Cazadores de Neutrinos

El Universo es misterioso y apasionante. Hay tantas cosas por descubrir que es muy probable que no lleguemos a conocer más que una ínfima parte de todo lo que nos rodea en todo el tiempo que exista la raza humana. En el todo el universo existen cosas que podemos observar directamente, teorizar sobre ellas, aplicar todo nuestro aparato matemático y decir que esa porción de nuestra observación se comporta de una determinada manera. Con esa teoría también podemos hacer predicciones futuras sobre comportamientos futuros. Sin embargo, también existen cosas que no podemos observar directamente y, en un principio, sólo podemos teorizar sobre ellas. Pero el ingenio humano y sus ansias de conocimiento no tiene límites y nos las apañamos para poder hacer visible lo invisible.

Al igual que en muchas otras ramas de la ciencia, la física tiene cosas que no se pueden observar y eso las hace más interesantes para los físicos. Nos volvemos locos por conocer lo que no vemos. Un ejemplo claro es la física de partículas. Cuando nos ponemos a investigar que hay en el interior de los átomos, que no vemos directamente, o que ocurre cuando dos átomos chocan a grandes velocidades (y grandes energías), se obtiene una cantidad de información que en muchos casos es desconcertante. Aparecen nuevas partículas más pequeñas que los mismos elementos constituyentes de los átomos. Su comportamiento difiere enormemente de lo que estamos acostumbrados en nuestra experiencia diaria. Aparecen nuevos misterios, que los vamos acumulando en la pila de misterios sin resolver. Los neutrinos son unos de esos misterios.

electron neutrinoNeutrino electrónico (Fuente: Particle Zoo)

Poco después del descubrimiento de la radiactividad, el gran Ernest Rutherford descubrió, en 1899, que una de las maneras en las que la radiactividad se manifestaba era a través de la emisión partículas cargadas negativamente con una carga igual a la del electrón. En un principio, a estas partículas se les llamó partículas beta, debido a que este tipo de radiactividad se conocía como radiactividad beta, hasta que realmente se identificaron con el electrón.

El descubrimiento de este tipo de radiactividad abrió una nueva línea de investigación.

A pesar de los esfuerzos para comprender este tipo de radiactividad, todavía era necesario conocer algunos ingredientes más que faltaban en el conocimiento del interior de los núcleos atómicos, cosa que sucedería después del descubrimiento de la radiactividad beta.

Los núcleos atómicos se componen de protones y neutrones. El protón fue descubierto por el propio Rutherford en 1919. La existencia del neutrón fue propuesta un año después por Rutherford para explicar por qué los núcleos atómicos no se desintegraban debido a la repulsión eléctrica de los protones. Otros muchos científicos, teorizaron sobre la existencia del neutrón en años posteriores y fue finalmente descubierto experimentalmente en 1932 por James Chadwick.

Una vez se conocían todos los elementos del núcleo atómico, se podía ya empezar a teorizar sobre la radiactividad beta. Las observaciones implicaban que el electrón era emitido por el núcleo, pero se sabía que los núcleos estaban formados sólo por neutrones y protones, así que era imposible que el electrón estuviera dentro del núcleo. La explicación que se dio fue que un neutrón del núcleo se transformaba en un protón emitiendo, al mismo tiempo, un electrón.

Sin embargo, de la ecuación de Einstein E = mc2, se esperaba que el electrón se llevara, en forma de energía cinética, la diferencia de masas entre el núcleo inicial y el núcleo resultante tras la emisión del electrón. Es decir, se esperaba que se conservara la energía. Pero esto no sucedía. La conservación de la energía es uno de los principios básicos de la física, así que cuando no se conserva pueden pasar dos cosas, o estamos haciendo algo mal o hay algo nuevo que todavía no conocemos. Esto último es lo que pasó.

En 1930, Wolfgang Pauli propuso la existencia de una partícula sin carga se emitía junto con el electrón, de manera que la energía total se conservara. Esta nueva partícula no se había detectado aún. Aunque Pauli le puso el nombre de neutrón, fue Fermi quien la renombró como neutrino, debido al descubrimiento del neutrón en 1932, cuando lo incorporó a su teoría sobre la radiactividad beta.

El problema es que el neutrino seguía sin ser descubierto. Incluso Pauli creía que había postulado una partícula que nadie podría detectar nunca ya que era tan pequeño, de hecho se pensaba que no tenía masa, y sin carga eléctrica que era imposible que interactuara con cualquier tipo de materia, incluso la de los instrumentos más sofisticados de la época.

