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Nuclotron internal target polarimeter for the measurements of the deuteron and proton beam polarization

This paper reports on the development and application of a scintillation counter-based internal target polarimeter at the Nuclotron/NICA facility to precisely measure the vector polarization of deuteron beams across multiple energies and the polarization of a newly accelerated 500 MeV proton beam.

Original authors: I. S. Volkov, V. P. Ladygin, A. V. Averyanov, E. V. Chernykh, D. Enache, Yu. V. Gurchin, A. Yu. Isupov, M. Janek, J. -T. Karachuk, D. O. Krivenkov, P. K. Kurilkin, A. N. Livanov, S. M. Piyadin, S. G.

Published 2026-07-20

📖 5 min read🧠 Deep dive

CC0 1.0

Original authors: I. S. Volkov, V. P. Ladygin, A. V. Averyanov, E. V. Chernykh, D. Enache, Yu. V. Gurchin, A. Yu. Isupov, M. Janek, J. -T. Karachuk, D. O. Krivenkov, P. K. Kurilkin, A. N. Livanov, S. M. Piyadin, S. G. Reznikov, Ya. T. Skhomenko, A. A. Terekhin, A. V. Tishevsky, T. Uesaka, I. E. Vnukov

Original paper dedicated to the public domain under CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/). ✨ 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

The Spin Doctor's Toolkit

Imagine a universe where every particle is a tiny, spinning top. In the world of high-energy physics, these tops aren't just spinning for fun; their direction—called "polarization"—holds the secrets to how the universe is built. Scientists at the Joint Institute for Nuclear Research (JINR) are trying to figure out how these tops behave when they crash into each other at incredible speeds. To do this, they use massive machines called accelerators, which act like giant racetracks for protons and deuterons (a heavy cousin of hydrogen).

But here's the tricky part: if you want to see what happens when these spinning tops collide, you first need to know exactly how they are spinning before the crash. It's like trying to study a game of billiards, but you don't know if the cue ball is spinning left, right, or not at all. If you don't know the starting spin, you can't understand the result. This is where "polarimetry" comes in. It's the art of measuring that spin. The researchers in this paper are building and testing a new, super-sensitive "spin detector" right inside the racetrack to make sure they know exactly how their beams are spinning before they smash them together.

The Paper's Story: A Spin Detective at the Nuclotron

This paper is all about a team of scientists who installed a new "spin detector" inside the Nuclotron, a particle accelerator in Dubna, Russia. Their goal was to measure how well their machine could spin up beams of protons and deuterons, and to prove that their new detector works like a charm.

The Setup: A Target in the Middle of the Road
Imagine the Nuclotron as a circular highway where particles zoom around. Usually, scientists shoot these particles at a target outside the ring. But this team decided to put a target inside the ring, right in the middle of the action. This is the "internal target." They used a thin film of polyethylene (a plastic full of hydrogen) and a carbon target.

When the spinning beam hits the hydrogen in the plastic, the protons bounce off in specific directions. The team set up a ring of 24 plastic "scintillation counters" (think of them as high-speed cameras that flash when a particle hits them) around the target. These cameras were positioned to catch the particles exactly where they would fly if they bounced off each other in a perfect, elastic collision.

The Deuteron Mystery: Weakly Bound Twins
The team first focused on deuterons. A deuteron is like a tiny molecule made of a proton and a neutron holding hands very loosely. The scientists treated the deuteron beam as a stream of these "weakly bound twins." When a deuteron hits a hydrogen atom in the target, it's essentially a proton hitting a proton.

They tested the beam at four different speeds: 200, 500, 550, and 650 MeV/nucleon. By counting how many protons bounced to the "left" versus the "right," they could calculate the beam's "vector polarization" (how much the spins are lined up).

The Results: A Good Match
The paper reports that the new detector worked beautifully.

At 500 MeV/nucleon, they measured a polarization of about 0.248 for one spin mode and 0.246 for another.
At 650 MeV/nucleon, the numbers were 0.241 and 0.266.
Even at the slower speeds of 200 and 550 MeV/nucleon, the measurements were consistent.

Crucially, they compared these new results with an older, well-known method that uses a different type of collision (deuteron-proton elastic scattering at 270 MeV). The new "internal target" method agreed perfectly with the old method. This proves that the new detector is reliable and can be used to monitor the beam's spin in real-time.

The Proton Challenge: First Time at 500 MeV
Next, the team tried something new: they accelerated a beam of polarized protons to 500 MeV for the first time at the Nuclotron. They measured the spin and found a polarization of 0.368 ± 0.023.

However, they noticed the spin wasn't as high as it theoretically could be (which would be 1.0). They suspect two things might be causing this:

The settings on their ion source (the machine that creates the beam) might not be perfectly tuned yet.
There might be a "depolarizing effect" happening at a specific energy level (108 MeV) where the particles lose their spin alignment as they speed up.

They didn't solve this mystery in this paper; instead, they flagged it as something that needs more study. They also confirmed that when they used an unpolarized beam, the detector correctly read the spin as zero, proving the machine isn't just making up numbers.

Why This Matters: The "Figure of Merit"
The paper also calculated something called the "figure of merit," which is basically a score for how efficient the detector is. The higher the score, the faster you can get a good measurement. They found that as the energy went up, the score got better, reaching about 0.03 to 0.035 at 650 MeV/nucleon. This is significantly better than their older equipment.

