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Low lying excitations in $^{150}$ Pm

This study investigates the low-lying excitations of the odd-odd nucleus $^{150}$ Pm using proton-induced reactions and $\gamma$ -ray spectroscopy to establish a new level scheme with 15 new levels, determine tentative spin-parity assignments and lifetimes, and interpret the results through large basis and projected shell model calculations, revealing a $1^-$ ground state, a nearby $2^-$ state, and a low-lying $6^-$ isomer.

Original authors: A. Pal, S. Basak, T. Bhattacharjee, Nazira Nazir, G. H. Bhat, S. Jehangir, D. Kumar, A. Saha, S. S. Alam, D. Banerjee, A. Das, A. Adhikari, A. Gupta, S. Das, A. Bisoi, Y. Sapkota, S. Sharma, S. Samant

Published 2026-06-25

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: A. Pal, S. Basak, T. Bhattacharjee, Nazira Nazir, G. H. Bhat, S. Jehangir, D. Kumar, A. Saha, S. S. Alam, D. Banerjee, A. Das, A. Adhikari, A. Gupta, S. Das, A. Bisoi, Y. Sapkota, S. Sharma, S. Samanta, S. Chatterjee, R. Raut, S. S. Ghugre, J. A. Sheikh

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the atomic nucleus not as a solid ball, but as a bustling, chaotic dance floor where protons and neutrons are the dancers. In most nuclei, these dancers move in perfect, predictable patterns. But in certain "transitional" nuclei, like the one studied in this paper (Promethium-150, or 150Pm), the dance floor is a bit unstable. It's right on the edge between a spherical shape (like a smooth ball) and a deformed shape (like a rugby ball).

This paper is essentially a report from a team of nuclear physicists who went to this specific dance floor to figure out exactly how the dancers are moving, what their spins are, and how long they stay in certain poses before changing.

Here is a breakdown of their journey and findings:

1. The Challenge: Catching a Ghost

Studying 150Pm is like trying to photograph a ghost that only appears for a few hours.

The Problem: This atom is unstable and doesn't exist naturally in large amounts. You can't just dig it up; you have to create it.
The Method: The team used a giant particle accelerator (a cyclotron) to shoot protons (tiny, fast bullets) at a target of Neodymium-150. When the protons hit the target, they knocked a neutron out and turned the atom into 150Pm.
The Detection: As these new atoms settled down, they emitted flashes of light called gamma rays. The team used a sophisticated array of five "Clover" detectors (which look like flower petals) and one special low-energy detector to catch these flashes. It's like having a team of high-speed cameras trying to catch the exact moment a dancer spins and the light they emit.

2. Sorting the Chaos: The "Two Groups" Discovery

The detectors recorded thousands of flashes. The tricky part was that the experiment also accidentally created a slightly different atom (Promethium-149), which was also flashing. It was like trying to sort a pile of mixed-up red and blue marbles.

The Trick: The team ran the experiment at two different speeds (beam energies). They noticed that the "red marbles" (149Pm) behaved one way, while the "blue marbles" (150Pm) behaved differently. By comparing the data from the two speeds, they could filter out the noise and isolate the true signals of 150Pm.
The Result: They successfully mapped out the "level scheme." Think of this as drawing a floor plan of a building. They found 15 new rooms (energy levels) and 16 new hallways (gamma ray transitions) that no one had seen before.

3. The Dance Moves: Spin and Parity

In nuclear physics, every energy level has a "spin" (how fast it's rotating) and "parity" (whether it's a mirror image of itself or flipped).

The Ground State: The most important question was: What is the "default" pose of this atom? Previous studies were arguing about whether it was a "1-minus" or "2-minus" pose.
The Verdict: By combining their new data with computer simulations (Shell Models), the team concluded the ground state is 1-minus.
The Close Neighbor: They also found a state very close to the ground state (only about 50 keV away, which is like a whisper in a loud room) that has a 2-minus spin.
The Isomer: They identified a "metastable" state (an isomer) with a 6-minus spin. Think of this as a dancer getting stuck in a difficult pose for a split second before snapping back.

4. Timing the Dance: Lifetimes

The team wanted to know how long the atom stays in these excited "rooms" before dropping down to the ground floor.

The Technique: They used a method called "Generalized Centroid Difference." Imagine two runners starting at slightly different times. By measuring the tiny difference in when they cross the finish line, you can calculate their speed.
The Finding: They measured the "lifetime" of two specific levels. Both were incredibly fast, lasting less than 2 nanoseconds (billionths of a second). This tells us these states are very unstable and drop down almost instantly.

5. The Computer Simulation: Did the Theory Match?

To make sense of their observations, the team ran two types of computer simulations:

The Spherical Model: This treats the nucleus like a smooth ball. It successfully predicted the ground state and the nearby low-energy states, confirming that single particles (protons and neutrons) are the main dancers here.
The Projected Shell Model: This treats the nucleus as a slightly squashed rugby ball. This model helped explain the "band structures"—groups of energy levels that look like a ladder. It suggested that these levels are formed by pairs of quasiparticles (proton-neutron couples) dancing together.

Summary

In simple terms, this paper is a detailed map of a very unstable, transitional atomic nucleus. The team:

Created the atom using a particle accelerator.
Filtered out background noise to find 15 new energy levels.
Determined that the atom's "default" state is a 1-minus spin.
Found a nearby 2-minus state and a higher 6-minus isomer.
Measured that these excited states vanish in less than 2 nanoseconds.
Confirmed that computer models of single particles and paired dancers can explain most of what they saw.

The paper concludes that while they have made a huge leap in understanding this specific nucleus, the "dance floor" is complex, and more high-resolution measurements are needed to fully understand the positive-parity states (the other side of the dance floor) that the current models couldn't quite predict.

