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Lise Meitner and O. R. Frisch (1969), Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction, Nature, 224, Nov. 1
This week, students created their own skim-read questions and some impressive examples are seen below, along with the best answers to the summary questions. Congratulations to the students of Polesworth School and Manchester High School for Girls for having the most entries!
What is an isotope?
Isotopes are atoms of an element with the same atomic number but different Mass Numbers. This means although they have the same number of protons, they have a different number of neutrons. Isotopes have the same chemical properties as they have the same electronic configuration but different physical properties. These atoms can be stable or unstable and in the case of the latter, they spontaneously decay over time to form more isotopes.
What is nuclear isomerism?
Nuclear isomers are nuclides which are identical (they have the same number of protons and neutrons in their nucleus) but are excited to different energy levels.
For a nuclide to have different nuclear isomers, it must have at least one metastable energy state along with its ground state. The ground state is the lowest possible energy configuration of the nucleus, and a metastable state is a state whose energy is higher than the ground state but which has more than a fleeting existence. Typically a metastable state has a lifetime of more than 10^-9 seconds.
Different nuclear isomers have different radioactive half-lives, which led to confusion in the search for transuranium elements – a sample of an element which had multiple half-lives was assumed to contain multiple nuclear isomers, instead of multiple isotopes.
What is neutron bombardment?
To initiate nuclear fission, a nucleus is bombarded with a high-energy neutron which results in the process of releasing additional neutrons. A neutron has no charge so it is not repelled when directed at the nucleus. Subsequently, they do need to be accelerated to high energies before undergoing the nuclear reaction meaning that it is less expensive. Nuclear fission can occur without being bombarded with neutrons although it is very rare.
What is neutron capture?
Neutron capture is type of nuclear reaction. A neutron is absorbed by a target nucleus then a gamma-ray photon is emitted in a discrete quantity. The target and product nuclei are both isotopes of the same element.
What is a resonance process?
When a target nucleus is bombarded with particles (as seen during nuclear fission), the nuclear reaction takes place with a high probability only if the energy of the particles is close to certain definite energy values- these energy values are referred to as resonance energies. The resultant nuclei from the nuclear reaction are referred to as nuclear resonance.
What are quantum-mechanical tunnel effects?
Due to the Heisenberg uncertainty principle, it is impossible to exactly pinpoint where a particle is, this means that quantum mechanics is probabilistic. As a result, quantum particles are modelled using wave functions, which provide the probability that the particle is at some location at a given time. According to classical physics, if the energy of a particle or wave is less than the energy of the barrier it is trying to cross, it simply won’t be able to cross it. However, because in quantum physics the particle is modelled as a wave function this turns out to not be the case. When the wave function meets a barrier, most of the wave function is reflected, but some of the wave function continues into the barrier. The amplitude of the wave function (and thus the probability of the particle being found there) decreases exponentially as it moves through the barrier, it is what is known as an evanescence wave. This means that so long as the barrier isn’t infinitely thick, the wave function can exit the other side of the barrier, meaning that there is a very small, but non-zero, probability that the particle is found on the other side of the barrier. This is known as quantum tunnelling. Even though the probability of the event occurring is very small, when there are a large number of particles in a system it is likely that tunnel effects will be observed.
What is eka-rhenium?
Eka-rhenium was how Bohrium was referred to before it’s official naming. Mendeleev had predicted that Bohrium would exist, and thus named it after rhenium as he hypothesised that it would have properties similar to that of Rhenium and be positioned near to it on the periodic table. ‘Eka’ was the prefix used to describe the first element in a chemical family, after its namesake. ‘Eka’ is from Sanskrit, meaning first.
What is surface tension?
This is due to forces between liquid molecules and is a property of a liquid. It allows it to resist an external force due to the cohesive nature of the water molecules.
What was the knowledge related to nuclear fission prior to this discovery?
