En física nuclear , la desintegración beta ( desintegración β ) es un tipo de desintegración radiactiva en la que una partícula beta ( electrón o positrón energético rápido ) se emite desde un núcleo atómico , transformando el nucleido original en una isobara de ese nucleido. Por ejemplo, la desintegración beta de un neutrón lo transforma en un protón mediante la emisión de un electrón acompañado de un antineutrino ; o, por el contrario, un protón se convierte en un neutrón por la emisión de un positrón con un neutrino en los llamadosemisión de positrones . Ni la partícula beta ni su (anti) neutrino asociado existen dentro del núcleo antes de la desintegración beta, sino que se crean en el proceso de desintegración. Mediante este proceso, los átomos inestables obtienen una relación más estable de protones a neutrones . La probabilidad de que un nucleido se desintegra debido a la beta y otras formas de desintegración está determinada por su energía de enlace nuclear . Las energías de unión de todos los nucleidos existentes forman lo que se llama la banda nuclear o valle de estabilidad . [1] Para que la emisión de electrones o positrones sea energéticamente posible, la liberación de energía ( ver más abajo ) o el valor Q debe ser positivo.
La desintegración beta es una consecuencia de la fuerza débil , que se caracteriza por tiempos de desintegración relativamente prolongados. Los nucleones se componen de quarks up y quarks down , [2] y la fuerza débil permite que un quark cambie su sabor mediante la emisión de un bosón W que conduce a la creación de un par electrón / antineutrino o positrón / neutrino. Por ejemplo, un neutrón, compuesto por dos quarks down y un quark up, se desintegra en un protón compuesto por un quark down y dos quarks up.
La captura de electrones a veces se incluye como un tipo de desintegración beta, [3] porque el proceso nuclear básico, mediado por la fuerza débil, es el mismo. En la captura de electrones, un electrón atómico interno es capturado por un protón en el núcleo, transformándolo en un neutrón, y se libera un neutrino electrónico.
Descripción
Los dos tipos de desintegración beta se conocen como beta menos y beta más . En la desintegración beta menos (β - ), un neutrón se convierte en un protón y el proceso crea un electrón y un electrón antineutrino ; mientras que en la desintegración beta más (β + ), un protón se convierte en un neutrón y el proceso crea un positrón y un neutrino electrónico. La desintegración β + también se conoce como emisión de positrones . [4]
La desintegración beta conserva un número cuántico conocido como número de leptones , o el número de electrones y sus neutrinos asociados (otros leptones son las partículas de muón y tau ). Estas partículas tienen un número de leptones +1, mientras que sus antipartículas tienen un número de leptones -1. Desde un protón o un neutrón tiene lepton número cero, β + decaimiento (un positrón, o antielectrón) debe ser acompañado con un neutrino electrónico, mientras que β - desintegración (un electrón) debe ir acompañado por un electrón antineutrino.
Un ejemplo de la emisión de electrones (β - decaimiento) es la decadencia de carbono-14 en nitrógeno-14 con una vida media de aproximadamente 5730 años:
- 14
6C
→ 14
7norte
+
mi-
+
ν
mi
En esta forma de desintegración, el elemento original se convierte en un nuevo elemento químico en un proceso conocido como transmutación nuclear . Este nuevo elemento tiene un número de masa A sin cambios , pero un número atómico Z que se incrementa en uno. Como en todas las desintegraciones nucleares, el elemento en descomposición (en este caso14
6C
) se conoce como el nucleido padre mientras que el elemento resultante (en este caso14
7norte
) se conoce como el nucleido hijo .
Otro ejemplo es la desintegración del hidrógeno-3 ( tritio ) en helio-3 con una vida media de aproximadamente 12,3 años:
- 3
1H
→ 3
2Él
+
mi-
+
ν
mi
Un ejemplo de emisión de positrones ( desintegración β + ) es la desintegración del magnesio-23 en sodio-23 con una vida media de aproximadamente 11,3 s:
- 23
12Mg
→ 23
11N / A
+
mi+
+
ν
mi
La desintegración β + también da como resultado la transmutación nuclear, y el elemento resultante tiene un número atómico que se reduce en uno.
