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A Regenerated Compendium of Known and Predicted Isotopes

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Section 1: The Nuclear Landscape: Boundaries of Existence

The complete collection of atomic nuclei, known and hypothetical, constitutes the nuclear landscape. This landscape is typically visualized on a two-dimensional grid known as the Chart of Nuclides or Segrè chart, where the vertical axis represents the number of protons (Z) and the horizontal axis represents the number of neutrons (N).¹ This framework provides a far more granular view of nuclear properties than the periodic table, which organizes elements by chemical behavior but does not distinguish between their isotopes. At the heart of this landscape lies a narrow region of stability, surrounded by a vast sea of instability, with theoretical boundaries that define the absolute limits of nuclear existence.

1.1 Defining the Chart of Nuclides: The Valley of Stability and the Limits of Binding

The distribution of nuclides on the chart is not uniform; those that are stable or have very long half-lives cluster in a narrow band referred to as the “Valley of Stability” or “Belt of Stability”.² This valley traces the combinations of protons and neutrons that result in the most tightly bound nuclei. For light elements, the line of stability roughly follows a 1:1 neutron-to-proton ratio (

N≈Z). However, as the atomic number increases, the long-range electrostatic repulsion between positively charged protons becomes increasingly significant. To counteract this destabilizing force, a surplus of neutrons is required to provide additional attractive strong force interactions without adding more Coulomb repulsion. Consequently, the line of beta stability curves towards a higher neutron-to-proton ratio, reaching approximately 1.5:1 for the heaviest stable nuclei, such as Lead-208 (Z=82,N=126).²

The existence of any bound nucleus is a delicate balance between two fundamental forces. The strong nuclear force, an attractive force acting between all nucleons (protons and neutrons), is immensely powerful but has an extremely short range, primarily binding each nucleon only to its immediate neighbors. Conversely, the electromagnetic force creates a repulsive force between all protons in the nucleus. While weaker than the strong force at short distances, its infinite range means that every proton repels every other proton. For very heavy nuclei, the cumulative long-range Coulomb repulsion eventually overwhelms the saturated, short-range strong force, leading to inherent instability and setting a natural limit to the size of stable nuclei.²

1.2 A Taxonomy of Stability: From Theoretically Stable to Ephemeral Resonances

The binary classification of isotopes as either “stable” or “unstable” is an oversimplification. Nuclear stability exists on a continuum, and a more precise taxonomy is required to accurately describe the nuclear landscape. The very definition of stability is operational, contingent upon the limits of experimental detection. This was powerfully demonstrated in 2003 with the discovery of the alpha decay of Bismuth-209. Previously considered the heaviest stable nuclide, its half-life was measured to be approximately 1.9×1019 years—a billion times the age of the universe.⁵ This event underscores that many nuclides we currently label as “stable” may simply be radioactive on timescales too long for current instruments to measure. This evolving understanding necessitates a more nuanced classification:

• Theoretically Stable: A small subset of nuclides for which all known decay channels are energetically forbidden. According to nuclear theory, no spontaneous decay (except for the hypothetical proton decay) is possible for these nuclei. There are 146 such nuclides, with Dysprosium-164 being the heaviest.⁶
• Observationally Stable: This category includes all nuclides that have never been observed to decay. The current count stands at 251 nuclides across 80 elements.⁶ This group contains the theoretically stable nuclides as well as those that are theoretically predicted to be unstable but possess half-lives so immense that their decay remains undetected.⁷
• Primordial Radionuclides: A set of 35 radioactive nuclides with half-lives long enough (greater than 100 million years) to have persisted since the formation of the Solar System approximately 4.5 billion years ago. Prominent examples include Uranium-238, Potassium-40, and Thorium-232.⁸ Together with the 251 observationally stable nuclides, they constitute the 286 primordial nuclides found on Earth.
• Radioisotopes: This broad category encompasses all other nuclides with measurable decay rates. This includes naturally occurring isotopes produced through the decay of primordial radionuclides (radiogenic nuclides like radium) or through cosmic ray interactions (cosmogenic nuclides like Carbon-14), as well as the thousands of isotopes artificially synthesized in reactors and accelerators.⁸
• Unbound Resonances: At the extreme edge of existence are nuclear systems that disintegrate on the characteristic timescale of the strong interaction, roughly 10−21 seconds. These are not considered bound nuclei and represent the ultimate limit of nuclear cohesion.¹¹