Todo cambió con la llegada de los reactores nucleares de fisión. La fisión nuclear utiliza núcleos muy pesados que cuando se fisionan se generan otros elementos cuyos núcleos tienen tal cantidad de neutrones (isótopos) que es imposible que sean estables, por lo que estos mismos neutrones se desintegran emitiendo electrones y (anti)neutrinos, es decir, emiten radiactividad beta. Aunque un solo neutrino es muy difícil de detectar, cuando se tienen muchos la probabilidad de detectar al menos uno aumenta.

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Central nuclear de Zorita ya cerrada (Fuente: propia)

Se empezaron a diseñar experimentos cada vez más grandes, con detectores cada vez más sofisticados, con la intención de detectar el neutrino. Pasaron muchos años desde que se postuló su existencia y se estableció el proceso de desintegración beta, hasta que se descubrió el neutrino. Fue en 1956 cuando Reines y Cowan consiguieron por fin detectar una señal clara que confirmó que el misterioso neutrino, había sido descubierto… y lo hicieron buscando la desintegración beta inversa.

Como hemos visto antes, la desintegración beta consiste en que un neutrón, se transforma en un protón emitiendo un electrón y un neutrino. En realidad, se trata de un antineutrino, ya que al combinar el formalismo cuántico con el relativista se encuentra que cada partícula tiene su antipartícula, es decir la misma partícula pero con cargas opuestas. En el caso del neutrino, al no tener carga, no está todavía muy claro si el neutrino y el antineutrino son la misma partícula (partículas de Majorana) o son diferentes (partículas de Dirac), pero eso lo dejamos para otra ocasión. La desintegración inversa consiste en que un antineutrino colisiona con un protón produciendo un neutrón y un positrón (la antipartícula del electrón, que es como un electrón pero con carga positiva).

Para llevar a cabo su descubrimiento, Reines y Cowan llenaron tanques con 400 litros de agua y disolvieron en ellos 40 kg de cloruro de cadmio (CdCl2). Estos tanques estaban a una profundidad 12 metros bajo la superficie para protegerlo de los rayos cósmicos que podían interferir en las medidas, y a 11 metros del centro del reactor de Savannah River donde se generaban los neutrinos. En la parte superior del tanque, por encima del nivel del agua, pusieron detectores de centelleo líquidos y tubos fotomultiplicadores por debajo para detectar la luz de centelleo. El positrón era detectado cuando se frenaba y colisionaba con un electrón del contenido del tanque, aniquilándose ambos y emitiendo dos rayos gamma que eran detectados al mismo tiempo por los detectores de centelleo y los tubos fotomultiplicadores. El neutrón era también frenado por el agua y capturado por el cadmio microsegundos después de la captura del positrón. En esta captura del neutrón varios rayos gamma eran emitidos también que eran detectados por los detectores de centelleo justo después de la detección de los dos rayos gamma resultantes de la aniquilación del positrón. Este retraso se podía predecir teóricamente, con lo que si lo que se medía experimentalmente coincidía con lo predicho, se demostraba que era el neutrino el que había desencadenado la reacción.

reinescontrols

Reines y Cowan en el centro de control (Fuente)

De esta manera comenzaba una nueva era en la investigación de lo desconocido, de lo pequeño e invisible, de los neutrinos. Pero aún quedaban muchas sorpresas, ya que lo que habían detectado era sólo una de las variedades de neutrinos, los neutrinos electrónicos. Más adelante otros experimentos detectarían otras variedades de neutrinos.

Pero no nos adelantemos…

Referencias:

T2K Experiment

First Detection of the Neutrino by Frederick Reines and Clyde Cowan

Neutrino. Frank Close. RBA Divulgación

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Neutrino Hunters

Universe is mysterious and exciting. There are so many things to discover that it is very likely that we will ever know only a negligible portion of everything around us during the humanity lifetime. In the universe there are things that we can directly observe, develop a theory about them, apply all our mathematical knowledge and say that the portion of the universe subject to our observation behaves in a specific way. With such a theory, we could also make future predictions about future behaviors. But, there are things that we cannot directly observe and, in principle, develop a theory about them. Notwithstanding, humanity inventiveness and willingness to know do not have limits and we manage to make unveil the invisible universe.

