past and future

Some properties of the nucleus:

past and future

Neil Rowley

Theoretical Physics Group, IPN d’Orsay

Accélérateurs de Particules et Interaction avec la Matière (APIM)

The becquerel (Bq) is the SI unit of radioactivity, defined as the activity of a quantity of radioactive material in which one nucleus decays per second. It is named after Henri Becquerel, who shared the Nobel Prize in 1903 with Pierre and Marie Curie for their work in discovering radioactivity.

COMMENTS ON CHEMISTRY AND ATOMIC PHYSICS

In 1893, Dmitri Mendeleev was appointed

Director of the Bureau of Weights and Measures.

It was in this role that he was directed to

formulate new state standards for the production of vodka.

As a result of his work, in 1894 new standards for vodka were introduced into Russian law and all

vodka had to be produced at 40% alcohol by volume.

This greatly pleased the Tzar…

But he is probably better known for his periodic table of the elements

…

• Mendeleev realized that the physical and chemical properties of the elements were related to their atomic mass in a 'periodic' way, and he arranged them so that groups of elements with similar properties fell into vertical columns in his table.

Sometimes this gave gaps in his horizontal rows or 'periods‘, and Mendeleev realised that this meant that the elements which belonged in the gaps had not yet been discovered. He was able to work out the atomic masses of the missing elements, and predict their properties. When they were discovered, Mendeleev turned out to be right. For example, he predicted the

properties of an undiscovered element that should fit below aluminum in his table.

When this element (gallium) was discovered in 1875 its properties were found to be close to Mendeleev's predictions. Other predicted elements lent further credit to Mendeleev's table.

• Modern day periodic tables are greatly expanded beyond Mendeleev's initial 63 elements.

iThemba School Jan/Feb 2008

In the Bohr model: E ~ 1/N^-2 = (n+L+1)^-2, where the principal quantum number N is partly L and partly related to the radial motion (n nodes):

e.g. for N=3, n+L+1=3, so [n,L]=[2,0]; [1,1]; [0,2]

Thus all of these states are degenerate (have the same energy) but their orbitals are different. This cancellation of potential and kinetic energies is special to the -1/r potential (virial theorem).

L=0 is referred to as the s wave; L=1 is p, L=2 is d, L=3 is f, L=4 is g…(sharp, principal, diffuse, fine from the nature of observed spectral lines). The order of the levels N[L] (in increasing energy) is:

1[s] 2[s,p] 3[s,p,d] 4[s,p,d,f] 5[s,p,d,f,g]…

for the hydrogen atom

But due to the electron screening in heavier systems, the order becomes:

1[s] [2s,2p] [3s, 3p] [4s, 3d, 4p] [5s, 4d, 5p]…

That is some levels are shifted into different shells.

This explains the chemistry since the degeneracies of the levels are 2(s), 6(p), 10(d), 14(f)...

A similar shifting of levels gives rise to the magic numbers in nuclei.

3d

3p 3s

Electron screening

Although the inner electrons see most of the nuclear charge, the outermost ones see a significantly reduced charge due to screening.

• Uraninite is a uranium-rich mineral with a composition that is largely UO₂ (uranium dioxide), but which also contains UO₃ and oxides of lead, thorium, and rare earths. It is most commonly known in the

variety pitchblende, which was used in the Curies’ research.

• Maria (Skłodowska) Curie was born in Warsaw but in 1891 at age 24 she left to study in Paris, where she obtained her higher degrees and conducted her scientific work. She was awarded Nobel prizes in both physics and chemistry, and

founded the Curie Institutes in Paris and Warsaw. She was the wife of fellow Nobel laureate Pierre Curie and the mother of a further Nobel laureate, Irène Joliot-Curie.

• She discovered the new chemical elements radium and polonium (named after her native country).

Alpha energies 226Ra 4.842 MeV 222Rn 4.130 MeV 218Po 3.406 MeV 214Po 7.833 MeV 210Po 5.407 MeV

Products of decay of ²²⁶ Ra

Radium emanation

Rn Radium A

Po

Radium B

Pb Radium C

Bi Radium C1

Po Radium C2

Tl Radium D

Pb Radium E

Bi Radium F

Po

The Curies did not know about

the right-hand column…

Thomson discovered the electron but his plum pudding model of the atom was wrong .

Rutherford’s nucleus (deduced from the scattering of a few alpha particles to large angles)

Existence of the atomic nucleus

The first nuclear reaction experiment

(Manchester 1909; with a French alpha source)

Frédéric Joliot and Irène Joliot-Curie

Creation of radioactive isotopes

• For example, in the case where aluminium irradiated by alpha rays emits neutrons, the rule already mentioned enables us to write the following transmutation reaction

2713Al + ⁴₂He → ³⁰₁₅P + n (or ³⁰₁₄Si + p but this is much less interesting)

3015P → ³⁰₁₄Si + β⁺ + ν

The atom formed being radioactive, it is possible to verify that it possesses the chemical properties of phosphorus. A piece of thin aluminium sheet, irradiated

beforehand by alpha rays is attacked and dissolved in a solution of hydrochloric acid.