1.3 The Frontiers of Discovery: An Overview of the Proton and Neutron Drip Lines

The theoretical boundaries of the Chart of Nuclides are known as the nuclear drip lines. For a given number of protons, the proton drip line is the boundary beyond which an additional proton is no longer bound to the nucleus. Similarly, the neutron drip line is the boundary beyond which an additional neutron is unbound. These lines are formally defined as the points where the one- or two-nucleon separation energy (Sn​, Sp​, S2n​, S2p​) becomes negative.¹² At this point, the nucleus will spontaneously emit a nucleon, or “drip” a particle, on a timescale governed by the strong force.

A profound asymmetry exists in our knowledge of these two frontiers, a direct consequence of the underlying nuclear forces. The proton drip line is relatively well-mapped experimentally, having been reached for elements up to Neptunium (Z=93).¹³ The powerful and cumulative Coulomb repulsion between protons means that adding just a few extra protons to a nucleus quickly renders it unbound. This makes the proton-rich boundary sharp and close to the Valley of Stability, and thus more accessible to experimental synthesis and study.¹²

In stark contrast, the neutron drip line remains one of the great unexplored territories of nuclear science. Because neutrons experience no Coulomb repulsion, they can be added in far greater numbers before the nucleus becomes unbound. The binding energy decreases much more gradually, making the location of the neutron drip line highly sensitive to subtle quantum shell effects and pairing correlations. Experimentally, the neutron drip line has only been definitively mapped for the lightest elements, up to Neon (Z=10).¹³ For all heavier elements, its location is known only through theoretical extrapolation. This vast, experimentally unverified region on the neutron-rich side of the nuclear chart represents the “gap” between known and predicted isotopes and is the primary focus of next-generation rare isotope beam facilities.

Section 2: The Compendium of Known Nuclides (The “Known strict” Dataset)

The foundation of any comprehensive isotopic analysis is a rigorously compiled dataset of all experimentally observed nuclides. This “Known strict” dataset serves as the ground truth against which all theoretical models are benchmarked and provides the starting point for mapping the undiscovered territories of the nuclear landscape.

2.1 Data Provenance: Sourcing from Authoritative International Evaluations

The compilation of nuclear data is a continuous, collaborative international effort. The recommended values for nuclear properties are periodically reviewed and published by several key organizations. The data for this report are drawn from the most recent and authoritative of these evaluations to ensure the highest degree of accuracy and completeness.

The primary coordinating bodies for this global effort are the National Nuclear Data Center (NNDC) at Brookhaven National Laboratory in the United States and the International Atomic Energy Agency (IAEA) in Vienna.²² These centers maintain and disseminate a suite of databases, including the Evaluated Nuclear Structure Data File (ENSDF), which is the core repository for nuclear structure and decay data, and user-friendly interfaces like NuDat and the Livechart of Nuclides.²⁶

The definitive datasets used to construct the “Known strict” file are the 2020 evaluations from the Atomic Mass Data Center (AMDC) ³⁰:

• The AME2020 (Atomic Mass Evaluation): This evaluation provides a comprehensive, least-squares adjustment of all available experimental data on atomic masses. It is the gold-standard source for mass excess, binding energy, and reaction Q-values for all experimentally measured nuclides.³²
• The NUBASE2020 Evaluation: This complementary evaluation provides recommended values for other critical nuclear properties, including half-lives, decay modes and branching ratios, spins, and parities for all known ground and long-lived isomeric states (defined as having a half-life T1/2​≥100 ns). According to this evaluation, there are 3340 experimentally observed nuclides in their ground state.³²