As well as in many science areas, physics has things that cannot be observed what makes them even more interesting for physicist. We are made to learn about what we cannot see. One example is particle physics. When we start to investigate what there is inside atoms, which we do not directly see, or what happens when two atoms collide at high speed (and high energy), we get such an amount of information that in many cases is disconcerting. Smaller particles, than the constituent elements of the atoms, appear. Their behavior largely differs from what we are used in our daily experience. New mysteries appear which we pile up while waiting for solving them. Neutrinos are one of these mysteries.

electron neutrino

Electron neutrino (Source: Particle Zoo)

Shortly after the discovery of radioactivity, Ernest Rutherford discovered, in 1988, that one of the possible manners that radioactivity showed up was through the emission of negatively charged particles with a charge equal to that of the electron. Initially, they were named as beta particles, following the name of this kind of radioactivity which was known as beta decay, until they were finally identified as electrons.

The discovery of this kind of radioactivity opened a new research area.

Besides the efforts to understand this kind of radioactivity, it was still necessary to discover some of the missing ingredients in our knowledge of atomic nuclei and this happened after the beta decay discovery

Atomic nuclei are made of protons and neutrons. The proton was discovered by Rutherford in 1919. The existence of the neutron was proposed by Rutherford one year later to explain why atomic nuclei did not disintegrate due to the electric repulsion of protons. Many other scientists theorized about the existence of neutrons after Rutherford and it was finally experimentally found in 1932 by James Chadwick.

Once all the elements of the atomic nucleus were known, it was then possible to start developing a theory for beta decay. Observations implied that the electron was emitted by the nucleus, but it was known that nucleus were only made up of neutrons and protons, to that it was impossible that the electron was inside the nucleus. The explanation that was accepted is that a neutron transforms into a proton emitting, at the same time, an electron.

However, from Einstein’s equation E = mc2, it was expected that the electron would carry off, in form of kinetic energy, the mass difference between the initial nucleus and the nucleus after the electron emission. It was expected that the energy was conserved. But this did not happen. Conservation of energy is one of the basic principles of physics and when it is not conserved it can happen either we are doing something wrong or there is something new that we do not know yet. The latter was the solution.

In 1930, Wolfgang Pauli proposed the existence of a new particle without electric charge emitted with the electron, so that total energy was conserved. This new particle had not been detected so far. Although Pauli gave it the name of neutron, it was Fermi who renamed is as neutrino, due to the discovery of neutron in 1932, when he incorporated it to his beta decay theory.

The problem was that the neutrino was still undiscovered. Even Pauli thought that he had postulated a particle that nobody could ever detect because it was so small, in fact it was thought that it did not have mass, and without electric charge that it was impossible for it to interact with any kind of matter, even with that of the most sophisticated instruments of that time.

Everything changed with the advent of nuclear fission reactors. Nuclear fission uses very heavy elements that when fission occurs, the resulting lighter elements, which nucleus has such an amount of neutrons (isotopes), cannot be stable and they disintegrate emitting electrons and (anti)neutrinos, that is, they emit beta radioactivity. Although one single neutrino is very difficult to be detected when there a lot of them, the likelihood of detecting at least one is increased.

CentralNuclear

 

Already closed Zorita Power Plant (Source: mine)

Larger experiments started to be developed, with more and more sophisticated detectors, whose intention was to detect the neutrino. Many years passed by, since it existence was postulated and the beta decay was described, until the neutrino was discovered. It was in 1956 when Reines and Cowan, managed to find a clear signal that confirmed that the mysterious neutrino had been discovered… and they did it looking for the inverse beta decay.

As we have seen before, the beta decay consists in a neutron which is transform in a proton emitting an electron and a neutrino. Actually, it is an antineutrino because when the quantum formalism is combined with the relativistic one, it is found that every particle has its own antiparticle, that is the same particle but with opposite electric charge. In the case of neutrinos, because they do not have electric charge, it is not clear yet whether the neutrino and antineutrino are the same particle (Majorana particles) or different ones (Dirac particle), but let’s leave this for another moment. The inverse beta decay consists in an antineutrino colliding with a proton giving as result a neutron and a positron (the electron antiparticle, which is like an electron but with a positive electric charge).

In their experiment, Reines and Cowan dissolved 40kg of Cadmium Chloride (CdCl2) in 400 liters of water in tanks. These tanks were at 12 meters underground, to be shielded from cosmic rays which may interfere in the data gathering, and 11 meters away from the Savannah River reactor, which was the antineutrinos source. Above the water there were liquid scintillators and below they installed photomultiplier tubes to detect the scintillating light. The positron was detected by its slowing down and annihilating with an electron of the tank content and thus emitting two gamma rays that were detected by the liquid scintillators and the photomultiplier tubes. The neutron was also slowed by the water and captured by the cadmium microseconds after the positron capture. In this neutron capture several gamma rays emitted and detected by the liquid scintillators just after the detection of the gamma rays produced by the annihilation of the positron. This delay was theoretically predicted and thus, if the experimental measure matched the prediction, it can be demonstrated that the neutrino had produced the reaction.

reinescontrols

Reines y Cowan in the control centre (Source)

In this way an era started for researching the unknown, the small and invisible, the neutrinos. But there were still many surprises to be found, because what they had detected was only one of the varieties of neutrinos, electron neutrinos. Later, other experiments would detect other varieties.