El espectro beta, o distribución de valores de energía para las partículas beta, es continuo. La energía total del proceso de desintegración se divide entre el electrón, el antineutrino y el nucleido de retroceso. En la figura de la derecha, se muestra un ejemplo de un electrón con 0,40 MeV de energía de la desintegración beta de 210 Bi. En este ejemplo, la energía de desintegración total es 1,16 MeV, por lo que el antineutrino tiene la energía restante: 1,16 MeV - 0,40 MeV = 0,76 MeV . Un electrón en el extremo derecho de la curva tendría la máxima energía cinética posible, dejando que la energía del neutrino sea solo su pequeña masa en reposo.
Historia
Descubrimiento y caracterización inicial
La radiactividad fue descubierta en 1896 por Henri Becquerel en uranio , y posteriormente observada por Marie y Pierre Curie en torio y en los nuevos elementos polonio y radio . En 1899, Ernest Rutherford separó las emisiones radiactivas en dos tipos: alfa y beta (ahora beta menos), basándose en la penetración de objetos y la capacidad de causar ionización. Los rayos alfa podrían detenerse con láminas delgadas de papel o aluminio, mientras que los rayos beta podrían penetrar varios milímetros de aluminio. En 1900, Paul Villard identificó un tipo de radiación aún más penetrante, que Rutherford identificó como un tipo fundamentalmente nuevo en 1903 y denominó rayos gamma . Alfa, beta y gamma son las tres primeras letras del alfabeto griego .
En 1900, Becquerel midió la relación masa / carga ( m / e ) de las partículas beta mediante el método de JJ Thomson utilizado para estudiar los rayos catódicos e identificar el electrón. Encontró que m / e para una partícula beta es lo mismo que para el electrón de Thomson y, por lo tanto, sugirió que la partícula beta es de hecho un electrón. [5]
En 1901, Rutherford y Frederick Soddy demostraron que la radiactividad alfa y beta implica la transmutación de átomos en átomos de otros elementos químicos. En 1913, después de que se conocieron los productos de más desintegraciones radiactivas, Soddy y Kazimierz Fajans propusieron independientemente su ley de desplazamiento radiactivo , que establece que beta (es decir,
β-
) la emisión de un elemento produce otro elemento un lugar a la derecha en la tabla periódica , mientras que la emisión alfa produce un elemento dos lugares a la izquierda.
Neutrinos
El estudio de la desintegración beta proporcionó la primera evidencia física de la existencia del neutrino . Tanto en la desintegración alfa como en la gamma, la partícula alfa o gamma resultante tiene una distribución de energía estrecha , ya que la partícula transporta la energía de la diferencia entre los estados nucleares inicial y final. Sin embargo, la distribución de energía cinética, o espectro, de las partículas beta medidas por Lise Meitner y Otto Hahn en 1911 y por Jean Danysz en 1913 mostró múltiples líneas sobre un fondo difuso. Estas mediciones ofrecieron el primer indicio de que las partículas beta tienen un espectro continuo. [6] En 1914, James Chadwick usó un espectrómetro magnético con uno de los nuevos contadores de Hans Geiger para realizar mediciones más precisas que mostraban que el espectro era continuo. [6] [7] La distribución de las energías de las partículas beta estaba en aparente contradicción con la ley de conservación de la energía . Si la desintegración beta fuera simplemente una emisión de electrones como se suponía en ese momento, entonces la energía del electrón emitido debería tener un valor particular y bien definido. [8] Para la desintegración beta, sin embargo, la amplia distribución de energías observada sugirió que la energía se pierde en el proceso de desintegración beta. Este espectro fue desconcertante durante muchos años.
Un segundo problema está relacionado con la conservación del momento angular . Los espectros de bandas moleculares mostraron que el espín nuclear del nitrógeno-14 es 1 (es decir, igual a la constante de Planck reducida ) y, más generalmente, que el espín es integral para núcleos de número de masa par y medio integral para núcleos de número de masa impar. Esto se explicó más tarde por el modelo protón-neutrón del núcleo . [8] La desintegración beta deja el número de masa sin cambios, por lo que el cambio de espín nuclear debe ser un número entero. Sin embargo, el espín del electrón es 1/2, por lo que el momento angular no se conservaría si la desintegración beta fuera simplemente una emisión de electrones.
