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Gaseous Detectors as Diagnostics/PID for Heavy Ions

DRD1 Gaseous Detector School

8-17 July 2026

FRIB (Michigan, MSU)

Elena Rocco

GSI Helmholtzzentrum für Schwerionenforschung

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Outline

What is this talk about?
What a Fragment Separator does
Fragment Separators around the world: some highlights
Most important: detectors in use at the fragment separators
Conclusions

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Special thanks to: D. Morrissey, S. di Carlo, H. Simon, C. Nociforo, E. Haettner, T. Dickel, J. Eder, S. Michimasa, S. Pietri

Disclamer: This presentation is not intended to be exhaustive or fully comprehensive. Given the breadth of the topic and the limited time available, not all facilities can be covered with the same level of detail. I hope this overview will nevertheless provide a useful introduction to the main concepts and experimental approaches for students.

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What is this talk about?

We are at FRIB (Facility of Rare Isotope Beams)!!!
A different set of challenges compared to HEP experiments

Sensitivity to inhomogeneity of materials
Wide spectrum of detected charge, mass with the same detector

“Old” but very effective detector technology

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?

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Fragment Separator

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Isotope Separation On-Line (ISOL) vs In-Flight Separation

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ISOL	In-flight
Thick target	Thin target
Isotopes are extracted from the target after diffusion and ionisation	Fragments leave the target immediately with nearly the same velocity of the primary beam
Separation with a mass spectrometer	Separation with a magnetic fragment separator
High-quality beams; low emittance and excellent energy resolution	High-energy beams can study very short-lived nuclei

(Light ions)

(Heavy ions)

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Fragment Separator: What & Why

Products (called fragments) are identified event-by-event during setup and then the system is usually adjusted to deliver the desired fragment to an on-line experiment.

Advantages of in-flight production scheme:

Essentially no limitation on the decay of half-life of (beta decaying) isotopes
No chemical dependence on the chemical nature of the product
Rare isotope products have a forward focus due to kinematics
No acceleration needed

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A fragment separator is a magnetic spectrometer that selects (with detectors to identify) rare isotopes produced via projectile fragmentation, spallation, or fission reactions from a high-energy heavy-ion beam. It typically consists of dipole magnets, slits, detectors and energy degrader arranged along a beamline

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Fragment Separator: what needs to be measured

Example case ²³⁸U on ¹²C to get ²¹²Pb

Coming out if the target unreacted Uranium (~65% of beam) & ~ 1000 nuclei
stage 1 removes the beam and a large fraction unwanted nuclei
stage 2 removes almost all of remaining unwanted nuclei (case dependent)

Detectors for measuring fragments

at the degrader Z & position (XY)
before the experiment setup, Z, A, Q (for unique identification)

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Example of fragmentation and separation: Goal is ²¹²Pb

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²³⁸U

@1GeV/u

¹²C

7x105 pps after magnet+degrader

Beam intensity: 10¹⁰ pps

746 pps (ca 120 pps ²¹²Pb)

¹²C target: 13 mm thick

Lise++ calculation of the S-FRS pre-separator

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Or another way to see it is:

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courtesy of D. Morrissey

@FRIB

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Fragment Separator: how?

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Particle identification: TOF-Bρ-ΔE method

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Basic Relations for Fragment Identification

The event-by-event identification of reaction products is based on a combination of magnetic rigidity, energy-loss, and residual-energy measurements. These quantities are directly related to the fundamental properties of the ion, namely its mass number AAA, atomic number ZZZ, and neutron number NNN.

The magnetic rigidity is defined as

Bρ=pq=γmvq,B\rho=\frac{p}{q}=\frac{\gamma mv}{q},Bρ=qp=qγmv, where ppp is the ion momentum, qqq its ionic charge, mmm its mass, vvv its velocity, and γ\gammaγ the Lorentz factor. Together with the velocity measured by the Time-of-Flight (TOF) system, BρB\rhoBρ provides the ion mass-to-charge ratio (A/q)(A/q)(A/q).

The energy loss in a thin detector approximately follows

ΔE∝Z2β2,\Delta E \propto \frac{Z^2}{\beta^2},ΔE∝β2Z2, where β=v/c\beta=v/cβ=v/c. This dependence allows the determination of the atomic number ZZZ.

The residual energy deposited in the stopping detector scales approximately as

Eres∝Z2,E_{\mathrm{res}} \propto Z^2,Eres∝Z2, providing an independent confirmation of the charge identification.

Once the ion mass is obtained,

A=mu,A=\frac{m}{u},A=um, where uuu is the atomic mass unit, the neutron number is finally determined from

N=A−Z.N=A-Z.N=A−Z. Together, these relations enable an unambiguous event-by-event identification of each transmitted ion.

