Case Study 2: Navigating NNDC — A Hands-On Guide to Nuclear Data

Introduction: The Most Important Bookmark in Nuclear Physics

The National Nuclear Data Center at Brookhaven National Laboratory (https://www.nndc.bnl.gov) is the most important single resource for any nuclear physicist. It hosts the evaluated databases (ENSDF, ENDF), the interactive tools (NuDat, Sigma), and the compilations (AME, NUBASE, Nuclear Wallet Cards) that contain essentially everything the field has measured over the past century.

Yet many nuclear physics students graduate without ever learning to use NNDC effectively. This case study is a guided tour. We will walk through three practical tasks — the kind of things a working nuclear physicist does routinely — using NNDC's tools.

Task 1: Looking Up a Nuclide on NuDat

The Scenario

You are reading a paper about the $N = 50$ shell closure and encounter a discussion of ${}^{88}\text{Sr}$ ($Z = 38$, $N = 50$). You need to quickly verify: (a) its ground-state properties, (b) its first few excited states, and (c) its decay data (if it decays).

Step-by-Step Walkthrough

Step 1: Go to https://www.nndc.bnl.gov/nudat3/

Step 2: On the interactive chart of nuclides, locate ${}^{88}\text{Sr}$. You can either: - Click directly on the chart at $Z = 38$, $N = 50$ - Use the search box to enter "Sr-88" or "88Sr"

Step 3: The NuDat page for ${}^{88}\text{Sr}$ displays:

Ground-state properties: - $J^\pi = 0^+$ (spin-parity of the ground state) - $T_{1/2}$: Stable (natural abundance: 82.58%) - $\Delta$ (mass excess): $-87.920\,\text{MeV}$

This tells us immediately that ${}^{88}\text{Sr}$ is a stable, even-even nucleus with the expected $0^+$ ground state.

Step 4: Click on "Levels" to see the nuclear level scheme. The first few excited states of ${}^{88}\text{Sr}$:

Key observation: The first $2^+$ state lies at 1836 keV. This is high — compare to ${}^{86}\text{Sr}$ ($E(2^+_1) \approx 1077\,\text{keV}$) and ${}^{90}\text{Sr}$ ($E(2^+_1) \approx 831\,\text{keV}$). The high $2^+_1$ energy in ${}^{88}\text{Sr}$ is direct evidence for the $N = 50$ shell closure: the nucleus is "stiff" against the quadrupole excitation that creates the first $2^+$ state. This is the same physics we discussed in Chapter 6 (shell model) and Chapter 8 (collective motion).

💡 Practical Tip: The energy of the first $2^+$ state in even-even nuclei is one of the most important observables in nuclear structure. A high $E(2^+_1)$ signals a magic number; a low $E(2^+_1)$ signals collectivity and deformation. You can survey $E(2^+_1)$ values across the chart of nuclides using NuDat's "Search and Plot" feature.

Step 5: Click on "Decay" to see decay radiation data. Since ${}^{88}\text{Sr}$ is stable, there is no decay data for the ground state. However, NuDat also provides information about nuclides that decay to ${}^{88}\text{Sr}$ — for example, ${}^{88}\text{Y}$ ($\beta^+$/EC decay, $T_{1/2} = 106.63\,\text{d}$) and ${}^{88}\text{Rb}$ ($\beta^-$ decay, $T_{1/2} = 17.773\,\text{min}$).

Step 6: Click on "Gammas" to see adopted gamma-ray data. This lists all known gamma-ray transitions in ${}^{88}\text{Sr}$ with energies, intensities, multipolarities, and mixing ratios.

What We Learned

In approximately three minutes, NuDat provided: ground-state properties, the level scheme, evidence for the $N = 50$ shell closure, and a gateway to the full evaluated data. This is the kind of lookup you will perform hundreds of times in a career.

Task 2: Finding Cross Section Data with Sigma

The Scenario

You are designing a neutron activation analysis experiment and need the thermal neutron capture cross section for ${}^{58}\text{Ni}(n,\gamma){}^{59}\text{Ni}$, as well as the energy dependence of the cross section from thermal to 1 MeV.

Step-by-Step Walkthrough

Step 1: Go to https://www.nndc.bnl.gov/sigma/

The Sigma tool provides interactive access to evaluated nuclear reaction data from the ENDF/B library.

Step 2: In the search interface: - Element: Ni - Mass number: 58 - Reaction: (n,gamma) or equivalently MT=102 in ENDF notation - Library: ENDF/B-VIII.0

Step 3: The tool returns the cross section $\sigma(E_n)$ plotted as a function of neutron energy. Key features you observe:

Thermal region ($E_n < 0.5\,\text{eV}$): The cross section follows the $1/v$ law: $\sigma \propto 1/\sqrt{E}$. At the standard thermal energy $E_n = 0.0253\,\text{eV}$:

$$\sigma_{\text{th}}(n,\gamma) \approx 4.6\,\text{b}$$

Resolved resonance region ($1\,\text{eV} < E_n < 100\,\text{keV}$): Sharp resonance peaks appear — these are individual compound nuclear states in ${}^{59}\text{Ni}^*$. Each resonance is characterized by a Breit-Wigner profile (Chapter 18) with parameters $E_r$ (resonance energy), $\Gamma_n$ (neutron width), $\Gamma_\gamma$ (gamma width), and $J^\pi$ (spin-parity).

Unresolved resonance and continuum region ($E_n > 100\,\text{keV}$): Individual resonances overlap and the cross section is represented as a smooth function from statistical model calculations.