De 1920 a 1927, Charles Drummond Ellis (junto con Chadwick y sus colegas) estableció además que el espectro de desintegración beta es continuo. En 1933, Ellis y Nevill Mott obtuvieron una fuerte evidencia de que el espectro beta tiene un límite superior efectivo en energía. Niels Bohr había sugerido que el espectro beta podría explicarse si la conservación de la energía fuera cierta solo en un sentido estadístico, por lo que este principio podría violarse en cualquier desintegración dada. [8] : 27 Sin embargo, el límite superior de las energías beta determinadas por Ellis y Mott descartó esa noción. Ahora, el problema de cómo explicar la variabilidad de la energía en los productos de desintegración beta conocidos, así como la conservación del momento y el momento angular en el proceso, se volvió agudo.
En una famosa carta escrita en 1930, Wolfgang Pauli intentó resolver el enigma de la energía de las partículas beta sugiriendo que, además de los electrones y protones, los núcleos atómicos también contenían una partícula neutra extremadamente ligera, a la que llamó neutrón. Sugirió que este "neutrón" también se emitió durante la desintegración beta (lo que explica la energía faltante conocida, el momento y el momento angular), pero simplemente aún no se había observado. En 1931, Enrico Fermi rebautizó el "neutrón" de Pauli por "neutrino" ("pequeño neutro" en italiano). En 1933, Fermi publicó su teoría histórica para la desintegración beta , donde aplicó los principios de la mecánica cuántica a las partículas de materia, suponiendo que se pueden crear y aniquilar, al igual que los cuantos de luz en las transiciones atómicas. Así, según Fermi, los neutrinos se crean en el proceso de desintegración beta, en lugar de estar contenidos en el núcleo; lo mismo ocurre con los electrones. La interacción de los neutrinos con la materia fue tan débil que detectarla resultó ser un desafío experimental severo. Se obtuvo más evidencia indirecta de la existencia del neutrino observando el retroceso de los núcleos que emitieron tal partícula después de absorber un electrón. Los neutrinos fueron finalmente detectados directamente en 1956 por Clyde Cowan y Frederick Reines en el experimento de neutrinos de Cowan-Reines . [9] Las propiedades de los neutrinos fueron (con algunas modificaciones menores) las predichas por Pauli y Fermi.
β+
decaimiento y captura de electrones
En 1934, Frédéric e Irène Joliot-Curie bombardearon aluminio con partículas alfa para efectuar la reacción nuclear.4
2He
+ 27
13Al
→ 30
15P
+ 1
0n
, and observed that the product isotope 30
15P
emits a positron identical to those found in cosmic rays (discovered by Carl David Anderson in 1932). This was the first example of
β+
decay (positron emission), which they termed artificial radioactivity since 30
15P
is a short-lived nuclide which does not exist in nature. In recognition of their discovery the couple were awarded the Nobel Prize in Chemistry in 1935.[10]
The theory of electron capture was first discussed by Gian-Carlo Wick in a 1934 paper, and then developed by Hideki Yukawa and others. K-electron capture was first observed in 1937 by Luis Alvarez, in the nuclide 48V.[11][12][13] Alvarez went on to study electron capture in 67Ga and other nuclides.[11][14][15]
Non-conservation of parity
In 1956, Tsung-Dao Lee and Chen Ning Yang noticed that there was no evidence that parity was conserved in weak interactions, and so they postulated that this symmetry may not be preserved by the weak force. They sketched the design for an experiment for testing conservation of parity in the laboratory.[16] Later that year, Chien-Shiung Wu and coworkers conducted the Wu experiment showing an asymmetrical beta decay of cobalt-60 at cold temperatures that proved that parity is not conserved in beta decay.[17][18] This surprising result overturned long-held assumptions about parity and the weak force. In recognition of their theoretical work, Lee and Yang were awarded the Nobel Prize for Physics in 1957. However Wu, who was female, was not awarded the Nobel prize.[19]
β - decaimiento
In
β−
decay, the weak interaction converts an atomic nucleus into a nucleus with atomic number increased by one, while emitting an electron (e−) and an electron antineutrino (
ν
e).
