- In this video, I'm gonna talk about the three major types
of nuclear radiation, alpha, beta and gamma,
distinguished by how powerful they are.
Alpha being easier to block than beta,
which is easier to block than gamma.
There are quite a few videos about what these radiations are
but almost nothing about how we know what they are.
For example, why was alpha and beta radiation discovered
so much before gamma radiation?
Or what did the discoveries of alpha, beta
and gamma radiation have to do with the discovery
that a beam in a vacuum tube called cathode rays
was really a beam of electrons?
Why did it take so long to discover that alpha radiation
was really a beam of helium nuclei?
And why did it take even longer to discover
that there's two kinds of beta radiation?
Ready for the real, if slightly convoluted story?
Let's go.
Let's start in the summer of 1898 when a 27-year-old
Cambridge grad student named Ernest Rutherford
read a very interesting article
in the science papers from Paris.
A woman named Marie Sklodowska Curie
had used a very sensitive electricity meter
to measure how radioactive uranium and thorium are.
As Rutherford was just finishing his PhD
on the electrical effects of x-rays,
it seemed like a good idea to use Curie's method
to study what was called Becquerel rays in the same way.
Therefore, Rutherford took a scale and put a layer
of uranium on a plate with a gap and measured the current
from the electrified air.
He then put a very thin layer of gold on the gap
to see if and how the radiation changed.
To his delight, the gold seemed to block some of the rays.
Two leaves of gold lowered the current even more.
Three pieces lowered the current too but not by as much.
When he switched to the thicker aluminum,
after a few pieces, there's basically a steady current,
no matter how much metal he added.
Rutherford quickly realized that there must be at least
two kinds of radiation.
One that was easily blocked and one that wasn't
and decided to name them after the Greek letters
for A and B, alpha and beta.
On September 1st, 1898, Rutherford published his conclusion.
Uranium radiation is complex and there are present
at least two distinct types of radiation.
One that is readily absorbed,
which will be termed for convenience, the alpha radiation
and the other of a more penetrative character,
which will be termed the beta radiation.
Rutherford missed that there is a third type of radiation
because gamma rays are very hard to detect with electricity
and his uranium did not produce much radiation
in the first place.
However, Rutherford did admit that,
"It is possible that other types of radiation
"either of small intensity or very great penetrating power
"may be present."
Yep.
Meanwhile, Marie Curie discovered a new, highly radioactive
substance in uranium ore that she named polonium
after her native country of Poland and her husband, Pierre
was so enthralled that he dropped his research
to work with her on this new field.
Then in December, 1898, Marie and Pierre Curie
published that there was a second substance
hidden in the ore that was even more radioactive.
So radioactive that they decided to name it radium.
Radium was special.
For example,
it seemed to violate the laws of thermodynamics.
It was always very hot and glowed bright enough
to read by without changing shape
or having any input energy source whatsoever.
Now everyone wanted a piece but only the Curies in France
and a German chemist named Friedrich Giesel,
who had started working on it when he read about polonium
had any, and while they were working on it,
they weren't sharing much
except to their closest associates.
Speaking of Geisel, in August, 1899,
some friends of Geisel named Elster and Geitel
decided that since cathode rays,
i.e. a beam of electrons are easily moved by a magnet,
it would be "interesting to determine
whether the conductivity caused by the Becquerel rays,
radio activity, can also be altered by a magnetic force."
They didn't notice anything from the weak uranium
but a piece of radium on loan from Geisel
seemed to have a magnetic effect.
This inspired Geisel and some other friends
named Meyer and Schweidler to then wonder if the magnet
actually slowed down the radiation from the radium
or deflected some of the rays.
They therefore decided to study the radiation from radium
under a magnetic field with photographs.
And by November, 1899, they found that the radiation
from radium was distinctly of two types.
One that was undisturbed by magnetic fields
and one that curved like cathode rays or beam of electrons.
Geisel and his friends also missed finding
the gamma radiation because gamma radiation
is hard to see with photographs.
