Learning from Dr. Enrico Fermi: An Attitude Toward Experiments

Introduction

The stars in the sky are bright lights, lights in the sky! Today, we are going to delve into a very famous quote by Dr. Fermi!

"There are two possible outcomes: if the result confirms the hypothesis, then you've made a measurement. If the result is contrary to the hypothesis, then you've made a discovery."

"There are two outcomes to an experiment. If the result confirms the hypothesis, then you have measured something. If the result is contrary to the hypothesis, then you have discovered something."

...Oh, just one thing to confirm, you aren't conducting experiments without even forming a hypothesis, are you? (smiles)

As you can see from this quote, you can't measure or discover anything in the first place if you haven't formed a hypothesis!

Now, to the main topic!

Chicago Pile-1, the world's first nuclear reactor built by Dr. Fermi

Dr. Fermiis a figure who is the model of a researcher, having achieved great results in both theoretical physicsand experimental physics. There are an extremely large number of scientific terms that bear his name, such as "Fermi...".

Such was Dr. Fermi. In fact, on December 2, 1942, in a small underground gymnasium at the University of Chicago, he was the person who created the world's first artificial nuclear reactor.

...Oh, you're asking if there are "non-artificial" nuclear reactors? Well, there are. But that is a topic for another lecture. Natural nuclear reactors are also interesting enough to make an entire article. Look forward to it.

Let's return to the main topic.

What is a critical reaction...

If left alone, neutrons will gradually undergo the following reactions, causing their numbers to decrease.

1. Being absorbed by other atomic nuclei (involving nuclear reactions such as neutron capture reactions and direct processes)

2. Neutrons are unstable atomic nuclei on their own. They beta-decay into protons (hydrogen nuclei) with a half-life of 15 minutes and disintegrate

On the other hand, when heavy elements like uranium absorb a single neutron, they undergo nuclear fission into two or more atomic nuclei, and in that process, they release two or more neutrons. In other words, you can produce more neutrons than are consumed in a single reaction.

However, in an environment where you are just leaving uranium there without thinking, the reactions shown above where neutrons decrease are overwhelmingly dominant, so the number of neutrons will gradually decrease, and eventually the reaction will converge.

Here, we consider ways to prevent neutron loss or to increase them further.

• Reduce the surface area of the neutron generation part (by arranging uranium in a spherical shape) to reduce neutron leakage.

• Reduce the energy of neutrons through scattering reactions to make them react more easily with uranium (moderator). (Neutrons are not lost in scattering reactions).

• Select moderators and other core structural materials that have few impurities that easily absorb neutrons (for example, boron is often mixed in as an impurity, but it has the property of absorbing neutrons extremely well).

By doing this, when the 'decreasing neutrons' and the 'newly born neutrons' are just balanced, the chain reaction where neutrons generated by fission cause the next new fission continues without needing to inject new neutrons.

This state is called 'critical'.
The core of a nuclear power plant is also a mechanism that keeps this critical state stable by human hands.

This is how the world's first artificial nuclear reactor was built.

Nowadays, because there is abundant experimental data, we can predict extremely accurately how to arrange uranium and graphite to reach criticality, but at that time, it was an era without high-precision experimental data on the probability of uranium fission. Moreover, while nuclear data is now published in a form that anyone can view, taking an open science stance ahead of all other fields, at that time, it was often kept secret because it could also be used for military research.

To prevent reaching criticality suddenly due to unexpected phenomena, rods made of cadmium, which absorbs neutrons extremely well, were placed, and uranium was arranged around them in a spherical shape while being surrounded by graphite containing almost no boron as an impurity.

However, there is a possibility of reaching criticality while stacking the uranium. Therefore, a neutron detector (BF3 proportional counter) was prepared, and it was confirmed that neutrons were not increasing exponentially, and it was stacked while saying, 'Okay, it hasn't reached criticality yet.'

However, as will be mentioned later, Dr. Fermi had a hypothesis at this point that it would not reach criticality.

Until just a little before this, the probability of nuclear reactions in the air was not known, so there was even a realistic fear that if a critical reaction occurred somewhere on Earth, all elements might undergo nuclear reactions, and the Earth might be destroyed.

Certainly, as long as there was no measurement data, it could not be said that such a thing would not happen.

