Boron-8

Properties and data of the isotope ⁸B.

Contents

General Data

Radioactive Decay

Radiation emissions

Formation

Isotones und Isobares

Literature

Boron-8 is a radioisotope of the chemical element boron, which, in addition to the element-specific 5 protons, has 3 neutrons in the atomic nucleus, resulting in the mass number 8. The very short-lived, unstable and therefore radioactive nuclide, which can only be produced artificially, has no practical significance; the study of ⁸B serves exclusively academic purposes and experimental research.

The proton-rich isotope was first described in 1950 as a product of the irradiation of ¹⁰BF₃ with protons (32 MeV) [1].

The ⁸B nucleus is of great interest for astrophysics, for example, as it is the main source of solar neutrinos with energies above 2 MeV; the boron neutrinos are far fewer in number, but their much higher energy makes detection easier. The β⁺/EE decay α spectrum occurring for this nuclide has been studied in detail on numerous occasions [3,4].

The nucleus of the proton-rich nuclide ⁸B is believed to have a 1-proton halo structure.

General data

Name of the isotope:Boron-8; B-8Symbol:⁸B or 85BMass number A:8 (= number of nucleons)Atomic number Z:5 (= number of protons)Neutrons N:3Isotopic mass:8.0246073(11) u (atomic weight of Boron-8)Nuclide mass:8.0218644 u (calculated nuclear mass without electrons)Mass excess:22.92155 MeVMass defect:0.040512608 u (per nucleus)Nuclear binding energy:37.73725128 MeV (per nucleus)
4.71715641 MeV (average binding energy per nucleon)Separation energy:S_N = 12.826(25) MeV (first neutron)
S_P = 0.1364(10) MeV (first proton)Half-life:771.17(94) msDecay constant λ:0.89882539590485 s^-1Specific activity α:6.766066428284 × 10⁺²² Bq g^-1
1828666602239 Ci g^-1Spin and parity:
(nuclear angular momentum)2+Isobaric spin: 1Magnetic dipole moment:μ(μ_N) = 1.0355(3)Quadrupole moment Q:+0.0643(14) barn (100 fm²)Nuclear g-factor:g_l = 0.51775Mirror nucleus:Lithium-8Year of discovery:1950

Radioactive Decay

Half-life T_½ = 771.17(94) ms respectively 7.7117 × 10^-1 seconds s.

Decay mode	Daughter	Probability	Decay energy	γ energy (intensity)
EC β⁺	⁸Be	100 %	17.9798(10) MeV
β⁺, α	⁴He

A newer and more precise value for the half-life was published in 2020 and adopted here [5]. The older value of T_1/2 = 0.770(3) s can be found in the relevant literature.

Formation

Boron-8 does not occur as a natural decay product of other radioisotopes; Corresponding starting nuclides are not known.

In stellar nucleosynthesis, the nuclide ⁸B occurs in the proton-proton III branch (p-p chain, T > 23 x 10 ⁶ °C) to:

³He + ⁴He → ⁷Be + γ; + 1.56 MeV,

⁷Be + ¹H → ⁸B + γ + 0.14 MeV.

This reaction plays an important role in nuclear astrophysics and has a direct impact on both the high-energy component of solar neutrinos [3].

Isotones and Isobars

The following table shows the atomic nuclei that are isotonic (same neutron number N = 3) and isobaric (same nucleon number A = 8) with Boron-8. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.

Z	Isotone N = 3	Isobar A = 8
1	⁴H
2	⁵He	⁸He
3	⁶Li	⁸Li
4	⁷Be	⁸Be
5	⁸B	⁸B
6	⁹C	⁸C
7	¹⁰N
8	¹¹O

External data and identifiers

Adopted Levels, Gammas:NuDat 8B

Literature and References

[1] - Luis W. Alvarez:
Three New Delayed Alpha-Emitters of Low Mass.
In: Physical Review, 80, 519, (1950), DOI 10.1103/PhysRev.80.519.

[2] - G. A. Korolev, A. V. Dobrovolsky, A. G. Inglessi et al.:
Halo structure of ⁸B determined from intermediate energy proton elastic scattering in inverse kinematics.
In: Physics Letters B, 780, 200-204, (2018), DOI 10.1016/j.physletb.2018.03.013.

[3] - R. Buompane et al.:
Test measurement of ⁷Be(p,γ)⁸B with the recoil mass separator ERNA.
In: The European Physical Journal A, 54, 92, (2018), DOI 10.1140/epja/i2018-12522-6.

[4] - S. Viñals et al.:
The experiments to determine the electron capture and β-decay of ⁸B into the highly excited states of ⁸Be.
In: Journal of Physics: Conference Series, 1643, 012130, (2020), DOI 10.1088/1742-6596/1643/1/012130.

[5] - S. Viñals et al.:
The Most Accurate Determination of the ⁸B Half-life.
In: Acta Physica Polonica B, 51, 3, (2020), DOI 10.5506/APhysPolB.51.717.

[6] - J. C. Zamora et al.:
Direct fusion measurement of the ⁸B proton-halo nucleus at near-barrier energies.
In: Physics Letters B, 816, 136256, (2021), DOI 10.1016/j.physletb.2021.136256.