential experimental neutron spectra (black points) and their decomposition (in assumption of three sources model) for proton-induced fission of

232Th at two proton energies Ep = 13 and 40 MeV

In our experiments on the proton induced fission we extractedpre-equilibrium neutrons, and after we applied standardprocedure (three source model) on deducing neutronmultiplicities: pre-fission, and two post-fission multiplicitiesfrom both fragments.

After having received the number of pre-equilibrium neutrons, we corrected our mass distributions.

poststpre2 2n,i nn n nn n n

stpre 2 stpre post 3/ 2 posti 1n n i i

M EM Ed M exp expdE d 4 (T ) T 2( T ) T=

< >< > ε ⎧ ⎫ ⎧ ⎫ε ε= − + −⎨ ⎬ ⎨ ⎬Ω π π⎩ ⎭ ⎩ ⎭

∑

n n n CN(FF) CN(FF) CN(FF) CN(FF) CN(FF)E 2 E E / A cos E / Aε = − Φ +

ΩΘ=110

φΘ=1080

φ

Θ=1080

φΘ=110

φ

Average preequilibrium , average statistical prescission and postsscission neutron multiplicities as well as average γ-ray multiplicity , average energy emitted by γ-rays and average energy per one gamma quantum as a function of mass and total kinetic energy (TKE) of fission fragments were measured in proton induced reactions.

70 80 90 100 110 120 130 140 150 160 1700,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

70 80 90 100 110 120 130 140 150 160 1700,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0p + 242Pu -> 243Am

mass [amu]

Ep=55MeVEp=20MeV

mass [amu]

p+ 238U -> 239Np

Ep=13MeV

Ep=55MeVEp=20MeV

– Ep = 20 MeV

– Ep = 55 MeV

70 80 90 100 110 120 130 140 150 160 1700123456789

70 80 90 100 110 120 130 140 150 160 1700123456789

[ M

eV ]

Ep=55 MeV Ep=20 MeV

Ep=55 MeV Ep=20 MeV

p+238U->239Np

mass [amu]

70 80 90 100 110 120 130 140 150 160 1700123456789

70 80 90 100 110 120 130 140 150 160 1700123456789

p + 242Pu -> 243Am

p+ 238U -> 239Np

Ep=55 MeV Ep=20 MeV

Ep=13 MeV

Ep=55 MeV Ep=20 MeV

mass [amu]

70 80 90 100 110 120 130 140 150 160 1700123456789

70 80 90 100 110 120 130 140 150 160 1700123456789

[ M

eV ]

p + 242Pu -> 243Am

p+ 238U -> 239Np

[ M

eV ]

Ep=55 MeV Ep=20 MeV

Ep=13 MeV

Ep=55 MeV Ep=20 MeV

mass [amu]

80 90 100 110 120 130 140 150 160 1700

1

2

3

4

5

6

7

8

9

80 90 100 110 120 130 140 150 160 1701,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

p+242Pu->243Am

Ep = 13 MeV Ep = 20 MeV Ep = 55 MeV

mass [ amu ]

Ep = 13 MeV Ep = 20 MeV Ep = 55 MeV

70 80 90 100 110 120 130 140 150 160 1700

1

2

3

4

5

6

7

8

9

70 80 90 100 110 120 130 140 150 160 1701,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

Ep = 20 MeV Ep = 55 MeV

mass [ amu ]

p+238U->239Np

Ep = 20 MeV Ep = 55 MeV

With help of average gamma-ray multiplicity we mayobserve the influence of shell effects (that isimportant, because our aim is to investigate the yield of doubly-magic 78Ni nucleus), the mean spinof the fragments.

With help of average neutron multiplicity we may obtainthe excitation of fragments, the level densityparameter.

90 100 110 120 130 140 150 160 170

0,6

0,8

1,0

1,2

1,4

1,6

1,890 100 110 120 130 140 150 160 170

0

1

2

3

4

5

6

7

8

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

[ M

eV ]

mass [ amu ]

, and as a function

of single fragment mass. is taken from the paper

Budtz-Jørgensen and H.-H. Knitter, Nucl. Phys. A490 (1988).

252Cf

70 80 90 100 110 120 130 140 150 160 1700123456789

70 80 90 100 110 120 130 140 150 160 1700123456789

p + 242Pu -> 243Am

p+ 238U -> 239Np

Ep=55 MeV Ep=20 MeV

Ep=13 MeV

Ep=55 MeV Ep=20 MeV

mass [amu]

80 90 100 110 120 130 140 150 160 1700,8

0,9

1,0

1,1

1,2

1,3

1,4

70 80 90 100 110 120 130 140 150 160 1700,8

0,9

1,0

1,1

1,2

1,3

1,4

mass [ amu ]

p+242Pu->243Am

[ M

eV ]

Ep = 55 MeV Ep = 20 MeV Ep = 13 MeV

p+238U->239Np

[ M

eV ]

Ep = 55 MeV Ep = 20 MeV

70 80 90 100 110 120 130 140 150 160 170

0,75

1,00

1,25

1,50

1,75

2,00

2,25

2,50

70 80 90 100 110 120 130 140 150 160 170

0,75

1,00

1,25

1,50

1,75

2,00

2,25

2,50

mass [amu]

Ep=55MeVEp=20MeV

[M

eV]

mass [amu]

p + 242Pu -> 243Am p + 238U -> 239Np

Ep=13MeV

Ep=55MeVEp=20MeV

[M

eV]

We observed the large enhancement of fission fragment yield for super asymmetric mode at AL < 80 in the proton-induced fission of 242Pu, 238U and 232Th at Ep= 55 MeV. Obtained results allow us to plan future experiments on the investigation of neutron-rich exotic nuclei with proton number around 28.

For the first time it has been obtained data on neutron and γ-rays emission in the coincidence with fission fragments for the reactions 232Th(p,f), 244Pu(p, f).The multiplicities and average energies of neutrons and γ-rays also confirm the suggestion about closed-shell structure of fission fragments with AL

E.M.Kozulin, A.A.Bogachev, M.G.Itkis, J.Kliman, G.N.Kniajeva, L.Krupa, A.Yu.Chizhov

Flerov Laboratory of Nuclear Reactions, JINR, Dubna, Russia

J.Äystö, W.A.Rubchenya, W.H.Trzaska, V.Lyapin, M. Sillanpää, S. Yamaletdinov, M. Mutterer

Department of Physics, University of Jyväskylä, Finland

E.VardaciINFN – Sezione di Napoli, Italy

A.M.StefaniniINFN - Laboratori Naziolali di Legnaro, Italy

S.V.KhlebnikovKhlopin Radium Institute, St. Petersburg, Russia

O. DorvauxInstitut de Recherches Subatomiques, CNRS-IN2P3, Strasbourg, France

F. HanappeUniversite Libre de Bruxelles, Belgium

Spokesperson: E. Kozulin