Technical Summary: Low Lying Excitations in 150Pm

Problem Statement
The nuclear structure of odd-odd nuclei in the $A \sim 150$ transitional region, specifically $^{150}\text{Pm}$ ( $Z=61, N=89$ ), remains poorly understood despite its significance in the quantum shape phase transition (QPT) from spherical vibrators to deformed rotors. This nucleus lies between the $X(5)$ symmetry exhibiting even-even neighbors $^{150}\text{Nd}$ and $^{152}\text{Sm}$ . Existing experimental data on $^{150}\text{Pm}$ is scanty, with conflicting reports regarding the ground state spin-parity ( $J^\pi$ ). While some studies suggest a $1^-$ ground state based on $\beta$ -decay intensity measurements, others propose a $2^-$ ground state based on $(^3\text{He}, t)$ reaction data, often assuming the absence of a nearby $1^-$ state. Furthermore, the existence and nature of low-lying isomers, which could impact nucleosynthesis via the slow neutron capture process, and the development of band structures in this odd-odd system require clarification. The scarcity of data is attributed to the difficulty in populating these states via stable targets and the lack of suitable $\beta$ -decay routes.

Methodology
The study utilized the proton-induced reaction $^{150}\text{Nd}(p,n)^{150}\text{Pm}$ at the Variable Energy Cyclotron Centre (VECC), Kolkata. A highly enriched ( $97.65\%$ ) $^{150}\text{Nd}$ target was irradiated with proton beams at two energies, 8 MeV and 9 MeV. The decay $\gamma$ -rays were detected using the Indian National Gamma Array (INGA), comprising five BGO-suppressed Clover HPGe detectors and one segmented planar Ge detector (LEPS).

Data analysis involved:

Excitation Function Analysis: Comparing singles $\gamma$ -ray yields at 8 MeV and 9 MeV to distinguish transitions belonging to $^{150}\text{Pm}$ from the contaminant $^{149}\text{Pm}$ , utilizing a normalization factor based on the 211 keV line of $^{149}\text{Pm}$ .
Coincidence Analysis: Constructing symmetric (Clover-Clover) and asymmetric (Clover-LEPS) $\gamma-\gamma$ matrices to establish level schemes and place new transitions.
Intensity Measurements: Determining relative intensities using total and gated projections of the $\gamma-\gamma$ matrix.
Spin-Parity Assignment: Employing limited angular distribution analysis (using detectors at $86.5^\circ$ , $93.5^\circ$ , and $125^\circ$ ) to extract angular distribution coefficients ( $a_2$ ) and infer multipolarities.
Lifetime Analysis: Estimating lifetimes for two excited levels using the Generalized Centroid Difference (GCD) technique with Ge detectors in the nanosecond range, calibrated against known lifetimes in $^{149}\text{Pm}$ and $^{150}\text{Sm}$ .
Theoretical Modeling: Performing Large Basis Spherical Shell Model (LBSM) calculations using NuShellX@MSU with a $^{132}\text{Sn}$ inert core to constrain ground state properties. Additionally, Projected Shell Model (PSM) calculations were conducted using a deformed mean field to interpret band structures and quasiparticle configurations.

Key Contributions and Results

Level Scheme Construction: The study identified and placed 16 new $\gamma$ -rays and 15 new energy levels in the level scheme of $^{150}\text{Pm}$ . Three previously known $\gamma$ -rays (76.6, 159.2, and 229.5 keV) were repositioned within the scheme.
Ground State Assignment: Based on angular distribution data, shell model predictions, and phenomenological arguments, the ground state of $^{150}\text{Pm}$ is assigned a spin-parity of $1^-$ . A closely lying $2^-$ state is proposed at approximately 50 keV (specifically 31 keV in shell model calculations), which may explain previous conflicting assignments.
Isomeric State: A low-lying $6^-$ isomeric state is identified. A second group of levels (including 177.2+X, 336.4+X, and 454.8+X keV) is tentatively placed on this isomeric state, distinct from the ground state band.
Band Structures: The data reveals emerging band-like structures. Negative parity bands (B1, B2, B3) and positive parity bands (B4, B5) were proposed. Band B1 is identified as the yrast sequence.
Lifetime Measurements: Lifetimes for the 128.3 keV and 177.2+X keV levels were estimated to be less than 2 ns using the GCD technique.
Theoretical Interpretation:
- Shell Model: The LBSM calculation successfully reproduced the $1^-$ ground state and the dominant proton-neutron configurations ( $\pi g_{7/2} \otimes \nu f_{7/2}$ ) for the low-lying negative parity states. However, it predicted a lower level density than observed and failed to predict low-energy positive parity states, likely due to model space truncations.
- Projected Shell Model: PSM calculations, incorporating deformation ( $\epsilon \approx 0.208$ ), provided a reasonable description of the rotational band structures and quasiparticle configurations, supporting the interpretation of the observed sequences as quasiparticle bands.

Significance
The paper claims that this work provides the most comprehensive low-lying level scheme for $^{150}\text{Pm}$ to date, resolving ambiguities regarding the ground state spin-parity by confirming the $1^-$ assignment. The identification of a low-lying $6^-$ isomer and the characterization of emerging band structures offer critical insights into the interplay between single-particle excitations and collective rotational behavior in odd-odd nuclei near the $N=90$ shape transition. The results serve as a benchmark for understanding the critical point symmetry in this mass region and highlight the necessity of high-resolution spectroscopy to fully map the structure of these transitional nuclei. The authors note that while the current data supports the proposed assignments, future measurements involving fusion-evaporation or transfer reactions with angular distribution and polarization capabilities are required for definitive spin-parity assignments.