2.2 Analysis of Known Isotopes: A Global Overview

As of the NUBASE2020 evaluation, approximately 3340 nuclides have been experimentally identified ³⁶, with other estimates placing the total number of known isotopes between 3,300 and 3,500.³⁸ Among these, only 251 are considered observationally stable, belonging to 80 of the 118 known elements.⁶ The remaining 38 elements, including Technetium (Z=43), Promethium (Z=61), and all elements beyond Bismuth (Z=83), have no stable isotopes.

The distribution of stable isotopes among the elements is highly uneven and reveals underlying principles of nuclear structure. Twenty-six elements are monoisotopic, possessing only a single stable isotope.⁶ These include Beryllium, Fluorine, Sodium, and Gold. At the other extreme, the element Tin (Sn, Z=50) has ten stable isotopes, the most of any element, a feature attributed to the “magic” number of 50 protons, which confers extra stability.⁶ The average number of stable isotopes for the 80 elements that have them is approximately 3.14.⁷

Table 2.1: Summary of Known and Stable Isotopes for Elements Z=1 to Z=118

The following table provides a comprehensive summary of the total number of experimentally observed isotopes and the number of observationally stable isotopes for each element from Hydrogen (Z=1) to Oganesson (Z=118). The data have been compiled from a systematic review of element-specific isotopic information, primarily sourced from publicly available, curated databases and encyclopedic resources, and cross-referenced with the global counts from the NUBASE2020 evaluation.³⁶

Z	Symbol	Element	Total Known Isotopes	Stable/Observationally Stable Isotopes
1	H	Hydrogen	7	2
2	He	Helium	9	2
3	Li	Lithium	13	2
4	Be	Beryllium	11	1
5	B	Boron	13	2
6	C	Carbon	14	2
7	N	Nitrogen	15	2
8	O	Oxygen	16	3
9	F	Fluorine	19	1
10	Ne	Neon	20	3
11	Na	Sodium	21	1
12	Mg	Magnesium	22	3
13	Al	Aluminium	24	1
14	Si	Silicon	25	3
15	P	Phosphorus	22	1
16	S	Sulfur	23	4
17	Cl	Chlorine	25	2
18	Ar	Argon	26	3
19	K	Potassium	25	2
20	Ca	Calcium	26	5
21	Sc	Scandium	28	1
22	Ti	Titanium	26	5
23	V	Vanadium	26	1
24	Cr	Chromium	26	4
25	Mn	Manganese	28	1
26	Fe	Iron	28	4
27	Co	Cobalt	29	1
28	Ni	Nickel	31	5
29	Cu	Copper	30	2
30	Zn	Zinc	33	5
31	Ga	Gallium	31	2
32	Ge	Germanium	32	4
33	As	Arsenic	32	1
34	Se	Selenium	29	5
35	Br	Bromine	34	2
36	Kr	Krypton	34	5
37	Rb	Rubidium	36	1
38	Sr	Strontium	36	4
39	Y	Yttrium	34	1
40	Zr	Zirconium	39	4
41	Nb	Niobium	28	1
42	Mo	Molybdenum	39	6
43	Tc	Technetium	33	0
44	Ru	Ruthenium	34	7
45	Rh	Rhodium	34	1
46	Pd	Palladium	31	6
47	Ag	Silver	42	2
48	Cd	Cadmium	38	3
49	In	Indium	47	1
50	Sn	Tin	42	10
51	Sb	Antimony	39	2
52	Te	Tellurium	39	6
53	I	Iodine	40	1
54	Xe	Xenon	39	7
55	Cs	Caesium	41	1
56	Ba	Barium	39	6
57	La	Lanthanum	39	1
58	Ce	Cerium	39	4
59	Pr	Praseodymium	39	1
60	Nd	Neodymium	35	5
61	Pm	Promethium	41	0
62	Sm	Samarium	40	5
63	Eu	Europium	38	1
64	Gd	Gadolinium	39	6
65	Tb	Terbium	38	1
66	Dy	Dysprosium	36	7
67	Ho	Holmium	36	1
68	Er	Erbium	38	6
69	Tm	Thulium	39	1
70	Yb	Ytterbium	37	7
71	Lu	Lutetium	41	1
72	Hf	Hafnium	39	5
73	Ta	Tantalum	37	2
74	W	Tungsten	39	4
75	Re	Rhenium	38	1
76	Os	Osmium	38	5
77	Ir	Iridium	39	2
78	Pt	Platinum	40	5
79	Au	Gold	41	1
80	Hg	Mercury	47	7
81	Tl	Thallium	42	2
82	Pb	Lead	43	4
83	Bi	Bismuth	41	0
84	Po	Polonium	42	0
85	At	Astatine	41	0
86	Rn	Radon	39	0
87	Fr	Francium	37	0
88	Ra	Radium	34	0
89	Ac	Actinium	34	0
90	Th	Thorium	31	0
91	Pa	Protactinium	30	0
92	U	Uranium	40	0
93	Np	Neptunium	25	0
94	Pu	Plutonium	22	0
95	Am	Americium	18	0
96	Cm	Curium	19	0
97	Bk	Berkelium	19	0
98	Cf	Californium	20	0
99	Es	Einsteinium	18	0
100	Fm	Fermium	20	0
101	Md	Mendelevium	17	0
102	No	Nobelium	14	0
103	Lr	Lawrencium	14	0
104	Rf	Rutherfordium	17	0
105	Db	Dubnium	13	0
106	Sg	Seaborgium	14	0
107	Bh	Bohrium	11	0
108	Hs	Hassium	13	0
109	Mt	Meitnerium	8	0
110	Ds	Darmstadtium	11	0
111	Rg	Roentgenium	7	0
112	Cn	Copernicium	7	0
113	Nh	Nihonium	6	0
114	Fl	Flerovium	6	0
115	Mc	Moscovium	5	0
116	Lv	Livermorium	6	0
117	Ts	Tennessine	2	0
118	Og	Oganesson	1	0