However, Dr. Fermi must have known at the time of this experiment that light elements like air are unlikely to undergo nuclear reactions, and even if they did, they would not easily emit neutrons, so criticality would not spread infinitely across the entire Earth.

Well, 35 tons worth (about 6 tons of metallic uranium and about 50 tons of uranium oxide) of uranium did not reach criticality even when all of it was stacked. This was thanks to the cadmium rods absorbing neutrons powerfully.

These cadmium rods work as control rods, which are still used today, to adjust the criticality by inserting and removing them. From here, the control rods are pulled out little by little to reduce neutron absorption and allow nuclear reactions to occur more and more.

The control rods were actually pulled out by hand.
In modern times, it would definitely be remote operation... It was a terrifying state where Dr. Fermi was also right next to it.

At that moment, Dr. Fermi said quietly.

'If you pull that control rod out another 47 millimeters, it will reach criticality.'

It was exactly as expected, the moment it was pulled back 47 millimeters, the number of neutrons detected increased exponentially—Chicago Pile-1 had reached criticality.

It was truly the moment when hypothesis and experiment aligned perfectly.

However... did someone see Cherenkov radiation? (sweat).
When high-energy particles pass through the eye, one can sometimes see a pale blue light.
However, it seems no such reports remain in the records of the participants at the time.

In any case... I would be too scared to do such an experiment now... I can only say I am truly glad that no one died.

Let's go back a little.

I want you to recall his words just before reaching criticality.
"47 millimeters more" was what he said.

While reactor physics has established theories today, Dr. Fermi was the first in the world to independently formulate the neutron diffusion equation, which serves as its foundation, and calculated the necessary amount of uranium and control rod conditions from actual measurement data. In fact, he had established a "hypothesis without a single millimeter of error" that they would not reach criticality while building it up, but would reach it if the control rods were pulled out to this point!

How do you think a scientist feels when the results match their expectations?
There are mainly two patterns.
"It's boring because it's as expected (no new discoveries)"
"As planned (smirk)"
I wonder which one Dr. Fermi was feeling at this time?

Now... after this, in order to establish the nuclear reactor as a science and technology, the series of experiments that began with Chicago Pile-1 (CP-1) were updated rapidly the following year with the improved Chicago Pile-2 and 3, and research progressed on coolants, shielding structures, safety systems, and so on.

In other words, all the commercial nuclear reactors currently serving as our energy sources are technologies that have been connected since this Chicago Pile-1.

Of course, those research results occupied an important position not only in peaceful use but also in nuclear weapons development in the Manhattan Project. We must also not forget that nuclear power can always lead to terrible consequences if used incorrectly.

Reference: The University of Chicago's website has detailed information, although it is in English! (The video of Fermi appearing on the top page and the young man pulling out the control rod by hand is super valuable! Perhaps this was the moment when "47 mm more" was said!)

There are two results from an experiment. If the result confirms the hypothesis, then you have measured something. If the result is contrary to the hypothesis, then you have discovered something.

Let's read these words again, keeping in mind that Dr. Fermi was the person who accurately read "47 millimeters ahead".

He must have formulated a thorough hypothesis every time. This is clear from the fact that even in the world's first artificial criticality experiment, it was something that could be predicted without a 1 mm error.

It is precisely because it is such a well-thought-out experiment that you can only call it a "discovery" if it contradicts the hypothesis.

At the beginning, I said, "You aren't doing experiments without even forming a hypothesis, right?" but if asked, "Are you doing experiments with as much careful hypothesis-forming as Dr. Fermi?", I might not be able to answer "Yes!" immediately either.

For someone like me, it cannot be denied that there are still "gaps in the hypothesis" for experiments that yield results contrary to the hypothesis.

Dr. Fermi must surely have had the experience of obtaining experimental results and immediately declaring with confidence, "It's a great discovery!"

Otherwise, this famous quote would never have come to be.

Let's follow Dr. Fermi's example, carefully examining and formulating hypotheses, and strive to plan experiments where, once results are obtained, we can immediately say, 'This is a great discovery!'

Summary

How was it?
I would be happy if the weight of the words feels different now compared to when you first read Dr. Fermi's quote before reading this article.

If you feel it has changed, please let me know in the comments section!

Heading off on a business trip to Hokkaido again today. I woke up early to write this article so I could sleep on the plane. As for proofreading... sorry, I didn't do much of it today. I might do it at the airport.
(The kind of post you edit after publishing...)