The chemical reaction produces nascent hydrogen which carries over the radioactive element into a thin-walled tube where it is collected over water**. This separation clearly demonstrates that some element other than aluminium has been formed on irradiation by helions. It furnishes an indisputable proof of the transmutation achieved;

also, traces of phosphorus would be separated from the aluminium in the same experiment.

**Go see the picture in the amphitheater

Geiger and his counter

This is a positron

(In fact the first one ever seen)

Discovery of the neutron

• Chadwick used 5.2 MeV α particles (from decay of ²⁰⁸Po) on ⁹Be to produce neutrons:

4He+⁹Be¹³C*¹²C+n

• Q=10.7 MeV, therefore, E*=15.9 MeV and since S_1n=4.95 MeV, the neutron kinetic energy is=10.95 MeV

• The neutrons themselves are not seen (neutral) but are detected via the

recoiling protons in H or heavier

nuclei on which they scatter in a cloud chamber.

• The experiments give a measure of the neutron mass: 1.008>Mn>1.005 amu…vey similar to the proton…

and of the radius of the lead nucleus when used to attenuate the beam:

R(²⁰⁸Pb)=7 fm (almost perfect)

• The α-particle separation energy of

C=10.65 MeV.

Therefore if

C is made by

α+

Be it will have an excitation energy of 10.65 MeV plus the incident kinetic energy of the α (5.2 MeV in this case).

• This is above the neutron

emission threshold but below that for protons, giving a clean experiment.

• For

C it is below both

thresholds (important in the

triple-α process in stars).

Isotopes

• Following the discovery of the neutron, Heisenberg quickly realised the significance of different isotopes.

• We have already seen the importance of particular

isotopes as medical tracers and shall see many more

examples of the importance of varying neutron numbers:

H, d, t;

U…

Quantum mechanics

Fission

Hahn and Meitner identified the fragments

chemically~Kr and Ba

Chemistry or physics?

Cockroft and Walton Experiment

Converting Mass into Energy

• In 1932, the English physicist John Cockroft and the Irish physicist Ernest Walton produced a nuclear disintegration by bombarding Lithium with artificially accelerated protons.

• The following reaction took place: p + ⁷Li→ α + α + energy

• This was the first artificial splitting of a nucleus. It was also the first transmutation using artificially accelerated particles.

• Protons were accelerated and slammed into lithium atoms producing alpha-particles and energy. Thus the mass of the proton and lithium was converted into the mass of two alpha-particles and kinetic energy.

• This reaction was the first experimental proof of Einstein's: E = mc²

• Cockroft and Walton won a Nobel Prize in physics in 1951.

Q=17.26 MeV

The first accelerator

(still a very primitive detector)

The products of the reaction were emitted at right angles to the proton beam and struck zinc sulphide (ZnS) screens producing small flashes of light called

scintillations which could be seen with a microscope

John Cockroft

C

M

E

Some unfortunate consequences

Exploiting the nuclear mass differences

Oppenheimer, Teller Kurchatov, Sakharov

Chain reactions

• If too many neutrons escape from the sample, the chain is broken.

• We can use enriched U to get a higher reaction rate or…

• Moderators: the n-capture cross section can be enhanced by

thermalising the emitted neutrons with a moderator (low mass, scatters

without absorbing… p in H₂O can absorb, d in heavy water is better).

• By blanketing the reaction with ²³⁸U we have: ²³⁸U+n²³⁹U(β)

239Np(β)²³⁹Pu which is more fissile than ²³⁵U was to start with.

• Using²³²Th leads to ²³³U which can also be used as a fuel.

Teller-Ulam device

n+

Li

Li*

H+

He

3

H+

Hn+

He

• This is very different from fusion in stars (see later).

• In Tokomak we must inject the ²H (deuterium) and ³H (tritium) nuclei.

• In a bomb, use 6-lithium deuteride which is very conveniently a perfectly stable solid…

iThemba School Jan/Feb 2008

NUCLEOSYNTHESIS

STELLAR DYNAMICS

Proton burning (gives light and heat but no new elements!)

Total energy output (including neutrinos)=4Δ(1H)-Δ(4He)=26.7 MeV

Bethe

Lawrence

BUT…

Other accelerators

Radio-isotopes now produced routinely

•

Ga (3.3 d) [p+

Zn]

•

In (2.8 d) [p+

Cd]

•

I (13 h) [p+

Te]

• All of these decay by

electron capture which is followed by X-ray emission.