It had only been 27 years since the proposal of the nuclear model of the atom by Ernest Rutherford in 1911, followed by the research of himself, Henri Becquerel, Marie Curie and Pierre Curie leading to the conclusion that a nucleus was able to undergo different forms of radioactive decay. In 1917, Rutherford was able to successfully transmute nitrogen into oxygen, by directing alpha particles into it, the first ever observation of a nuclear reaction. 15 years later, two of Rutherford’s colleagues, Ernest Walton and John Cockcroft, achieved a fully artificial nuclear reaction and transmutation by splitting a lithium-7 nucleus into two alpha particles using artificially accelerated protons. In 1932, James Chadwick discovered the neutron, and the first artificial fusion reaction was achieved by Mark Oliphant. Neutron bombardment was first conducted by Enrico Fermi in 1934, with his discoveries of new radioactive elements leading onto the experiments performed by Hahn, Meitner and Strassmann.
What is nuclear fission?
Nuclear fission is a process on atomic level with which a heavy atom is being split into lighter atoms under the emission of energy. This can be initiated with the acceleration of a neutron into the nucleus of an atom (for example uranium), so that it is separated into multiple smaller atoms (for example barium and krypton). It can be compared to the destruction of a droplet of water or other fluids under the influence of an immense force. Once the nucleus is separated, the remaining nuclei accelerate away from each other (due to the equal charges of both nuclei). This is the formation of different elements from the original element.
Describe, in as clear a way as possible, what the troubling results were surrounding the neutron bombardment of uranium, and explain why they were troubling prior to the ‘nuclear fission’ explanation.
Before the theory of nuclear fission was developed, scientists believed that bombarding uranium with neutrons would either have no effect or that it would absorb the neutron, creating a heavier atom. Rather than this, the neutron bombardment of uranium resulted in the formation of an atom with chemical properties similar to barium, therefore it was presumed to be radium (since radium is in the same group as barium but has an atomic number closer to uranium). However after closer analysis, the element formed was found to be barium itself, in contradiction to the hypotheses proposed. Without an understanding of nuclear fission, scientists had no explanation for this, since barium's atomic mass is nowhere close to that of uranium, therefore seems impossible to be borne from it.
How did the second paper (Lise Meitner and the Discovery of Fission) help you to better understand Meitner and Frisch’s original paper?
By expanding on the timeline and the thoughts around the original discovery of fission, it brought to light the incredibly complex process that led to the first paper. Seeing years—even a lifetime—of work in a few sentences doesn’t emphasise the agonising that went into the paper. Even the background behind the assumptions that made it difficult to discover fission helped in understanding why it was such a groundbreaking discovery. It’s hard to see the full, global impact of the original paper without taking a step back and seeing it in a contextualised way.
What did you learn from the second paper (Lise Meitner and the Discovery of Fission) about the relationship between science and society?
The second paper truly showed the barriers that society placed on science at the time of the second world war - the rise of authoritarian governments and anti-Semitic groups as well as the underlying sexism historically in the field. Due to her faith Meitner was forced out of her laboratory, a place where she had a great reputation. She had to quickly rush to Stockholm in order to evade the Nazis. Although Meitner was the leader of the project in the discovery of nuclear fission and directed Otto Hahn on how the experiment should be conducted, Meitner, after directing the project in secret, was hidden from any source of credit, even after the opportunity came for all her credit to be revealed after the Nazis were defeated. This is a truly shocking action against her, but this is not simply an exception. For example, Cecilia Payne, the discoverer of the chemical composition of the sun, faced oppression in her field of work all through her life and in her PhD thesis she was forced to neglect her discovery as it did not agree with what the male scientists believed at the time.
ANSWERS TO THE REST OF THE QUESTIONS (can be downloaded as a PDF here)
(P1, C1) Why do certain atoms emit radiation?
Because their nuclei are unstable and can become more stable (but not necessarily completely stable) by emitting radiation. You can think of this from an energy perspective too – by emitting radiation, the resulting nuclei reaches a lower energy state which is more stable (but not necessarily completely stable as the new nucleus could still be radioactive).
