Fission (a brief introduction) Stephan Pomp Department of physics and astronomy Uppsala University

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1 Fission (a brief introduction) Stephan Pomp Department of physics and astronomy Uppsala University

2 10 years ago I was not particulary interested in fission. Now I find it fascinating. How come? Courtesy S. Chiba a complete nuclear physics lab

3 Th, U, Pu, Uranium Lead Fission products Cesium Iron Fission products cover almost half the chemical elements found in nature!

landscape De-excitation: neutron emission, Other Later: Fission yields (FY):

4 Outline History and some literature sources A walk through the fission process: Phenomenology, time scale Yields, kinematics, Fission modes and potential energy landscape De-excitation: neutron emission, Other Later: Fission yields (FY): Definitions, trends, model codes, experiments Note: focus mostly on neutron-induced binary fission

Not only Otto Hahn Her paper, "On Element 93" [1934!

than uranium elements in his proofs, rather than only down to lead.

be isotopes of known elements but would not be neighbors of the irradiated element.

5 Not only Otto Hahn Her paper, "On Element 93" [1934!] suggested a number of possibilities, centering around Fermi's failure to chemically eliminate all lighter than uranium elements in his proofs, rather than only down to lead. Ida Noddack it is conceivable that the nucleus breaks up into several large fragments, which would of course be isotopes of known elements but would not be neighbors of the irradiated element. Lise Meitner was in Sweden and was lonely so I offered to come and visit her Listen to Hahn, Frisch and many others here: Otto Frisch On lost opportunities etc: Herrmann, NPA 502 (1989) 141

6 Historically important papers - O. Hahn and F. Strassmann, Naturwissenschaften 26 (1938) L. Meitner, Nature 143 (1939) O.R. Frisch, Nature 143 (1939) N. Bohr and J.A. Wheeler, Phys. Rev. 56 (1939) D.L. Hill and J.A. Wheeler,, Phys. Rev. 89 (1953) If you have read Bohr-Wheeler and Hill-Wheeler you understand fission. Nothing much has happened since (F. 2012) Nja Fun fact: Richard Feynman and Kip Thorne were students of John Archibald Wheeler. Wheeler is co-author of Gravitation (1973), with Kip Thorne and others...

7 Literature: some suggestions Fission in general: N. Bohr and J.A. Wheeler, The mechanism of nuclear fission, Phys. Rev. 56 (1939) 426. R. Vandenbosch and J. Huizenga, Nuclear fission, Academic Press, C. Wagemans, The nuclear fission process, CRC Press, A.N. Andreyev et al., Nuclear fission: a review of Experimental Advances and Phenomenology, Rep. Prog. Phys, 81 (2017). On measurement techniques etc.: e.g. PhD theses available for free on the web: Ali Al-Adili, PhD thesis, Uppsala, Andrea Mattera, PhD thesis, Uppsala Kaj Jansson, PhD thesis, Uppsala and of course many other papers and theses from many other universities and labs

8 And a movie... on youtube: MNOqdLpI

9 We may have spontanous fission (e.g. 252 Cf), or Some terminology induced fission (typically a incoming neutron, but also protons, photo-fission,...). Incoming particle forms, together with the target nucleus, a compound system with a certain excitation energy. n + A X -> A+1 X* Competition with other reaction channels (e.g. Emission of pre-fission neutrons). Possibly fission occurs, typically into a light and a heavy fragment (which are also excited). A+1 X* -> L* + H*, A L + A H = A 0 The point when the neck between the nascent fragments breaks is called scission.

10 What do you know about fission? Is this a good picture?

11 Fission: time line and observables Cross section, fission yields (at various stages), prompt neutron multiplicity and energy spectra prompt gamma multiplicity and energy spectra

12 Nuclear binding energy region of the most stable nuclei (highest binding energy per nucleon) Fusion Fission Helium Fission products B 1 MeV/A Fe, Co, Ni, Cu, Uranium i.e. about 200 MeV energy release per fission (roughly the same for all actinides)

13 Energy release and time scale Kinetic energy of fragments (total kinetic energy, TKE): typically 85 % of total energy release (but large variations!) Kinetic energy to prompt neutrons: about 5 MeV (then we have fission products) Energy to prompt gammas: about 7 MeV Energy to beta decay etc: about 10 % (cumulative..) Note: neutrinos take away about 8 MeV See S. Prussin, Nuclear Physics for Applications, p. 401 for a detailed list Figure from C. Wagemans (ed.), The Nuclear Fission Process (Boca Raton, 1991)