β−
decay generally occurs in neutron-rich nuclei.[22] The generic equation is:
- A
ZX
→ A
Z+1X′
+
e−
+
ν
e[1]
where A and Z are the mass number and atomic number of the decaying nucleus, and X and X′ are the initial and final elements, respectively.
Another example is when the free neutron (1
0n
) decays by
β−
decay into a proton (
p
):
n
→
p
+
e−
+
ν
e.
At the fundamental level (as depicted in the Feynman diagram on the right), this is caused by the conversion of the negatively charged (−1/3 e) down quark to the positively charged (+ 2/3 e) up quark by emission of a W− boson; the
W−
boson subsequently decays into an electron and an electron antineutrino:
d
→
u
+
e−
+
ν
e.
β + decaimiento
In
β+
decay, or positron emission, the weak interaction converts an atomic nucleus into a nucleus with atomic number decreased by one, while emitting a positron (
e+
) and an electron neutrino (
ν
e).
β+
decay generally occurs in proton-rich nuclei. The generic equation is:
- A
ZX
→ A
Z−1X′
+
e+
+
ν
e[1]
This may be considered as the decay of a proton inside the nucleus to a neutron:
- p → n +
e+
+
ν
e[1]
However,
β+
decay cannot occur in an isolated proton because it requires energy, due to the mass of the neutron being greater than the mass of the proton.
β+
decay can only happen inside nuclei when the daughter nucleus has a greater binding energy (and therefore a lower total energy) than the mother nucleus. The difference between these energies goes into the reaction of converting a proton into a neutron, a positron and a neutrino and into the kinetic energy of these particles. This process is opposite to negative beta decay, in that the weak interaction converts a proton into a neutron by converting an up quark into a down quark resulting in the emission of a
W+
or the absorption of a
W−
. When a
W+
boson is emitted, it decays into a positron and an electron neutrino:
u
→
d
+
e+
+
ν
e.
Captura de electrones (captura K)
In all cases where
β+
decay (positron emission) of a nucleus is allowed energetically, so too is electron capture allowed. This is a process during which a nucleus captures one of its atomic electrons, resulting in the emission of a neutrino:
- A
ZX
+
e−
→ A
Z−1X′
+
ν
e
An example of electron capture is one of the decay modes of krypton-81 into bromine-81:
- 81
36Kr
+
e−
→ 81
35Br
+
ν
e
All emitted neutrinos are of the same energy. In proton-rich nuclei where the energy difference between the initial and final states is less than 2mec2,
β+
decay is not energetically possible, and electron capture is the sole decay mode.[23]
If the captured electron comes from the innermost shell of the atom, the K-shell, which has the highest probability to interact with the nucleus, the process is called K-capture.[24] If it comes from the L-shell, the process is called L-capture, etc.
Electron capture is a competing (simultaneous) decay process for all nuclei that can undergo β+ decay. The converse, however, is not true: electron capture is the only type of decay that is allowed in proton-rich nuclides that do not have sufficient energy to emit a positron and neutrino.[23]
Transmutación nuclear
If the proton and neutron are part of an atomic nucleus, the above described decay processes transmute one chemical element into another. For example:
13755Cs → 13756Ba + e− + νe (beta minus decay) 2211Na → 2210Ne + e+ + νe (beta plus decay) 2211Na + e− → 2210Ne + νe (electron capture)
Beta decay does not change the number (A) of nucleons in the nucleus, but changes only its charge Z. Thus the set of all nuclides with the same A can be introduced; these isobaric nuclides may turn into each other via beta decay. For a given A there is one that is most stable. It is said to be beta stable, because it presents a local minimum of the mass excess: if such a nucleus has (A, Z) numbers, the neighbour nuclei (A, Z−1) and (A, Z+1) have higher mass excess and can beta decay into (A, Z), but not vice versa. For all odd mass numbers A, there is only one known beta-stable isobar. For even A, there are up to three different beta-stable isobars experimentally known; for example, 124
50Sn
, 124
52Te
, and 124
54Xe
are all beta-stable. There are about 350 known beta-decay stable nuclides.[25]
Competition of beta decay types