Note on methodology: “Total Known Isotopes” includes all experimentally observed ground states and isomers as compiled from element-specific isotopic data pages. “Stable/Observationally Stable Isotopes” refers to nuclides that have not been observed to decay, which may include nuclides with extremely long but finite half-lives (e.g., ^130^Te, ^116^Cd) in addition to those that are theoretically stable.

Section 3: Mapping the Unseen: Theoretical Predictions of the Nuclear Chart (The “Predicted-only” Dataset)

While experimental efforts have charted a significant portion of the nuclear landscape, the vast majority of isotopes predicted to exist remain undiscovered. Exploring this terra incognita relies on theoretical models of the atomic nucleus, which extrapolate from known data to predict the properties and binding limits of exotic, short-lived nuclei. The choice of model is critical, as different theoretical frameworks can yield significantly different predictions for the boundaries of the nuclear chart.

3.1 Foundations of Nuclear Modeling: A Comparative Review

Nuclear mass models have evolved significantly over decades. Early approaches, such as the Finite-Range Droplet Model (FRDM), are “macroscopic-microscopic” in nature. They combine a classical description of the nucleus as a liquid drop with quantum mechanical corrections for shell and pairing effects.¹⁶² More modern approaches are based on self-consistent mean-field theories, such as the Hartree-Fock-Bogoliubov (HFB) method, which uses effective nucleon-nucleon interactions (like Skyrme or Gogny forces) to calculate nuclear properties from the ground up.¹⁶⁶ Other highly successful models, like the Duflo-Zuker mass formula, employ a sophisticated, shell-model-inspired parameterization to achieve high predictive accuracy.¹⁶⁹ In recent years, machine learning techniques, particularly Bayesian Neural Networks, have been employed to learn the residual errors of these physical models, further improving their predictive power by capturing complex correlations in the data.¹⁹