14 Fission of a liquid drop FISSION is unusual among nuclear processes. The division of a many-particle system into two equal fragments is beyond explanation in terms of the movement of a single nucleon, or any small number of nucleons. In evidence is the collective behavior of the nucleus as a whole. This behavior has been idealized in the liquid drop model. The nuclear substance is compared with a nearly incompressible fluid,. D.A. Hill and J.A. Wheeler, Phys. Rev. 89 (1953) Historical note: the shell model was developed (Maria Goeppert Mayer and others) around

15 Trends I Light mass peak shifts as compound mass increases Heavy mass peak remains centered around A 140. Effect of closed shells around 132 (50+82) and 78 (28+50)

16 Estimate: kinetic energy from Coulomb repulsion Neutron-induced fission of U-235. Calculate the kinetic energy of the fission fragments assuming: - Symmetric fission - Spherical fission products in contact with zero kinetic energy - Consider fragments as point charges A CN = 236; A 1 = A 2 = 118; Z 1 = Z 2 = 46 d = 2 r A 0 1/3 A very deformed nucleus. E kin = 1 Z1 e Z2 e Z1 Z2 4πε 0 d 1.44MeV d [ fm] 250MeV Note: Z 1 Z 2 ZCN 2 2

17 Kinematics (before prompt neutron emission) E E H L m v H H = ml vl = m m L H using momentum conservation: v v H L = m m L H A A E E H L m m L H Figure from Bertulani, Nuclear Physics in a Nutshell

More on kinematics Average TKE very well known. Depends on system. Typically about 170 MeV. Total kinetic energy as function of mass, TKE(A), depends on mass split.

18 More on kinematics Average TKE very well known. Depends on system. Typically about 170 MeV. Total kinetic energy as function of mass, TKE(A), depends on mass split. Light fragment: higher kinetic energy (about 100 MeV) Heavy fragment: lower kinetic energy (about 70 kev) Velocities: Typically about 1cm/ns Note the narrower light mass peak. Fig from C. Wagemans, The nuclear fission process, page 331.

19 TKE and fission modes Fig from C. Wagemans, The nuclear fission process, page 347. Observed structures (e.g. dip at symmetry) are an early hint on different scission configurations, so-called fission modes. Fig from C. Wagemans, The nuclear fission process, page 350.

20 From: Brosa et al., Phys. Rep. 197 (1990) 167 S. Pomp, Zermatt, From: Jan Ali 2018 Al-Adili, PhD thesis, Uppsala, 2013

Pairing (and asymmetry) matters Neutron separation energies: 233 U: 5.755 MeV 234 U: 6.844 MeV 235 U: 5.298 MeV 236 U: 6.545 MeV 237 U: 5.

21 Pairing (and asymmetry) matters Neutron separation energies: 233 U: MeV 234 U: MeV 235 U: MeV 236 U: MeV 237 U: MeV 238 U: MeV 239 U: MeV 239 Pu: MeV 240 Pu: MeV 241 Pu: MeV 242 Pu: MeV Barriers are at about 6.0 or so MeV

22 Fission of 235 U: No threshold Fission of 238 U: Threshold at about 1.2 MeV Similar at second chance (n,nf): Threshold for this reactions is about 0.5 MeV lower in 238 U(n,nf) than 235 U(n,nf). [Consider: what is the fissioning system?]

23 (n,f) cross section examples From (very convenient!)

24 from the TALYS manual: Of course: everything competes! σ for various interactions depends on - which channels are open - total σ - emission/decay probabilities which in turn depend on - nuclear potential - transmission probability - nuclear structure/level densities -

25 Structure of the fission barrier(s) Fig from Ali Al-Adili, PhD thesis, Uppsala 2013 Second well: Fission or shape isomer Well established! Quantum number of transition states influence angular distr.

26 Sub-barrier fission of 240 Pu Fig from C. Wagemans, The nuclear fission process, page 77. Level spacing of class I states: about 14 ev. Level spacing of class II states: about 450 ev. Cross section increases when levels match and class II states act as doorway states.

27 Just another figure Fig from Bolsteri et al., Phys. Rev. C 5 (1972) Ground state Fission barriers (saddle points) Neck breaks (thickness about 1 fm)

The complete nuclear physics lab (aka fission) Fission is a slow, dynamic process which can be viewed as nuclear shape evolution. Fission comprises many aspects of nuclear physics. E.g.