Usually unstable nuclides are clearly either "neutron rich" or "proton rich", with the former undergoing beta decay and the latter undergoing electron capture (or more rarely, due to the higher energy requirements, positron decay). However, in a few cases of odd-proton, odd-neutron radionuclides, it may be energetically favorable for the radionuclide to decay to an even-proton, even-neutron isobar either by undergoing beta-positive or beta-negative decay. An often-cited example is the single isotope 6429Cu (29 protons, 35 neutrons), which illustrates three types of beta decay in competition. Copper-64 has a half-life of about 12.7 hours. This isotope has one unpaired proton and one unpaired neutron, so either the proton or the neutron can decay. This particular nuclide (though not all nuclides in this situation) is almost equally likely to decay through proton decay by positron emission (18%) or electron capture (43%) to 64
28Ni
, as it is through neutron decay by electron emission (39%) to 64
30Zn
.[26]
Stability of naturally occurring nuclides
Most naturally occurring nuclides on earth are beta stable. Those that are not have half-lives ranging from under a second to periods of time significantly greater than the age of the universe. One common example of a long-lived isotope is the odd-proton odd-neutron nuclide 4019K, which undergoes all three types of beta decay (
β−
,
β+
and electron capture) with a half-life of 1.277×109 years.[27]
Reglas de conservación para la desintegración beta
Baryon number is conserved
where
- is the number of constituent quarks, and
- is the number of constituent antiquarks.
Beta decay just changes neutron to proton or, in the case of positive beta decay (electron capture) proton to neutron so the number of individual quarks doesn't change. It is only the baryon flavor that changes, here labelled as the isospin.
Up and down quarks have total isospin and isospin projections
All other quarks have I = 0.
In general
Lepton number is conserved
so all leptons have assigned a value of +1, antileptons −1, and non-leptonic particles 0.
Angular momentum
For allowed decays, the net orbital angular momentum is zero, hence only spin quantum numbers are considered.
The electron and antineutrino are fermions, spin-1/2 objects, therefore they may couple to total (parallel) or (anti-parallel).
For forbidden decays, orbital angular momentum must also be taken into consideration.
Liberación de energía
The Q value is defined as the total energy released in a given nuclear decay. In beta decay, Q is therefore also the sum of the kinetic energies of the emitted beta particle, neutrino, and recoiling nucleus. (Because of the large mass of the nucleus compared to that of the beta particle and neutrino, the kinetic energy of the recoiling nucleus can generally be neglected.) Beta particles can therefore be emitted with any kinetic energy ranging from 0 to Q.[1] A typical Q is around 1 MeV, but can range from a few keV to a few tens of MeV.
Since the rest mass of the electron is 511 keV, the most energetic beta particles are ultrarelativistic, with speeds very close to the speed of light.
β− decay
Consider the generic equation for beta decay
- A
ZX
→ A
Z+1X′
+
e−
+
ν
e.
The Q value for this decay is
- ,
where is the mass of the nucleus of the A
ZX
atom, is the mass of the electron, and is the mass of the electron antineutrino. In other words, the total energy released is the mass energy of the initial nucleus, minus the mass energy of the final nucleus, electron, and antineutrino. The mass of the nucleus mN is related to the standard atomic mass m by
- .
That is, the total atomic mass is the mass of the nucleus, plus the mass of the electrons, minus the sum of all electron binding energies Bi for the atom. This equation is rearranged to find , and is found similarly. Substituting these nuclear masses into the Q-value equation, while neglecting the nearly-zero antineutrino mass and the difference in electron binding energies, which is very small for high-Z atoms, we have
This energy is carried away as kinetic energy by the electron and neutrino.
Because the reaction will proceed only when the Q value is positive, β− decay can occur when the mass of atom A
ZX
is greater than the mass of atom A
Z+1X′
.[28]
β+ decay
The equations for β+ decay are similar, with the generic equation
- A
ZX
→ A
Z−1X′
+
e+
+
ν
e
giving
- .