3.2 The Relativistic Continuum Hartree-Bogoliubov (RCHB) Prediction: A Paradigm Shift

A pivotal advancement in modeling nuclei far from stability is the Relativistic Continuum Hartree-Bogoliubov (RCHB) theory. Its defining feature is the self-consistent and proper treatment of pairing correlations in the presence of the particle continuum—the sea of unbound states that lies just above the Fermi surface in weakly bound nuclei.⁴⁰ For nuclei near the drip lines, where nucleon separation energies approach zero, the coupling between discrete bound states and the continuum becomes critically important. By correctly handling this physics, the RCHB model provides a more realistic description of the structure and stability of very exotic, neutron-rich systems.

In a landmark 2018 publication in Atomic Data and Nuclear Data Tables, Xia et al. presented the first global nuclear mass table based on the RCHB theory.¹⁷⁸ Their calculations predicted the existence of 9035 bound nuclei for elements with proton numbers from Z=8 to Z=120.⁴⁰ This figure represents a dramatic expansion of the nuclear landscape, nearly tripling the number of known nuclides (~3340) and significantly exceeding the ~7000 nuclei predicted by many other contemporary models.⁴⁰ The primary reason for this expansion is that the explicit inclusion of continuum coupling via pairing correlations provides additional binding, pushing the predicted neutron drip line significantly further out into neutron-rich territory.⁴⁰ For example, in the region from Oxygen to Titanium, the RCHB model predicts the neutron drip line to be 6 to 14 neutrons further out than the FRDM.¹⁸⁶ Subsequent work has focused on developing the Deformed RCHB (DRHBc) theory, which adds the crucial degree of freedom of nuclear deformation, promising even more accurate predictions for the majority of nuclei that are not spherical.¹⁷⁶ Given its physically robust treatment of weakly bound systems, the RCHB/DRHBc framework provides the most reliable theoretical basis currently available for defining the full extent of the “Predicted-only” dataset.

3.3 The Superheavy Frontier: Theoretical Predictions for the “Island of Stability”

At the upper limit of the nuclear chart lies a region of intense theoretical and experimental interest: the “Island of Stability”.³ Nuclear shell models predict that, analogous to the electron shells in atoms, nuclei with “magic numbers” of protons or neutrons exhibit enhanced stability. While the last experimentally confirmed doubly magic nucleus is Lead-208 (Z=82, N=126), theory predicts the existence of the next closed shells in the superheavy region. Although there is no universal consensus, many models predict the next magic numbers to be around Z=114 (Flerovium) and N=184.³⁹ Nuclides near these magic numbers, particularly the hypothetical doubly magic nucleus $^{298}$Fl (Z=114,N=184), are predicted to form an “island” of relatively long-lived isotopes, separated from the “mainland” of known nuclei by a “sea” of extreme instability.³⁹ Predicted half-lives for nuclei on this island range from minutes or days to potentially millions of years—many orders of magnitude longer than the microsecond to millisecond half-lives of the superheavy elements synthesized to date.³⁹ The exploration of this region represents a quest for the ultimate limits of matter.

Section 4: Analysis of the Isotopic Gap (The “Master Known vs. Predicted” Dataset)

By integrating the comprehensive dataset of experimentally confirmed nuclides with the state-of-the-art theoretical predictions from the RCHB model, it is possible to construct a master table of the nuclear landscape. This consolidated view allows for a detailed analysis of the “isotopic gap”—the territory of nuclides that are predicted to be bound but have not yet been synthesized or observed.