28 The complete nuclear physics lab (aka fission) Fission is a slow, dynamic process which can be viewed as nuclear shape evolution. Fission comprises many aspects of nuclear physics. E.g. structure of very deformed nuclei far from stability Nuclear shape evolution with Langevin equations; courtesy S. Chiba A.V. Karpov et al., J. Phys. G: Nucl. Part. Phys. 35 (2008)

29 A 3-D view of a potential landscape From: A.V. Karpov et al., J. Phys. G: Nucl. Part. Phys. 35 (2008)

30 higher energy of incident neutron Trends II Yields generally asymmetric But towards higher energies the symmetric component Increases (and gets wider). I.e. shell effects less important at high excitation energies. 238 U 232 Th PhD thesis V. Simutkin, Uppsala 2010

31 Trends III Figure from K.H. Schmidt et al., Nucl. Data Sheets 131 (2016) 107

32 Next: De-excitation Q = TKE + TXE Q-value is straight forward to calculate for a given mass split. But how is the energy shared? Generally the essence of physics: energy is conserved but takes different forms...

33 Q = TKE + TXE Energy release goes to: Kinetic energy Deformation of fragments Excitation of fragments TKE (Total Kinetic Energy) TXE (Total Excitation Energy) (fragment TXE -> neutrons, beta, neutrinos, )

34 Schematic view of de-excitation Very simplified picture... O. Litaize et al., EPJ A 51:177 (2015)

35 The sawtooth Modeled with the FIFRELIN code: O. Litaize et al., EPJ A 51:177 (2015)

36 Nubar as function of energy of incoming neutron: ν(e n ) Fig. from Ali Al-Adili, PhD thesis, Uppsala Screenshot from Ramona Vogts talk: MNOqdLpI

37 Hot topic: which fragments do emit more neutrons as excitation energy increases? Fig from Ali Al-Adili, PhD thesis, Uppsala Current discussion: More from heavy fragment? Why? Transfer of excitation energy? See GEF code and papers by K.H. Schmidt and B. Jurado-

38 Fission sources: 252 Cf(SF) Energy distribution of prompt neutrons (PFNS) Prompt neutrons from spontaneous fission: neutrons per fission Watts spectrum: NN EE = ee EE aa sinh (a= 1.18 MeV; b = MeV -1 ) T 1/2 = years BR(SF) = 3.09 % 2.314x10 6 n/s/mg 4,316 n/s/µci bbbb Source: Radev and McLean, Neutron-sources for standard testing, LLNL-TR

39 Many neutron sources (difficult to measure PFNS) In (n,f) at least 5 sources of neutrons: Incoming beam and associated background. Pre-fission, scission, prompt, and delayed neutrons.

40 Timescale and neutron emission to sec T 1/2 = 55.9 sec precusor Figs from Bertulani, Nuclear physics in a nutshell..

41 Prompt gamma emission 252 Cf 235 U O. Litaize et al., EPJ A 51:177 (2015)

Other observables: e.g. angular distributions PhD thesis A. Al-Adili, Uppsala 2013 Strong fluctuations in the angular distributions in 234 U(n,f) close to vibrational resonance.

42 Other observables: e.g. angular distributions PhD thesis A. Al-Adili, Uppsala 2013 Strong fluctuations in the angular distributions in 234 U(n,f) close to vibrational resonance. Resonances connected to the so-called transition states at the saddle. Also changes in mass and energy distributions where observed. May be connected to structure in potential energy landscape (fission barrier heights).

, PRC 90 (2014) 064611 252 Cf(SF) New theoretical ideas: Energy sorting mechanism based on the constant temperature

43 Some challenges Questions: Where does the excitation energy go? When does the freeze-out of fragment properties occur? Do scission neutrons exist? Linked to that: Prompt neutrons? Isomeric yields? A. Göök et al., PRC 90 (2014) Cf(SF) New theoretical ideas: Energy sorting mechanism based on the constant temperature model Schmidt and Jurado, PRC 83 (2011) (R) 237 Np(n,f) GEF code K.-H. Schmidt et al., Nucl. Data Sheets 131 (2016) Excitation energy

44 All questions answered?

Nuclear Fission Fission discovered by Otto Hahn and Fritz Strassman, Lisa Meitner in 1938

Nuclear Fission Fission discovered by Otto Hahn and Fritz Strassman, Lisa Meitner in 1938 Fission Readings: Modern Nuclear Chemistry, Chapter 11; Nuclear and Radiochemistry, Chapter 3 General Overview of Fission Energetics The Probability of Fission Fission Product Distributions Total Kinetic