However, in this equation, the electron masses do not cancel, and we are left with
Because the reaction will proceed only when the Q value is positive, β+ decay can occur when the mass of atom A
ZX
exceeds that of A
Z-1X′
by at least twice the mass of the electron.[28]
Electron capture
The analogous calculation for electron capture must take into account the binding energy of the electrons. This is because the atom will be left in an excited state after capturing the electron, and the binding energy of the captured innermost electron is significant. Using the generic equation for electron capture
- A
ZX
+
e−
→ A
Z−1X′
+
ν
e
we have
- ,
which simplifies to
- ,
where Bn is the binding energy of the captured electron.
Because the binding energy of the electron is much less than the mass of the electron, nuclei that can undergo β+ decay can always also undergo electron capture, but the reverse is not true.[28]
Espectro de emisión beta
Beta decay can be considered as a perturbation as described in quantum mechanics, and thus Fermi's Golden Rule can be applied. This leads to an expression for the kinetic energy spectrum N(T) of emitted betas as follows:[29]
where T is the kinetic energy, CL is a shape function that depends on the forbiddenness of the decay (it is constant for allowed decays), F(Z, T) is the Fermi Function (see below) with Z the charge of the final-state nucleus, E=T + mc2 is the total energy, p=√(E/c)2 − (mc)2 is the momentum, and Q is the Q value of the decay. The kinetic energy of the emitted neutrino is given approximately by Q minus the kinetic energy of the beta.
As an example, the beta decay spectrum of 210Bi (originally called RaE) is shown to the right.
Fermi function
The Fermi function that appears in the beta spectrum formula accounts for the Coulomb attraction / repulsion between the emitted beta and the final state nucleus. Approximating the associated wavefunctions to be spherically symmetric, the Fermi function can be analytically calculated to be:[30]
where p is the final momentum, Γ the Gamma function, and (if α is the fine-structure constant and rN the radius of the final state nucleus) S=√1 − α2 Z2, η=±Ze2c⁄ℏp (+ for electrons, − for positrons), and ρ= rN⁄ℏ.
For non-relativistic betas (Q ≪ mec2), this expression can be approximated by:[31]
Other approximations can be found in the literature.[32][33]
Kurie plot
A Kurie plot (also known as a Fermi–Kurie plot) is a graph used in studying beta decay developed by Franz N. D. Kurie, in which the square root of the number of beta particles whose momenta (or energy) lie within a certain narrow range, divided by the Fermi function, is plotted against beta-particle energy.[34][35] It is a straight line for allowed transitions and some forbidden transitions, in accord with the Fermi beta-decay theory. The energy-axis (x-axis) intercept of a Kurie plot corresponds to the maximum energy imparted to the electron/positron (the decay's Q value). With a Kurie plot one can find the limit on the effective mass of a neutrino.[36]
Helicidad (polarización) de neutrinos, electrones y positrones emitidos en desintegración beta
After the discovery of parity non-conservation (see History), it was found that, in beta decay, electrons are emitted mostly with negative helicity, i.e., they move, naively speaking, like left-handed screws driven into a material (they have negative longitudinal polarization).[37] Conversely, positrons have mostly positive helicity, i.e., they move like right-handed screws. Neutrinos (emitted in positron decay) have negative helicity, while antineutrinos (emitted in electron decay) have positive helicity.[38]
The higher the energy of the particles, the higher their polarization.
Tipos de transiciones de desintegración beta
Beta decays can be classified according to the angular momentum (L value) and total spin (S value) of the emitted radiation. Since total angular momentum must be conserved, including orbital and spin angular momentum, beta decay occurs by a variety of quantum state transitions to various nuclear angular momentum or spin states, known as "Fermi" or "Gamow–Teller" transitions. When beta decay particles carry no angular momentum (L = 0), the decay is referred to as "allowed", otherwise it is "forbidden".
Other decay modes, which are rare, are known as bound state decay and double beta decay.
Fermi transitions
A Fermi transition is a beta decay in which the spins of the emitted electron (positron) and anti-neutrino (neutrino) couple to total spin , leading to an angular momentum change between the initial and final states of the nucleus (assuming an allowed transition). In the non-relativistic limit, the nuclear part of the operator for a Fermi transition is given by
with the weak vector coupling constant, the isospin raising and lowering operators, and running over all protons and neutrons in the nucleus.