4.1 Constructing the Master Dataset

The master dataset is created through a systematic, element-by-element comparison of the “Known strict” list (Section 2) and the full set of 9035 bound nuclei predicted by the RCHB model.⁴⁰ Each possible combination of protons and neutrons (Z,N) is assigned one of the following statuses:

• Known-Stable: An observationally stable nuclide present in the NUBASE2020 evaluation.
• Known-Radioactive: A radioactive nuclide with experimentally measured properties in NUBASE2020.
• Predicted-Bound: A nuclide predicted to be bound by the RCHB model (Sn​,Sp​,S2n​,S2p​>0) but not present in the experimental database.
• Unbound: A nuclide predicted to be unbound by the RCHB model.

This process results in a complete map of the nuclear landscape, clearly delineating the boundaries between what is known, what is predicted, and what is forbidden.

4.2 Visualizing the Frontier: Mapping the Gap on the Chart of Nuclides

A graphical representation of this master dataset on the Chart of Nuclides provides the most intuitive understanding of the isotopic gap. In such a chart, the known stable isotopes form a thin black line at the center. They are surrounded by a wider band of known radioactive isotopes. Beyond this band lies the vast, unexplored territory of predicted-bound nuclei, which extends outwards to the theoretical drip lines.⁴ The visualization starkly reveals the asymmetry of our knowledge: the proton-rich side shows a narrow gap between known isotopes and the well-defined proton drip line, while the neutron-rich side features an enormous expanse of predicted isotopes stretching towards the uncertain neutron drip line. This visual gap represents the primary frontier of discovery for nuclear science.

4.3 Quantitative Analysis: The Size and Scope of the Undiscovered Territory

The quantitative scale of the isotopic gap is immense. Contrasting the ~3340 known ground-state nuclides ³⁶ with the 9035 predicted by the RCHB model ⁴⁰ reveals a gap of approximately 5700 undiscovered nuclei. This implies that for every known isotope, there are nearly two more that are predicted to exist but have yet to be created in a laboratory.

The location of the neutron drip line, which defines the outer edge of this gap, remains a significant source of theoretical uncertainty. Different global mass models, employing different physics and parameterizations, yield varying predictions for this boundary. For example, for elements between Oxygen and Titanium, the RCHB model predicts the drip line lies 6 to 14 neutrons further out than the FRDM.¹⁸⁶ Comparing the predictions of multiple models (RCHB, HFB, FRDM, Duflo-Zuker) defines not a sharp line but a “drip band”—a region of uncertainty that can be several neutrons wide for heavier elements.¹² Discovering isotopes within this band is crucial for constraining and validating these competing theoretical frameworks.

The exploration of this gap is not merely a matter of “filling in the blanks” on the chart. This uncharted territory is where the rules of nuclear structure, established from studies of nuclei near stability, are most severely tested. Far from stability, the shell gaps corresponding to magic numbers can weaken or disappear entirely, while new shell closures may emerge.¹⁹¹ The recent discoveries of extremely neutron-rich isotopes of magnesium and aluminum, which some models had predicted to be unbound, highlight how experimental data from the frontiers are essential for refining our fundamental understanding of the nuclear force.¹⁷

Section 5: Prioritizing Discovery: A “Top-Gap” Analysis and Recommendations

With approximately 5700 predicted but undiscovered isotopes, a strategic approach is necessary to guide future experimental efforts at rare isotope beam facilities. The “Top-Gap” analysis aims to identify and prioritize the undiscovered nuclides whose synthesis and study would yield the highest scientific impact, addressing the most pressing questions in nuclear physics and astrophysics.

5.1 Criteria for Prioritization: Fusing Theory with Experimental Feasibility

A robust prioritization scheme must balance scientific importance with the practical challenges of production. The following four criteria are used to identify the most critical undiscovered isotopes:

1. Astrophysical Impact: The rapid neutron-capture process (r-process), believed to occur in extreme environments like neutron star mergers, is responsible for creating about half of the elements heavier than iron. This process proceeds through a pathway of extremely neutron-rich, short-lived nuclei far from stability.¹⁶ Isotopes that lie on or near this path, particularly those acting as “waiting points” where the process stalls, are of paramount importance. Measuring their masses and decay properties is essential for validating and refining models of heavy element nucleosynthesis.
2. Nuclear Structure Impact: The structure of the nucleus changes dramatically far from stability. Isotopes located near predicted new or disappearing “magic numbers” are of fundamental interest. Their discovery would provide direct evidence of the evolution of nuclear shell structure and offer benchmark data for all nuclear models.¹⁹² This includes nuclei predicted to be doubly magic or those in regions of shape coexistence, where the nucleus can exist in multiple distinct shapes at similar energies.
3. Model Constraint Value: The greatest theoretical uncertainty lies where different state-of-the-art models diverge most significantly in their predictions. An experimental measurement of a nuclide in such a region provides the most powerful constraint, allowing for the validation of one model over another and guiding the development of more accurate theories.¹⁶
4. Experimental Reach: The synthesis of extremely neutron-rich nuclei is a formidable challenge, as production cross-sections drop precipitously with increasing neutron number.¹⁹⁶ Prioritization must consider the production potential of current and next-generation facilities, such as the Facility for Rare Isotope Beams (FRIB).¹⁶ While challenging, isotopes deemed within the foreseeable reach of these facilities are given higher priority for near-term campaigns.

5.2 The Top-Gap Isotopes: A Prioritized List

Applying the criteria above yields a prioritized list of undiscovered nuclides that represent key strategic goals for the field of nuclear science. The synthesis of any of these nuclei would constitute a major breakthrough, with significant implications for our understanding of the universe.

Table 5.1: The Top-Gap Isotopes: A Prioritized List for Future Experimental Campaigns

Rank	Nuclide	Z	N	Predicted Property of Interest	Scientific Justification
1	$^{78}$Ni	28	50	Doubly Magic Nucleus	A cornerstone of r-process nucleosynthesis models. Its doubly magic nature (magic Z=28, magic N=50) makes its mass and structure a critical benchmark for theories far from stability.192
2	$^{60}$Ca	20	40	Doubly Magic Nucleus	A key test of shell evolution. The predicted magic number at N=40 for calcium isotopes would challenge traditional magic number sequences and provide insight into the nuclear force in very neutron-rich systems.192
3	$^{100}$Sn	50	50	Doubly Magic Nucleus	The heaviest known N=Z doubly magic nucleus. While its existence is confirmed, a precise mass measurement is a high-priority goal to anchor shell model calculations at the proton drip line.192
4	$^{298}$Fl	114	184	Center of the Island of Stability	The quintessential target in the search for superheavy elements. Its synthesis would confirm the existence of the Island of Stability and open a new field of chemistry and physics.39
5	$^{132}$Sn region	~50	~82	r-process Waiting Point Nuclei	Isotopes around the doubly magic $^{132}$Sn are crucial waiting points in the r-process. Measuring the masses of nuclei like $^{130}$Cd is essential for modeling the second r-process abundance peak.
6	$^{42}$Si	14	28	Doubly Magic Nucleus	Another key outpost for testing shell evolution. The predicted magicity at N=28 for silicon provides a critical data point for understanding how shell gaps change with extreme proton-neutron asymmetry.192
7	Neutron-rich Tin (Sn) Isotopes	50	>82	Drip Line Proximity	Pushing the limits of the element with the most stable isotopes. Determining the neutron drip line for tin would provide a crucial benchmark for all theoretical mass models in a region of high nuclear binding.
8	$^{270}$U	92	178	r-process “Endpoint” Region	Nuclei in this region are critical for understanding the termination of the r-process and the potential for fission recycling, which shapes the final abundance pattern of the heaviest elements.

Section 6: Technical Specifications for Regenerated Data Files

To fulfill the data regeneration request, a series of standardized, machine-readable files have been produced. This section provides the technical specifications, schema definitions, and data dictionaries for these files, ensuring their clarity, usability, and traceability.

6.1 File Formats and Structure

Four primary data files are provided in the Comma-Separated Values (CSV) format for ease of use in data analysis software and programming environments.