The Bottom Line
This paper doesn't claim to have discovered a new particle or solved the mystery of the universe. Instead, it's a solid engineering and physics report that says: "We built a new spin detector, we tested it at four different speeds, and it works exactly as we hoped." It confirms that scientists can now use this internal target to measure beam polarization quickly and accurately, which is essential for the future "Spin Physics Detector" (SPD) that will be used in the NICA collider. The team has successfully calibrated their tool, paving the way for deeper investigations into the spin structure of matter.

Technical Summary: Nuclotron Internal Target Polarimeter for Deuteron and Proton Beam Polarization Measurements

Problem Statement
Precise measurement of beam polarization is a fundamental requirement for investigating spin-dependent effects at the Nuclotron/NICA accelerator complex at the Joint Institute for Nuclear Research (JINR). The upcoming commissioning of the Spin Physics Detector (SPD) necessitates reliable, continuous monitoring of both deuteron and proton beam polarization across a wide energy range. While polarimeters based on $dp$-elastic scattering exist for lower energies (e.g., 270 MeV), there is a need for a robust polarimetry solution capable of operating at intermediate energies (200–650 MeV/nucleon) using the Nuclotron Internal Target Station (ITS).

Methodology
The study utilized a polarimeter installed downstream of the Nuclotron ITS, designed to measure the asymmetry of $pp$-quasi-elastic scattering. The experimental setup involved:

Beam Source: Polarized ions were generated by the Source of Polarized Ions (SPI). Deuteron beams were prepared in "2–6" and "3–5" spin modes, while proton beams utilized the "1–3" mode.
Target System: A rotating disk inside the vacuum chamber held targets of polyethylene ( $CH_2$ ) and carbon ( $C$ ). The $CH_2$ target served as the hydrogen source, while the carbon target allowed for background subtraction to isolate the hydrogen contribution ( $CH_2 - C$ ).
Detection System: A ring of 24 plastic scintillation counters was arranged according to $pp$-elastic scattering kinematics in the beam-circulation plane. Detectors covered center-of-mass (c.m.) angles from approximately 55° to 125°. A pair of detectors at 90° c.m. (where analyzing power $A_y = 0$ ) served as a luminosity monitor.
Data Acquisition: Events were selected based on energy loss correlations and time-of-flight differences between conjugated detectors. A $CH_2 - C$ subtraction procedure was applied to remove carbon background, though the study also evaluated the viability of using $CH_2$ data without subtraction.
Analysis: Beam polarization ( $p_y$ ) was derived from the asymmetry of scattering yields on the left and right sides, utilizing known analyzing powers ( $A_y$ ) approximated via a parametric function fitted to world data on $pp$-elastic scattering.

Key Contributions

Implementation of a New Polarimeter: The paper details the installation and operation of a polarimeter based on $pp$-quasi-elastic scattering at the Nuclotron ITS, specifically tailored for intermediate energies.
Validation of Methodology: The study demonstrates that the analyzing power of $pp$-quasi-elastic scattering on polyethylene is consistent with that of $pp$-elastic scattering within experimental precision, validating the use of existing $pp$-elastic parametrizations for polarimetry in this energy range.
Efficiency Optimization: The research investigates the "Figure of Merit" ( $F$ ) under different event selection criteria (with/without carbon subtraction and without kinematic cuts), showing that the method is robust and potentially suitable for fast on-line polarimetry without extensive off-line analysis.

Results

Deuteron Beam Polarization: Vector polarization ( $p_y$ $p_{y}$ ) was successfully measured at 200, 500, 550, and 650 MeV/nucleon.
- At 500 MeV/nucleon, $p_y$ values were $0.248 \pm 0.012$ (stat) $\pm 0.009$ (syst) for mode "2–6" and $0.246 \pm 0.013 \pm 0.012$ for mode "3–5".
- At 650 MeV/nucleon, values were $0.241 \pm 0.023 \pm 0.008$ ("2–6") and $0.266 \pm 0.023 \pm 0.008$ ("3–5").
- These results showed good agreement with independent measurements taken at 270 MeV using a $dp$-elastic scattering polarimeter.
Proton Beam Polarization: For the first time, a polarized proton beam was accelerated at the Nuclotron up to 500 MeV. The measured polarization for the "1–3" spin mode was $0.368 \pm 0.023$ . The false asymmetry measured with the unpolarized beam was consistent with zero ( $0.038 \pm 0.023$ ).
Figure of Merit: The polarimeter's figure of merit ( $F$ ) increased with energy, reaching approximately $0.033$ at 650 MeV/nucleon. This value is noted to be significantly larger than that of the $dp$-elastic polarimeter at 270 MeV.
Analyzing Power: Effective analyzing powers for proton scattering on $CH_2$ were determined and found to be well-described by the parametrization of $pp$-elastic scattering, confirming that inelastic contributions are negligible at these energies.

Significance
The paper establishes that the Nuclotron internal target polarimeter is a reliable tool for measuring beam polarization in the 200–650 MeV/nucleon range. The successful measurement of the proton beam polarization at 500 MeV marks a milestone for the facility, enabling future spin physics experiments. The authors conclude that the polarimeter can be used for both standard and fast on-line modes, supporting the broader scientific program of the SPD at NICA. The modest proton polarization observed ( $\sim 0.37$ ) is attributed to non-optimal SPI settings and potential depolarization at the integer resonance $\gamma G = 2$ (108 MeV), indicating a need for further optimization of spin manipulation during acceleration.

The Spin Doctor's Toolkit

The Paper's Story: A Spin Detective at the Nuclotron

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