Yes, it's hard to see with electricity and photographs,
that's why it took so long to find.
Also since alpha particles are very heavy, like 8,000 times
as massive as beta, the magnetic fields used
weren't strong enough to move the alpha particles much.
And therefore they thought the alpha particles
that they were detecting were non-deviable.
Inspired by the Germans in January, 1900, Pierre Curie
set up an electrical system to quantitatively study
the magnetic effect on radiation and validated
that some of the radiation from radium was divertable
with the magnet and some was not.
And was also the first to determine that,
"The deflected rays are the most penetrating."
Therefore in France and Germany,
it seemed more descriptive to describe the two types
of radiation as non-deviable instead of alpha
and deviable instead of beta.
Two months later, in March of 1900, Marie and Pierre Curie
reported that the deviable rays i.e. the beta radiation
from radium contained negative charges and concluded that
"The deviable rays from radium,
"as with a beam of the cathode ray carry electricity."
This conclusion that the deviable rays from radium
were like a beam of cathode rays
were what aroused the interest of a shy chemist
named Paul Villard and led him to discover the gamma ray.
See three years earlier in 1896, Rutherford's advisor,
JJ Thompson had determined that if he increased
the vacuum for cathode ray tubes,
he could get the cathode rays to move
with electrically charged plates and not just by magnet.
Previously, the trace amount of air in the cathode ray tube
had acted like a Faraday cage and protected the beam
from being moved electrically.
Thompson then balanced the electrical force
and the magnetic force and used this electromagnetic
gauntlet to determine the speed of the beam.
He then used the speed of the beam to determine the mass
over the charge of the beam and found that the beam was made
of teeny, tiny, negatively charged particles
that were eventually called electrons.
However, there was an experiment that seemed
to invalidate Thompson's electron theory.
When the cathode rays traveled through thin metal materials,
they make it through the same electromagnetic gauntlet,
which implied if Thompson was correct,
the cathode rays are little particles
that tunnel through a metal but keep the same speed,
which seemed impossible.
Villard who was pro-electron theory
wondered if it could all work out.
If the cathode ray didn't tunnel through the metal,
as much as cause the point of contact to be a new source
of cathode rays.
To validate his theory, Villard conducted a new experiment
where he had part of a cathode ray hit an insulated sheet
at an angle and found the beam was split in two.
Where the part of beam that hit the metal
emerged perpendicular to the surface and ended up
at a distance to the center.
It was in the middle of these experiments with cathode rays,
that Villard read the Curie's paper about the deviable beams
from the radium and how it was similar to cathode rays.
He then wondered if these beams would act
like a cathode ray if it hit a piece of metal.
Luckily, Villard was working in the same building
as Henri Becquerel, who had discovered radiation
in the first place with uranium
and who had been supplied radium from the Curies.
Villard than borrowed some radium from Becquerel
and put the radium in a lead container with a small hole
in it so that the radiation came out in the beam
and then had half the beam of radiation
hit a metal sheet of 45 degrees.
And he recorded the response on a photographic plate
that was wrapped in black paper,
thus blocking out the alpha particles.
He got a very strange result.
He did get an image from some of the beam
moving perpendicularly from the metal.
However, a trace amount of the beam
seemed to go straight through the thin metal sheet.
It immediately became clear to Villard that there must be
two types of radiation that can go through paper.
One that acts like a cathode ray, i.e. a beam of electrons,
and one of the can travel straight through thin metal,
like an x-ray.
Villard then wondered if both kinds of radiation
could be moved with magnets.
So he removed the metal in the middle and added a magnet.
He then found two distinct beams on the photograph.
One that was unmoved by the magnet
and one that was moved a few centimeters.
However, the unmoved beam could not be diminished
even after going through several sheets of paper
and an aluminum strip and even the thin piece of lead.
After Villard pointed it out,
other scientists validated Villard's discovery.