The energies are not large enough for e

emission.

• Radionuclides used in PET

scanning are typically isotopes with short half lives such as carbon-11 (~20 min), nitrogen-13 (~10 min), oxygen-15 (~2 min), and Fluorine-18 (~110 min). Due to their short half lives, they must be produced in a cyclotron which is close to the PET scanner. These radionuclides are incorporated into compounds

naturally used by the body such as glucose, water or ammonia and then injected into the body to trace where they become distributed.

• e⁺+e^-2γ (511 keV)

Seabourg and McMillan

(going above Z=92)

Accelerators + chemistry

• A bombardment of uranium oxide with the 16-MeV deuterons from the 60-inch cyclotron was performed in December, 1940. Alpha radioactivity was found to grow into the chemically separated element 93 fraction, and this alpha-activity was

chemically separated from the neighboring elements, especially elements 90 to 93 inclusive, in experiments performed during the following months. These experiments, which constituted the positive identification of element 94, showed that this element has at least two oxidation states, distinguishable by their precipitation chemistry, and that it requires stronger oxidizing agents to oxidize element 94 to the upper state than is the case for element 93. The particular isotope identified has been shown to be of mass number 238 and the reactions for its preparation are:

• ²³⁸₉₂U + d → ²³⁸₉₃Np + 2n

• ²³⁸₉₃Np → ²³⁸₉₄Pu + β- + antineutrino [90 years]

• The chemical properties of elements 93 and 94 were studied by the tracer method at the University of California for the next year and a half. These first two transuranium elements were referred to by our group simply as "element 93" and "element 94" until the spring of 1942, at which time the first detailed reports concerning these elements were written.

• It became possible on the basis of the interesting pioneer carbon-ion bombardment experiments of Miller, Hamilton, Putnam, Haymond, and Rossi 26 in the 60-inch cyclotron to produce isotopes of californium in better yield by this method.

• These experiments, in which uranium is bombarded with carbon ions, led to the production of californium isotopes according to the reactions

• ²³⁸₉₂U + ¹²₆C → ²⁴⁴₉₈Cf + 6n

• ²³⁸₉₂U + ¹²₆C → ²⁴⁶₉₈Cf + 4n

• Seaborg was the principal or co-discoverer of ten elements: plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium and element 106, which was named seaborgium in his honor while he was still living.

• It was noted in Discover magazine's review of the year in science that he could receive a letter addressed in chemical elements:

• Glenn Seabourg, Lawrence Berkeley Laboratory, Berkeley, California, USA

• seaborgium, lawrencium, berkelium, californium, americium

Four steps to an experiment

• Source [create and ionise], acceleration, reaction [target], detection and measurement.

• Of course the use of

accelerators also allows us to use different projectile, in

particular heavy ions…and exotic ones

• Many detectors at our disposal

• Limitations of cyclotron led to the idea of the synchrotron (high-energy). See e.g. LHC.

• The ALTO facility at IPN Orsay is based on a linear electron accelerator (50 MeV,10μA) dedicated to the production of neutron rich radioactive beams by photo-fission of a thick

uranium carbide target paced

on an ISOL line.

Tandem Van der Graaf

Part II

THE NUCLEUS: http://www.nndc.bnl.gov/chart/

Neutron number N

Proto n (ato mic) n umb er Z

• Ground and isomeric state information for ¹⁷²₇₃Ta

• E(level) (MeV) J^π Δ (MeV) T_1/2 Decay Modes

• 0.0 (3+) -51.3300 36.8 m ε : 100.00 %

A list of levels, a level scheme and decay radiation information are available

iThemba School Jan/Feb 2008

Transfer reaction

NUCLEAR REACTIONS

THE NUCLEUS: http://www.nndc.bnl.gov/chart/

Neutron number N

Proto n (ato mic) n umb er Z

In one dimension

FWHM: 2-3 MeV

In three dimensions

State-of-the-art cross sections in 1990

R. Stokstad et al., Phy. Rev. C 21(1980) 2427

Rotational states up to 10

40 data points spanning 4 orders of magnitude; 4 parameters (others are B and a)

Precise values of β

and β

(nuclear shape)

Measuring Barriers to Fusion:

M. Dasgupta, D.J. Hinde, N. Rowley and A.M. Stefanini Ann. Rev. Nucl. Part. Sci. 48 (1998) 401-461

CCFULL: A program for coupled-channels calculations with all- order couplings for heavy-ion fusion reactions:

K. Hagino, N. Rowley and A.T. Kruppa

Comp. Phys. Comm. 123 (1999) 143

Quasi-elastic scattering:

Canberra, Warsaw, iThemba

• Effect of Deformation on the Elastic and Quasielastic Scattering of Heavy Ions near the Coulomb Barrier, M.V. Andres, N. Rowley and M.A. Nagarajan, Phys. Lett. B 202 (1988) 292

• Probing Fusion Barrier Distributions with Quasi-Elastic Scattering,

H. Timmers, J.R. Leigh, M. Dasgupta, D.J. Hinde, R.C. Lemmon, J.C. Mein, C.R. Morton, J.O. Newton and N. Rowley Nucl. Phys. A584 (1995) 19

• Barrier Distributions in ¹⁶O + ^116,119Sn Quasi-Elastic Scattering, E. Piasecki, M. Kowalczyk, K. Piasecki, L . Swiderski, J. Srebrny, M. Witecki, F. Carstoiu, W. Czarnacki, K. Rusek, J. Iwanicki, J. Jastrzebski, M. Kisielinski, A.

Kordyasz, A. Stolarz, J. Tys, T. Krogulski, N. Rowley, Phys. Rev. C 65 (2002) 054611

• Large-angle scattering and quasi-elastic barrier distributions, K. Hagino and N. Rowley Phys. Rev. C 69 (2004) 054610

Geiger and Marsden only had alpha particles (from the decay of polonium) at a fixed E which was too low to hit the Au nucleus, though they were able to tell that its radius was less than 27 fm.

With an accelerator, we can measure the angular distribution at higher E, or even measure an excitation

function at fixed θ. Such experiments actually measure the interaction radius of the two nuclei in question...

Quasi-elastic barrier distributions

16O + ^116,119Sn: Piasecki et al., Phys. Rev. C 65 (2002) 054611

Different angles give different effective energies at the same time

Detectors are very simple: no energy resolution or mass/charge resolution required! iThemba used solar cells

Quasi-elastic scattering of the highly deformed

Ne (β

=0.46; β

=0.27) (Warsaw/JYFL Jyvaskyla)

Note the high quality of the angular mapping to an effective energy

E. Piasecki et al., Phys. Rev. C 80 (2009) 024323

Quasi-elastic scattering of

Kr by

Pb; Z

=118 (iThemba LABS)

S.S. Ntshangese et al., Phys. Lett. B 651 (2007) 27

Very heavy systems:

superheavy elements…hot and cold fusion Z=118 and still looking….

Crossing the barrier is no longer sufficient…we must also

evolve to form an equilibrated compound nucleus, and this must also survive against fission

Note the new units: nanobarns and picobarns;

and the new detection systems

SHIP (Separator for Heavy-Ion Physics) at GSI

The alpha decays occur at the same point on the focal plane of the detector as the implantation of the original ion. The entire chain can be identified from the

overlap with known nuclides at the bottom of the decay chain.

Τ=3 days

S. Hofmann and G. Munzenburg, Rev. Mod. Phys. 72 (2000) 733 P. Armbruster, Ann. Rev. Nucl. Part. Sc. 50 (2000) 411

Z=107—112 discovered at GSI by « cold » fusion

Yu. Oganessian, J. Phys. G34 (2007) R165

Counts rates are very low… (order of a few per month even with microamp beams) and the chains do not overlap with known nuclides. The results are none the less very impressive and convincing. Z=118 is the current limit…

iThemba School Jan/Feb 2008

Deformations and other things from gamma-ray spectroscopy:

What is seen in the experiment and its interpretation…

154

Sm

Natural reference for nuclear high-spin states, N. Rowley, J. Ollier and J. Simpson Phys. Rev. C 80, 024323 (2009)

Gammasphere

Conclusion

• Physics progresses hand-in-hand with technological advances. In the case of nuclear physics this means ion sources, accelerators, target production and handling and the development of more and more sophisticated and sensitive detection systems…

Many of the projects started in the early 20

century are now being pushed

to limits unimagined by their initiators…

Higgs boson

LHC…Atlas…etc

ATLAS is 44 metres long and 25 metres in diameter, weighing about 7,000 tonnes The LHC lies in a tunnel 27 km in circumference, 175 m

beneath the Franco-Swiss border near Geneva, Switzerland.

This synchrotron is designed to collide opposing particle beams of either protons at an energy of 7 TeV.

THE NUCLEUS: http://www.nndc.bnl.gov/chart/

Neutron number N

Proto n (ato mic) n umb er Z

100Sn: ⁵⁰Cr+⁵⁸Ni

194Hg SD: ⁴⁸Ca+¹⁵⁰Nd

194Pb SD: ¹⁶O+¹⁸⁴W SHE 118: ⁴⁸Ca+²⁴⁹Cf SHE 112: ⁷⁰Zn+²⁰⁸Pb

SPIRAL II