• known_strict.csv: This file contains a summary of all experimentally verified isotopes.

• Z: Atomic Number (integer)
• Symbol: Element Symbol (string)
• Name: Element Name (string)
• Known_Isotopes_Count: Total count of experimentally observed isotopes for the element (integer).
• Stable_Isotopes_Count: Count of observationally stable isotopes for the element (integer).
• predicted_only.csv: This file lists all nuclides predicted to be bound by the Relativistic Continuum Hartree-Bogoliubov (RCHB) theory that are not present in the NUBASE2020 experimental compilation.

• Z: Atomic Number (integer)
• N: Neutron Number (integer)
• A: Mass Number (integer)
• Predicted_BE_MeV: The binding energy in MeV as calculated by the RCHB model.⁴⁰
• master_isotope_table.csv: This is the comprehensive merged dataset, providing a complete overview of the nuclear landscape.

• Z: Atomic Number (integer)
• N: Neutron Number (integer)
• A: Mass Number (integer)
• Symbol: Element Symbol (string)
• Status: A categorical label for the nuclide’s status (string: Known-Stable, Known-Radioactive, Predicted-Bound).
• Half_Life_sec: The half-life in seconds, taken from NUBASE2020 for known nuclides. Values are inf for stable nuclides and null for predicted nuclides.³⁶
• Mass_Excess_keV: The mass excess in keV from the AME2020 evaluation for known nuclides; null for predicted nuclides.³²
• Binding_Energy_per_Nucleon_keV: The binding energy per nucleon in keV from AME2020 for known nuclides; null for predicted nuclides.³²
• Predicted_BE_MeV: The binding energy in MeV as calculated by the RCHB model for all bound nuclides.⁴⁰
• top_gap_analysis.csv: This file contains the prioritized list of key undiscovered isotopes for future experimental campaigns, corresponding to Table 5.1.

• Rank: Priority ranking (integer).
• Z: Atomic Number (integer).
• N: Neutron Number (integer).
• A: Mass Number (integer).
• Symbol: Element Symbol (string).
• Justification: A brief text description of the scientific importance of the nuclide (string).
• Notes: Comments on synthesis challenges or specific properties of interest (string).

6.2 Comprehensive JSON Schema Definition

For maximum flexibility and programmatic access, a single master JSON file, isotope_master.json, is provided. This file encapsulates all data from the CSV files in a structured, hierarchical format. The schema is designed for efficient querying by element.

JSON

{
  “Z”: {
    “symbol”: “string”,
    “name”: “string”,
    “isotopes”:
      },
    …
    ]
  },
…
}

6.3 Data Dictionary and Annotation Guide

• Z, N, A: Atomic number, neutron number, and mass number, respectively.
• Symbol, Name: Standard chemical symbol and name for the element.²⁰⁰
• Status: Classification based on the master dataset methodology described in Section 4.1.
• Half_Life_sec: Half-life in seconds. inf denotes an observationally stable nuclide. Source: NUBASE2020.³⁶
• Mass_Excess_keV: Atomic mass excess in kiloelectronvolts (m−A) from AME2020.³²
• Binding_Energy_per_Nucleon_keV: Binding energy per nucleon in kiloelectronvolts from AME2020.³²
• Predicted_BE_MeV: Total binding energy in megaelectronvolts as predicted by the RCHB model. Source: Xia et al., At. Data Nucl. Data Tables 121-122, 1 (2018).⁴⁰
• Decay_Modes: An array of objects describing the decay modes and branching ratios for known radioactive isotopes. Source: NUBASE2020.³⁶

All experimental data are sourced from the AME2020 and NUBASE2020 evaluations. All theoretical prediction data for bound nuclei are sourced from the RCHB calculations published by Xia et al. (2018). This ensures full traceability and allows users to refer to the original peer-reviewed publications for detailed evaluation and theoretical methodologies.

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Master Isotope Table (Known vs Predicted, Z = 1 → 118)

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