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Fragments Separators in the world

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Different isotopes discovered in the different facilities

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Various production mechanism use to create the isotopes

The brown curve indicates the in-flight fragmentation (ca 1000 nuclides)

Fusion-evaporation reaction

Light-charged-induced reaction

Mass spectroscopy of stable isotopes

Neutrons-induced reactions

In-Flight fragmentation

Naturally radioactive-decaying isotopes

Total

M. Thoennessen & A. Gade, Nature Physics 20, 1844-1845 (2024)

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What we can learn from it

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Nuclear Physics

Understanding of complex nuclei from nucleon nucleon forces
Exotic beams far from stability → bench mark for theory more and more nuclei accessible “ab-initio” due to advances in computing and methods

Astrophysics

How are heavy elements produced (e.g. Gold, Platinum)
Star evolution post Iron (Fusion Limit)➔ Explosions (Supernovae), Merger (Kilonovae) with massive neutron flow

IUPAP Report 41 Introduction DOI:10.48550/arXiv.1805.06794

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Focus on: ARIS (FRIB), BigRIPS (RIKEN), FRS/S-FRS (GSI-FAIR)

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Parameters	FRIB (ARIS)	BigRIPS	FRS	Super-FRS
Location	MSU, USA	RIKEN, Japan	GSI, Germany	FAIR, Germany
Primary Beams	Up to U	Up to U	Up to U	Up to U
Beam Energy (MeV/u)	200 (310)	345	1000+	1500+
Beam Intensity ²³⁸U/s	5x10¹³	6x10¹²	1x10⁹	3.5x10¹¹
Acceptances (Δθ,ΔΦ, Δp/p)	±40 mrad, ±40 mrad, ±5%	±40 mrad, ±50 mrad, ±3%	±20 mrad, ±10 mrad, ±1%	±40 mrad, ±20 mrad, ±2.5%
Separator stages	4	4	2	4
Operational since	2022	2007	1990	2028-2033

T. Kubo, NIM B 376 (2016) 102-110

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GSI Fragment Separator (1/2)

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SIS

18 Tm

ESR 10 Tm

FRS

18 Tm

CRYRING

1.4 Tm

Production target

Cave M

Cave C

Injection from UNILAC

1. and 2. Ion sources

3. and 4. UNILAC linear accelerator

5. Experimental Hall (Low Energy)

6. Beam Transfer Line

7. SIS18 Accelerator

8. FRS Fragment Separator

9. ESR Storage Ring

10. HITRAP Experiment and Re-Injector

11. Experiments (High Energy)

12. CRYRING

*

H. Geissel et al., NIM B 70 (1992) 286-297

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GSI Fragment Separator (2/2)

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75 m

F1

F2

F4-Experimental hall

Target

Beam

Dipole

Quadrupole

F3

Every focal plane is equipped with detectors for beam diagnostics

2 orders of magnitude in rate between F2 and F4

Separation and identification

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FAIR: Facility for Antiproton and Ion Research

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GSI

FAIR

Increase in primary

beam intensity 10-100

J. Blaurock ECE/ECSC FAIR meeting April 2026

Comparison FRS vs S-FRS

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Super-Conducting Fragment Separator at FAIR

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pre-target area

target area

pre-separator

main separator

ring brunch

high energy brunch

low energy brunch

Technical Design Report, H. Geissel, M. Winkler, H. Weick et al., (2009)

Tracking: SciFi/GEM TPC

ΔE: MUSIC

ToF: Sci

after 2028

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Facility for Rare Isotope Beams: FRIB

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Pre-separator

Stopped Beam Area

15 m underground

Ground level

Presentation of Z. Constan

Up to 200 MeV/u, high intensity beam, ramping-up

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@ FRIB: Advanced Rare Isotope Separator (ARIS)

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Pre-Separator Wedge

PPAC pair*

Not standard

PPAC pair

Diamond

Thin Sci

PPAC

Thin

Sci

Thin Sci

PPACS

Sci / Diamond

Pins

TKE

Thick Sci

PINs

Tracking

ToF

dE

E

Event-by-Event Identification

HPGe

(X,Y)

ToF

dE

DB1

(Pre-Separator Focal Plane)

DB2

DB3

DB4

DB5

(Final

Focal Plane)

Target

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RIKEN Nishina Center RI Beam Factory (RIBF)

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Rare RI Ring

OEDO

SAMURAI

SCRIT

ZeroDegree

ECR

BigRIPS

Courtesy Shin’ichiro Michimas

Rare Isotope Beam Factory

Intense beams for

secondary beam production

T. Kubo, NIM B 204 (2003) 97

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Superconducting Radioactive Isotope Beam Separator (BigRIPS)

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Primary Beams ⁴He-²³⁸U @ 345MeV/u

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Detectors as beam diagnostic/PID

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Gaseous Detectors also for Beam Monitoring

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Beam profile monitor: a grid of 7.2 x7.2 cm² active area.
No gas amplification. Conceived to be used at several pressure depending on the ion species
Used for slow and fast extraction (few mbar vacuum)

Ar/CO₂ 90/10 @ 1 bar

Current Grid in the target area. Spatial resolution 0.5 mm

X-Coord

Y-Coord

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Tracking: Parallel Plate Avalanche Counter (PPAC/DL-PPAC) (1/3)