Step 4: Use the "Retrieve" or "Download" button to get numerical cross section data. Sigma provides data in several formats: - Tabular (energy, cross section) — suitable for importing into Python or other analysis codes - ENDF-6 format — the standard interchange format for nuclear data - Plotly interactive plot — for quick visualization

Step 5: To compare with experimental data, use EXFOR (also accessible through the IAEA Nuclear Data Services). EXFOR compiles all experimentally measured cross sections, allowing you to compare the ENDF/B evaluation to the underlying measurements.

What We Learned

The Sigma tool gives immediate access to any reaction cross section in the ENDF/B library, with visualization and download capabilities. For the activation analysis experiment, we now know the thermal cross section (4.6 b) and can plan our irradiation time and detector requirements accordingly.

🔗 Cross-Reference: This workflow directly extends the reaction physics from Chapter 17 (cross sections), Chapter 18 (resonances and the Breit-Wigner formula), and Chapter 30 (experimental techniques).

Task 3: Exploring an ENSDF Evaluation

The Scenario

You are preparing a journal club presentation on a new measurement of excited states in ${}^{132}\text{Sn}$ ($Z = 50$, $N = 82$ — doubly magic). Before reading the new paper, you want to know: what was previously known about the level structure of ${}^{132}\text{Sn}$?

Step-by-Step Walkthrough

Step 1: Go to https://www.nndc.bnl.gov/ensdf/

Step 2: Search for $A = 132$ (the mass number). The ENSDF evaluation for $A = 132$ covers all isobars: ${}^{132}\text{In}$, ${}^{132}\text{Sn}$, ${}^{132}\text{Sb}$, ${}^{132}\text{Te}$, etc.

Step 3: Select the ${}^{132}\text{Sn}$ adopted levels. The evaluation presents:

Nuclear properties: - $J^\pi = 0^+$ (ground state) - $T_{1/2} = 39.7 \pm 0.8\,\text{s}$ ($\beta^-$ emitter) - Doubly magic: $Z = 50$ (proton magic), $N = 82$ (neutron magic)

Known excited states (prior to your new paper): The first $2^+$ state lies at 4041 keV — the highest known $E(2^+_1)$ for any nucleus with $A > 16$. This extraordinarily high energy is the strongest evidence for the doubly-magic character of ${}^{132}\text{Sn}$ and was a major experimental achievement, first measured at ISOLDE and confirmed at HRIBF/ORNL.

Step 4: The ENSDF evaluation includes: - Adopted levels: The evaluator's best values for excitation energies and $J^\pi$ assignments, with references to all supporting measurements - Comments: Narrative text explaining the evaluator's reasoning for adopted values, discussing discrepancies between measurements, and noting uncertainties in assignments - Gamma-ray data: Energies, intensities, multipolarities, and mixing ratios for all known transitions - References: Complete bibliography of all papers contributing data for this nuclide

Step 5: Check the evaluation date. ENSDF evaluations are updated on a rolling basis (one mass chain at a time). The evaluation for $A = 132$ may be several years old, meaning very recent measurements may not be included. This is where XUNDL becomes useful — it contains recent experimental data that has been compiled but not yet formally evaluated.

Step 6: Read the evaluator's comments. For a doubly-magic nucleus like ${}^{132}\text{Sn}$, the comments discuss: - The shell model interpretation of the low-lying states (particle-hole excitations across the $Z = 50$ and $N = 82$ gaps) - Comparison with the other doubly-magic nuclei (${}^{208}\text{Pb}$, ${}^{48}\text{Ca}$, ${}^{56}\text{Ni}$) - Experimental techniques used (beta decay, Coulomb excitation, transfer reactions with radioactive beams) - Known uncertainties and ambiguities in spin-parity assignments

What We Learned

The ENSDF evaluation provided a complete picture of the prior knowledge about ${}^{132}\text{Sn}$'s level structure, including evaluator commentary that contextualizes the data. Armed with this background, you can now read the new paper and immediately identify which states are newly measured, whether the new energies agree with previous measurements, and whether new spin-parity assignments confirm or challenge the existing scheme.

Putting It All Together: The Complete Workflow

The three tasks in this case study represent the typical workflow when a nuclear physicist encounters a new nucleus or reaction:

NuDat (quick overview)
  ↓
ENSDF (detailed level scheme, evaluated data)
  ↓
Sigma/ENDF (reaction cross sections if needed)
  ↓
XUNDL (most recent measurements)
  ↓
AME (masses, binding energies, separation energies)
  ↓
Literature search (arXiv, INSPIRE, Google Scholar)

Each step takes 2–10 minutes. The entire workflow takes less than an hour and provides a comprehensive picture of the current state of knowledge for any nuclide.

💡 Advice: Bookmark these tools. Use them every time you encounter a new nucleus in a paper, a talk, or a problem set. The more you use them, the faster and more effective you become. Within a year of regular use, you will navigate NNDC as fluently as you navigate a search engine.

A Note on Data Quality

Nuclear data evaluations are performed by expert evaluators — typically experienced nuclear physicists who specialize in a particular mass region. The evaluation process is rigorous: every published measurement is reviewed, discrepancies are investigated, and adopted values are determined by weighted averages or by selecting the most reliable measurement.

However, evaluations are not infallible: - They may lag several years behind the latest publications - The evaluator's judgment in resolving discrepancies may be contested - Some spin-parity assignments are tentative (marked with parentheses in ENSDF) - Very recent facilities (FRIB, for instance) may produce data for nuclides that have no prior evaluation at all

Always treat evaluated data as the best available starting point, not as the final word. When a new measurement disagrees with the evaluation, that disagreement is interesting — it may reveal new physics or a problem with a prior measurement.