(P1, C1) Radioactive substances are those that emit alpha, beta, gamma radiation or neutrons from their nuclei. Discuss what each of these types of radiation is and any of their properties.
Alpha: a helium nucleus (two protons and two neutrons) - it is the most ionising and least penetrating form of radiation.
Beta: a fast-moving electron – it is mildly ionising and mildly penetrating
Gamma: high energy electromagnetic radiation – it is weakly ionising but highly penetrating
Neutrons: single neutrons ejected from the nucleus. They themselves are not ionising as they have no charge and don’t interact directly with electrons, but they are indirectly ionising as their interactions can lead to further ionising events. Neutrons are more penetrating than alpha and beta radiation. Depending on the material they are travelling through, they can be more penetrating that gamma radiation too (if the atomic number of the material they travel through is large).
(P1, C1) Why do we say that radiation is a random process?
Because we can’t predict when a particular nucleus will decay.
(P1, C1) What is the atomic number and mass number for the most common form of uranium? How many protons and neutrons must it have?
Atomic number: 92
Mass number: 238
Number of protons: 92 (atomic number)
Number of neutrons: 146 (mass number minus atomic number)
When they say that ‘four radioactive substances were produced’, this isn’t from a single uranium nucleus decaying. The sample of uranium that was bombarded with neutrons contained billions of atoms. The different nuclei could decay in different ways, leading to seemingly four different products that were radioactive (and possibly more that weren’t radioactive).
(P1, C1) How might the experimentalists have detected that there were radioactive substances produced?
Using a Geiger counter to monitor the activity over time.
(P1, C1) What is a nuclear isomer?
Two nuclei with the same atomic number and mass number (so the same number of protons and neutrons) but their nuclei are in different states of ‘excitation’ (which means some of the protons or neutrons have more energy than is typical). Eventually the ‘excited’ (higher energy) nucleus will decay by giving up its energy in the form of gamma radiation. However, these excited states may be very long-lived such that they are often termed quasi-stable.
At this point in history, scientists were expecting that, when bombarded with neutrons, a nucleus would do one of two things:
- absorb the neutron (and possibly even more than one) and then go through a series of decays (alpha, beta, gamma, neutron emission) until we got to something stable.
(P1, C1) “It was always assumed that these radioactive bodies had atomic numbers near that of the element bombarded”. Given the two bullet points above, why is this?
Because the only known options were to:
· absorb a neutron (so mass number increases by 1) then
o Do nothing
o Emit alpha radiation (mass number decreases by 4 and atomic number decreases by 2)
o Emit beta-minus radiation (atomic number increases by 1)
o Emit beta-plus radiation (atomic number decreases by 1)
o Emit gamma radiation (no change)
o Emit neutron (mass number decreases by 1)
All of these would leave the resulting atom very close to the original atom.
(P1, C1) “only particles with one or two charges were known to be emitted” - what are they referring to here?
Predominantly alpha (Charge of +2) and beta (charge of +1 or –1) radiation.
(P1, C1) ‘eka-osmium’ is now called Plutonium. Looking at the periodic table, why would osmium and ruthenium be expected to have similar properties?
Because osmium and ruthenium are both in group 8 of the periodic table.
(P1, C1) If something is “isotopic with radium”, then it is an isotope of radium. What does this mean?
An isotope of radium has the same number of protons (88) but a different number of neutrons.
(P1, C1) If bombardment of uranium (Z=92) lead to something “chemically similar to barium” (Z=54), what two reasons would lead them to initially conclude it was an isotope of radium (Z=88)?
Because radium is chemically similar to barium as they’re both in group 2 of the periodic table.
Radium is close, in terms of its mass number and atomic number, to uranium, so the decay of uranium could go through known channels (alpha, beta and neutron emission).
(P1, C1&C2) Why were Hahn and Strassman “forced” into a conclusion that they probably weren’t happy with?
Because they would have expected the products of the bombardment of uranium to be close to it in the periodic table. They were expecting it to be an isotope of radium, but found that the resulting nucleus could not be distinguished from barium, leading them to conclude that it must be an isotope of barium itself.