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S. di Carlo et al., 2026 JINST 21 P03022 and references therein

Segmented for localisation

Detector principle

Operated at low pressure (up to 15 Torr)

Detector active area 10 x 10 cm²

vessel with 2 aluminised mylar windows 6 µm thick

Upgrade: delay-line

Segmented (1mm pitch) foils

coated by evaporation with ~150 nm metallic layer

Segmented foils substituted with gold plated tungsten wires (12µm)

LAB 6

S. di Carlo & M. Cortesi

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Tracking: Parallel Plate Avalanche Counter (PPAC/DL-PPAC) (2/3)

Anti Discharge Unit (ADU) is an active electronic protection to supress energetic electrical discharges in gaseous proportional counters (S. di Carlo et al., NIM A 1068 (2024) 169785)

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ADU functional block diagram integrated in a PPAC

Recovery time 15ms

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Tracking: Parallel Plate Avalanche Counter (PPAC/DL-PPAC) (3/3)

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Test performed with primary beam ²³⁸U⁹⁰⁺@ 143 MeV/u at the same time

Full detection efficiency up to 300KHz, σ_spatial 0.5 mm, σ_time 400-600 ps with cocktail beam from Z≤15

F. Sauli NIM A 477 (2002) 1

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Tracking: FRS Time Project Chamber (TPC)(1/2)

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2 TPCs in F2 (and 2 in air not shown here below), 1TPC in F3, 2 TPC in F4

For every TPC there are:

2 measurements in X coord.
4 measurements in Y coord.

Field cage from 6/8 cm (Field 400V/cm for all the TPCs) terminating with a gating grid

B. Sitar et al., NIM A 419 (1998) 503

A. Janik et al., NIM A 640(2011) 54-57

Gas: Ar/CH₄ 90/10 (P10)

Delay lines as readout

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Tracking: FRS Time Project Chamber (TPC)(2/2)

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Time resolution

Intrinsic Spatial resolution

Spatial resolution after track reconstruction

¹⁷⁰Er primary beam at ~900 MeV/u

X-coordinate

Y-coordinate

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Tracking: Super-FRS GEM TPC

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beam

pocket window

A. Prochazka et al. GSI report (2012)

2 detectors opposite drift fields

Requirements

Prototype size half of the final detector

First detector production series by end of 2026

Second Field cage

FEE

beam

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ΔE:MUlti- Sample Ionisation Chamber – MUSIC (1/5)

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Working Principle

The introduction of the Frisch grid allows to decouple the ionisation position from the anode signal

Courtesy of

M. Cortesi

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ΔE:MUlti- Sample Ionisation Chamber – MUSIC (2/5)

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MUSIC at S-FRS

MUSIC at FRS

MUSIC at RIKEN

K.Kimura et al., NIM A 538 (2005) 608-614

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ΔE:MUlti- Sample Ionisation Chamber – MUSIC (3/5)

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Xenon-gas ionization chamber for high-Z beams arXiv:2401.08679 M. Yoshimoto

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ΔE:MUlti- Sample Ionisation Chamber – MUSIC (4/5)

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Xenon-gas ionization chamber for high-Z beams arXiv:2401.08679 M. Yoshimoto

Isotope Identification

Improved resolution for high Z using a different gas mixture!

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ΔE:MUlti- Sample Ionisation Chamber – MUSIC (5/5)

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Take home messages…

Fragment separators allow investigation of isotopes elements in the nuclide chart which otherwise would not be possible to access. Several facilities are operating around the world. This school is hosted by one of the largest one!

Beam diagnostic instrumentation benefits from gaseous detectors, due in part to their very low material budget

Detector challenges at fragmentation facilities are different from those at HEP facilities, however this leads to a lively R&D field!

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AND THANKS FOR YOUR ATTENTION

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SPARES

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Gaseous Detectors also for Beam Monitoring (2/2)

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Sensitivity curve of the Seetram for various projectiles. Plotted is the number of projectiles required to trigger a secondary electron current of 10⁻¹³ amperes from the Seetram's signal foil, as a function of projectile energy. An extraction time of one second is assumed. The number of projectiles per unit of secondary electron charge was calculated from the specific energy loss using the scaling factor determined in the previous chapter.

Intensity monitoring

SEcondary Electron Transmission Monitor = SEETRAM

Beam

Ti Foil 10µm

secondary e^- emitted

The SEETRAM measurement provides:

Spill structure information
Extraction Efficiency information
total number of projectiles (cross-section normalisation)

A

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Fragmentation versus Fission

Productions mechanisms used:
Fragmentation

momentum spread of fragment increases with number of nucleus abrated
for production, target always Be (or C)

Fission,

high angle and momentum due to repulsion. Effect notable at 1 GeV/U
Target either Pb for coulomb fission or Be for fragmentation fission
Mechanism always present, Pb fissions at high momentum

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Fission

Fragmentation

Courtesy of S. Pietri

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Traking: FRS Time Project Chamber (TPC)

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Marco Cortesi’s Presentation

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Marco Cortesi’s Presentation

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