However, most people were far more interested
in alpha and beta radiation and even Villard moved on
to other subjects.
Now, we go back to Ernest Rutherford.
Back in 1898, when Rutherford was publishing
about alpha and beta radiation,
he moved from England to Canada to be professor
at McGill University,
which had what he called a swell lab so that he could
"knock the shine out of the Yankees."
In Canada, Rutherford started doing amazing work.
For example, he discovered a new radioactive gas
and with that radioactive gas discovered the half-life
of radiation and with his good friend, Frederick Soddy,
Rutherford discovered that often radiation
involves one element transforming into another element
in an effect called transmutation.
Soddy recalled, "I can remember quite well standing there
"transfixed as though stunned by the colossal import
"of the thing and blurting out,
"'Rutherford, the Thorium is disintegrating
"and transmuting itself into an argon gas.'"
Rutherford replied, "For Mike's sake, Soddy,
"don't call it transmutation.
They'll have our heads off as alchemists.
By the way, Rutherford and his friends discovered all this
with radium so weak it couldn't even light up
an x-ray screen.
For example, for years, Rutherford was convinced
that alpha radiation was made of charged particles,
which means they should be moved by a magnet
but it was incredibly difficult to measure
as they are so heavy and their radium was so weak
and therefore produced so few alpha particles.
In fact, it wasn't until February of 1903,
when Rutherford was able to experimentally demonstrate
that alpha particles from his weak radium were
"Deviable by a strong magnetic and electric field.
"The deviation is in the opposite sense of the cathode rays.
"So the radiation must consist of positively charged bodies
"projected with great velocity."
This was also the paper that Rutherford decided
for first time to name the third type of radiation,
gamma rays as gamma is the third letter
in the Greek alphabet and he wanted to start his paper
with a short description of the three types of radiation.
Marie Curie obviously read Rutherford's paper
because four months later,
when she presented her PhD thesis,
she not only included that Rutherford had found
that alpha particles are movable by magnets,
she also, for the first time,
named the three types of radiation,
"according to the notation adopted by Mr. Rutherford,
"by the letters alpha, beta and gamma."
Mere months after publishing her thesis,
the Nobel committee announced that Marie Curie was being
awarded a fourth of the 1903 Nobel prize for physics.
The public and the newspapers fell over themselves
in excitement.
Suddenly everyone wanted to know about radium
and the Curie's thesis became the Bible
of radioactive research.
And the three times types of radiation
were referred to as alpha, beta and gamma,
as they've been referred to ever since.
Let's take a moment and look at Marie Curie's
phenomenal 1903 PhD thesis.
In it, she said,
"The alpha rays are very slightly penetrating
"and the magnetic field acts very slightly upon them.
"The beta rays are less absorbable as a whole
"than the proceeding ones.
"They're deflected by a magnetic field
"in the same manner as cathode rays.
"The gamma rays are penetrating rays
"unaffected by magnetic field
"and comparable to Rontgen rays i.e. x-rays."
These definitions are still considered correct today
and the only major thing she missed was that alpha particles
are composed of helium nuclei and to discuss that,
we need to go back to Ernest Rutherford
and his friend, Frederick Soddy.
See about a month before Marie Curie defended her thesis,
Frederick Soddy moved from Canada to England
to work with a famous chemist named William Ramsay.
Years later, Soddy recalled
"Our trouble here was the same as in Montreal.
"We had quite insufficient amount of radiation
"for our investigation.
"Then by most extraordinary chance,
"the whole future prospect was changed."
What happened was while going for a walk,
Soddy casually happened to see a shop with pure radium
from professor Geisel in Germany on sale.
A few weeks later, Rutherford visited
and Soddy wasted no time about telling him
about the extraordinary find.
Soddy recalled that Rutherford,
"was absolutely bowled over and became as excited
"as a school boy over the coming holidays.
"With 30 precious milligrams of pure radium bromide,
"we bounded back to the laboratory and we both immediately
"repaired to the dark room with some metal foils
"and a bit of x-ray screen.