This is surprising as it would have involved a very long chain of known decays (alpha, beta and neutron emission).
(P1, C2) Meitner and Frisch draw an analogy to a liquid drop. Concentrating simple on a liquid, what keeps a droplet together?
Surface tension. At a liquid-air interface, the liquid molecules are more attracted to one another than to the air, leading to an inward force that makes it seem like the water is encased by some elastic membrane.
(P1, C2) What similarities do Meitner and Frisch draw between the uranium nucleus and a ‘liquid drop’?
If you shake a liquid drop sufficiently violently, it will divide itself into smaller drops. Meitner and Frisch suggest that, rather than emitting nuclear radiation, the nucleus of uranium splits in a similar way to a liquid drop: into two smaller parts.
(P1, C2) What do you think the authors mean by “stability of form”?
Much like a drop of liquid, which can be destabilised into forming two smaller droplets, the uranium nucleus isn’t so strong that it will continue to hold itself in one large piece, but can be disrupted into splitting into two pieces of similar size. It might bring to mind the idea of surface tension.
(P1, C2) Given that we’re discussing nuclear radiation and the behaviour of nuclei, why is it perhaps not surprising that the size of the smaller pieces of uranium may be dictated “partly on chance”?
Because the laws that govern the nucleus are quantum mechanical laws and are probabilistic by their very nature. The emission of radiation is a random process, so it might not be surprising if the splitting of a nucleus into smaller parts might also have some randomness to it in the size of the resulting pieces.
(P1, C2) Why will the two nuclei that are formed when uranium splits repel one another?
Because they are nuclei, they contain just protons and neutrons and therefore have positive charge. Like charges repel and so the nuclei will repel one another.
(P1, C2) What is the definition of fission? And hence what is the definition of nuclear fission?
Fission: division or splitting up into two or more parts
Nuclear fission: the division of splitting of the nucleus into two or more parts.
Remember that the article continues below the horizontal line on the second page.
(P2, C1) What do they mean when they say “After division, the high neutron/proton ratio of uranium will tend to readjust itself by beta decay to the lower value suitable for lighter elements.”
As Uranium has a mass number of 238 and an atomic number of 92, for every proton in uranium, there are ~2.6 neutrons. This ratio is much larger than for lighter elements. So when uranium breaks into two parts, at least one part will have a lot more neutrons than is typical for however many protons there are. This atom will then be unstable and will go through beta decays (neutron turning into a proton and releasing an electron and an antielectron neutrino) to adjust the balance.
(P2, C1) Look at the periodic table and explain why krypton “might decay through rubidium, strontium and yttrium to zirconium”
Each step would be a single beta decay as each step is an increase of 1 in the atomic number (on each step, we might end up at an isotope of each atom though due to the excess neutrons).
In the paragraph that begins “It is possible, and seems to us rather probable”, Meitner and Frisch are explaining the previous puzzling experimental evidence through this new lens of nuclear fission. Fermi and friends had previously detected radioactive nuclei with short half-lives as some of the products of the uranium reaction. Rather than having to ascribe these values to new, heavy elements, they are better understood as half-lives of much lighter elements (that were already known) but simply weren’t expected to be seen.
The final few paragraphs involved Meitner and Frisch giving their opinions on other confusing experimental results: explaining the neutron bombardment of thorium through a fission paradigm and shining some light onto the isotopes of uranium produced.
Remember, reading a paper isn't like reading a piece of fiction or a newspaper article. Don't get frustrated if it doesn't immediately make sense - you might need to do a little research of your own to understand some of the ideas. This article gives you an idea of how scientists read differently.
Each question refers to a specific part of the paper e.g. Page 2, Column 3 is written as (P2, C3).
Next week, we'll publish solutions to the questions and the best submitted summaries from students across the country.
We're going to be looking at space weather. Specifically, we're going to be looking at how understanding and predicting space weather is crucial in a political context.
This page from the Met Office describes a little about what space weather is.
This video gives an overview of some of the key vocabulary around space weather.