But let’s not get ahead of ourselves…

References:

T2K Experiment

First Detection of the Neutrino by Frederick Reines and Clyde Cowan

Neutrino. Frank Close. RBA Divulgación

 

 

 

Radiation and Radioactivity (I)

Often, when we talk about the danger of nuclear energy we talk about the danger of radiation. Also, when we talk about the danger, not yet demonstrated by the way, of mobile telephones we talk about radiation too. Is it correct to use the word radiation in both cases? Is it always the same radiation?

Historically, radiation is a combination of oscillating electric and magnetic fields propagating in a medium. In other words, it is a moving wave. Opposite to sound waves which need a physical medium to propagate, electromagnetic waves, radiation, can also propagate in the vacuum such as the outer space.

As a side note, due to the vacuum is empty, if we ignore quantum fluctuations that continuously create particle and antiparticle pairs of course, and thus there is nothing that helps the propagation of sound waves, the big explosions heard when the starship Enterprise shoots a photon torpedo against a Klingon’s bird of prey should not be possible to be heard, but Star Trek is Star Trek and this fact can be forgiven.

These electromagnetic waves are characterised by an energy that depends on its wavelength (distance between two points with the same phase, such as crests) or its frequency (number of waves per second).

A wave

Wavelength and frequency are inversely related, so if the wavelength is high or there distance between two crests is large its frequency is low or there are few waves per second and vice versa. It we take the frequency as the reference magnitude, we have that the higher the frequency the greater the energy.

c=λν and E=hν

Mathematical relations between wavelength λ and frequency ν, and between the energy E and the frequency ν. c is the speed of light which is constant with an approximate value of 300.000 km/s and h is the Plank constant with a value of  6,63 x10-34 joules per second and that, as it names indicates, is constant.

Up to now, this does not say much about radiation, thus to go deeper we have to know that every body with a temperature T above absolute zero, which is equivalent to -273.15 Celsius (pure water freezes at 0 Celsius), emits radiation. If we take into account that the third principle of thermodynamics tells us that it is impossible to reach the absolute zero in a finite number of steps, we come to the conclusion that everything emits radiation! And if everything emits radiation, we have a problem, a big one, if every radiation is of the same kind! Because if it is of the same kind as the mobile telephone radiation and of the same kind of the one that escapes from nuclear power plants, since everything emits radiation we should have disappeared from the earth a long time ago.

Keep calm because it is not as bad. As I mentioned earlier, radiation is a wave that propagates with a frequency and thus it has an energy. Depending on the frequency we have a type of radiation or another and this is known as the electromagnetic spectrum.

Now is when things become more interesting. Electromagnetic radiation is classified as non-ionizing radiation and ionizing radiation. Non ionizing radiation is the one that doesn’t carry enough energy (its frequency is not high enough) so as to liberate one or more electrons from the atoms or molecules it collides with, while the ionizing radiation has enough energy to liberate such electrons. I guess we can now deduce what radiation is the bad on, cannot we?

The limit between what it is known as ionizing and non-ionizing radiation is not, however, very clear. If we have in mind that the ionization energy of a hydrogen atom is 13.6 eV (1 EV or electronvolt is the energy that an electron requires so overcome an electric potential barrier of 1 volt), the radiation that makes the hydrogen to lose an electron will be ionizing. Notwithstanding, matter, including ourselves, is made of different chemical elements which make that there is a wide range of ionization energies. In any case if we consider the hydrogen ionization energy as the lowest limit, by looking at the electromagnetic spectrum we have that for radiation with energies in the ultraviolet range and above towards gamma rays, it will be considered as ionizing radiation. Anyway, there is not an agreement on this limit yet, because as I said everything depends on the material the radiation is colliding with considering the ultraviolet frequency range as a lower limit.

We have seen that the electromagnetic radiation is everywhere and cannot avoid it; in fact, we are a large radiation source. Because we are visible to others, it means that we are emitting radiation, in this case in the visible frequency range. Additionally, we usually have a body temperature above 36 Celsius and thus we are also emitting radiation in the infrared frequency range. Therefore, is radiation dangerous? The answer is, in general terms, no. What we need to do is to use the correct terms when we speak and say that what it is dangerous is the ionizing radiation, especially at high energies.