"The effect was terrific, it was like a person born blind,
"suddenly being given sight.
"For though Rutherford had made a special study
"of the Becquerel rays, this was the first time
"he had ever seen them.
"Now he had a visual demonstration
"of what he had found out in the dark, so to speak."
Rutherford then kindly lent his 30 milligrams of radium
to Soddy while he went on a European vacation.
And while Rutherford was celebrating with the Curies
after Madame Curie finished her PhD thesis,
Soddy and Ramsay demonstrated with their two different
radium samples that radium emanates helium gas.
Rutherford, Soddy and Ramsay were then fully convinced
that alpha particles were really charged helium atoms
but it was incredibly difficult to prove it
even with their stronger radium.
In fact, it took until November, 1908
for Rutherford to conclusively prove that alpha particles
are charged helium atoms and that,
"The charge is twice the unit charge
"carried by the hydrogen atom
"set free in electrolysis of water."
Just a few months after that, in mid 1909,
Rutherford was asked by his coworker named Heinz Geiger
for a suggestion on a good, easy project for a new student
and Rutherford suggested that he could use
some of his radium and "see if any alpha particles
"can be scattered through a large angle
"of a super thin piece of metal."
Now Rutherford just wanted to get the student
used to the equipment as he was convinced
that alpha particles are way too heavy and energetic
to be bounced off a thin metal.
However, a few days later, he was shocked to learn
that some of the alpha particles did bounce
off a thin piece of metal.
Rutherford thought about it for over a year and decided that
"This scattering backwards must be the result
"of a single collision and when I made the calculations,
"I saw it was impossible to get anything of that order
"of magnitude, unless you took a system
"in which the greater part of the mass of the atom
"was concentrated in a minute nucleus."
And thus the idea of the nucleus was born.
With the idea of the nucleus,
it became clear that the alpha particles
were really helium nuclei.
One comment about beta radiation.
It turns out that the beta radiation
that they were experimenting with was only one kind
of beta radiation.
See in 1934, Marie and Pierre Curie's daughter, Irene
and her husband, Frederic Joliot, conducted an experiment
where they bombarded aluminum with alpha particles
and found that the resulting material emanate radiation
that was positive but had the weight of an electron.
Now, the existence of a positive electron or positron
had been discovered two years earlier in cosmic rays.
But this was the first example of emission of positrons
from radioactive decay.
For this reason,
beta radiation is usually split into two types.
Beta negative decay, the more common kind
where an electron is emitted and the more rare,
beta positive decay where positron is emitted.
And that is how alpha, beta and gamma radiation
was discovered and named and how we know
that alpha radiation is made of helium nuclei.
Beta is made of electrons or positrons and gamma is composed
of high energy, massless electromagnetic radiation.
Now to understand how radiation works in the nucleus,
we need to talk about the neutron,
which was discovered and named by, you guessed it,
my friend, Ernest Rutherford
and I'm gonna talk about that very soon, I promise.
But first I wanna talk about something else.
I wanna talk about helium.
I was quite surprised to learn that the discovery of helium
didn't predate the discovery of radiation by very much.
In fact, helium was only discovered on earth
by Frederick Soddy's boss, William Ramsay,
three years before Rutherford discovered alpha particles.
Before that, it was just found in the sun,
which is why it is named
after the Greek God of the sun, Helios.
But wait, how did they discover an element in the sun?
How did Ramsay discover it in earth?
What does it have to do with Kansas
and the tragedy of the Hindenburg
and how did it end up in party balloons?
This fascinating history is next time
on The Lightening Tamers.
Thanks for watching and thanks to my patrons
for supporting me.
If you wanna be thanked too, there's a link down below.
If you wanna know more about Marie Curie,
I have two videos about her.
If you wanna know more about Ernest Rutherford,
which you do because he's awesome,
I have two videos about him.
If you wanna know more about JJ Thompson
and the discovery of electrons, I have a video about him.
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