Gamow–Teller transitions
A Gamow–Teller transition is a beta decay in which the spins of the emitted electron (positron) and anti-neutrino (neutrino) couple to total spin , leading to an angular momentum change between the initial and final states of the nucleus (assuming an allowed transition). In this case, the nuclear part of the operator is given by
with the weak axial-vector coupling constant, and the spin Pauli matrices, which can produce a spin-flip in the decaying nucleon.
Forbidden transitions
When L > 0, the decay is referred to as "forbidden". Nuclear selection rules require high L values to be accompanied by changes in nuclear spin (J) and parity (π). The selection rules for the Lth forbidden transitions are:
where Δπ = 1 or −1 corresponds to no parity change or parity change, respectively. The special case of a transition between isobaric analogue states, where the structure of the final state is very similar to the structure of the initial state, is referred to as "superallowed" for beta decay, and proceeds very quickly. The following table lists the ΔJ and Δπ values for the first few values of L:
Forbiddenness | ΔJ | Δπ |
---|---|---|
Superallowed | 0 | no |
Allowed | 0, 1 | no |
First forbidden | 0, 1, 2 | yes |
Second forbidden | 1, 2, 3 | no |
Third forbidden | 2, 3, 4 | yes |
Modos de decaimiento raros
Bound-state β− decay
A very small minority of free neutron decays (about four per million) are so-called "two-body decays", in which the proton, electron and antineutrino are produced, but the electron fails to gain the 13.6 eV energy necessary to escape the proton, and therefore simply remains bound to it, as a neutral hydrogen atom.[39] In this type of beta decay, in essence all of the neutron decay energy is carried off by the antineutrino.
For fully ionized atoms (bare nuclei), it is possible in likewise manner for electrons to fail to escape the atom, and to be emitted from the nucleus into low-lying atomic bound states (orbitals). This cannot occur for neutral atoms with low-lying bound states which are already filled by electrons.
Bound-state β decays were predicted by Daudel, Jean, and Lecoin in 1947,[40] and the phenomenon in fully ionized atoms was first observed for 163Dy66+ in 1992 by Jung et al. of the Darmstadt Heavy-Ion Research group. Although neutral 163Dy is a stable isotope, the fully ionized 163Dy66+ undergoes β decay into the K and L shells with a half-life of 47 days.[41]
Another possibility is that a fully ionized atom undergoes greatly accelerated β decay, as observed for 187Re by Bosch et al., also at Darmstadt. Neutral 187Re does undergo β decay with a half-life of 41.6 × 109 years,[42] but for fully ionized 187Re75+ this is shortened to only 32.9 years.[43] For comparison the variation of decay rates of other nuclear processes due to chemical environment is less than 1%.
Double beta decay
Some nuclei can undergo double beta decay (ββ decay) where the charge of the nucleus changes by two units. Double beta decay is difficult to study, as the process has an extremely long half-life. In nuclei for which both β decay and ββ decay are possible, the rarer ββ decay process is effectively impossible to observe. However, in nuclei where β decay is forbidden but ββ decay is allowed, the process can be seen and a half-life measured.[44] Thus, ββ decay is usually studied only for beta stable nuclei. Like single beta decay, double beta decay does not change A; thus, at least one of the nuclides with some given A has to be stable with regard to both single and double beta decay.
"Ordinary" double beta decay results in the emission of two electrons and two antineutrinos. If neutrinos are Majorana particles (i.e., they are their own antiparticles), then a decay known as neutrinoless double beta decay will occur. Most neutrino physicists believe that neutrinoless double beta decay has never been observed.[44]
Ver también
- Neutrino
- Betavoltaics
- Particle radiation
- Radionuclide
- Tritium illumination, a form of fluorescent lighting powered by beta decay
- Pandemonium effect
- Total absorption spectroscopy
Referencias
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Bibliografía
- Tomonaga, S.-I. (1997). The Story of Spin. University of Chicago Press.
- Tuli, J. K. (2011). Nuclear Wallet Cards (PDF) (8th ed.). Brookhaven National Laboratory.
enlaces externos
- The Live Chart of Nuclides - IAEA with filter on decay type
- Beta decay simulation [1]