ERC Consolidator Grant · 2019–2025

INITIUM

Negative-ion transport for low-diffusion optical TPCs.

INITIUM developed negative-ion drift for optically read out gaseous time projection chambers (TPCs). Primary electrons attach to an electronegative gas component, and the resulting negative ions transport the charge with lower diffusion. This helps preserve recoil topology over long drift distances.

See how negative-ion drift works

View the official project record

Exploded technical drawing of the 0.4 cubic metre INITIUM and CYGNO04 detector demonstrator — Exploded technical drawing of the 0.4 m³ demonstrator.

Project at a glance

Project scope and funding

Name: INITIUM
Full name: an Innovative Negative Ion TIme projection chamber for Underground dark Matter searches
Programme: Horizon 2020 · ERC Consolidator Grant
Grant agreement: 818744 · DOI 10.3030/818744
Principal Investigator: Elisabetta Baracchini
Coordinator: Gran Sasso Science Institute
Beneficiaries: GSSI and INFN
Start date: 1 March 2019
ERC grant end date: 31 August 2025
Total cost / EU contribution: €1,995,719
Experimental location: Laboratori Nazionali del Gran Sasso

From proposal to demonstrator. INITIUM was proposed as a 1 m³ negative-ion TPC. The final objective became a 0.4 m³ demonstrator adapted to the available underground space and designed as a detector scale-up step.

This project received funding from the European Union’s Horizon 2020 research and innovation programme under ERC Consolidator Grant Agreement No. 818744.

The tracking limit

A long drift can blur a short track.

A low-energy interaction can leave a millimetre-scale ionization track. Diffusion broadens the drifting charge distribution, progressively reducing the spatial information carried by the original recoil.

Short driftLong drift

Diffusion grows in three dimensions: a longer drift increases both transverse spread and broadening along the drift direction.

Negative-ion drift

Negative-ion drift reduces charge diffusion

The diagram follows one sampled ionization track through electron drift and negative-ion drift. Scroll timing is normalized; it is not a detector timing measurement.

Qualitative transport model

01 · Primary ionization

Transport sequence

100%

01
Primary ionization
A particle creates primary electrons along an ionization track. Both lanes start from the same sampled charge distribution.
02
Electron attachment
In the INITIUM mixture, the primary electrons attach rapidly to SF₆. The resulting negative ions then carry the charge.
03
Charge transport
Free electrons drift faster and broaden more. Negative ions drift more slowly, with lower longitudinal and transverse diffusion under the conditions represented here.
04
GEM amplification
High fields near the GEM holes detach electrons from the negative ions and initiate an electron avalanche. CF₄ scintillation produces light in both transport regimes.
05
Optical readout
Scientific cameras image the avalanche light, while PMTs record its time development. Both panels show the same initial event after transport.

Design considerations

Negative-ion drift can suppress diffusion without a magnetic field. The gas mixture, attachment and detachment processes, GEM gain and acquisition window must be optimized for slower and potentially multi-species charge arrival.

Transport comparison

Electron drift and negative-ion drift in a shared optical TPC

CYGNO and INITIUM share detector architecture, prototypes, infrastructure and analysis methods. Their principal difference here is the carrier used to transport the primary ionization charge.

Detector question	CYGNO electron drift	INITIUM negative-ion drift
Shared readoutTriple-GEM amplification · scientific cameras for transverse topology · PMTs for time information
Charge transport	Primary electrons remain the drifting carriers	Primary electrons attach to an electronegative additive and drift as negative ions
Gas strategy	He/CF₄-based electron-drift mixtures	He/CF₄ with a controlled electronegative component; INITIUM studies use SF₆
Arrival and diffusion	Higher-mobility charge arrives earlier; longitudinal and transverse diffusion increase with drift distance	Lower-mobility charge arrives later; collisional thermalization can keep diffusion near the thermal limit under suitable fields
Topology at long drift	Diffusion progressively broadens fine spatial structure	Reduced diffusion retains more of the original ionization topology
Detector design	Field, gas, gain and triggering are optimized while managing long-drift diffusion	Attachment, mixture stability, high-field detachment, amplification and slower or multi-species arrival must be optimized together

Prototype programme

From mixture studies to CYGNO04

01 · Mixtures

MANGO

10 × 10 cm² area · 1–15 cm drift

One sCMOS camera and one PMT for gas-mixture, amplification and negative-ion drift studies.

02 · Integration

LEMOn

20 × 24 cm² area · 20 cm drift

One sCMOS camera and one PMT for intermediate-scale optical detector development.

03 · Underground

LIME

33 × 33 cm² area · 50 cm drift

One sCMOS camera and four PMTs in a 50-litre underground module for long-term operation, shielding and background studies.

Hall F underground infrastructure prepared for the 0.4 cubic metre demonstrator at LNGS — Hall F infrastructure for the 0.4 m³ detector at LNGS.

04 · Scale-up

CYGNO04

0.4 m³ underground demonstrator

Explore CYGNO04

Results

Selected programme results

01

Atmospheric-pressure NID

MANGO observed optical negative-ion drift at 900 ± 7 mbar in He:CF₄:SF₆.

02

Underground operation

LIME tested long-term underground operation, detector services, shielding and background monitoring.

03

Optical and timing readout

sCMOS images record transverse topology; PMT waveforms provide time information along the drift direction.

04

Low-energy topology

The programme studied tracking, directionality and particle identification for electron, nuclear-recoil and alpha populations.

05

Background measurements

Internal and external backgrounds were measured and compared with detector simulations in the optical volume.

06

Detector scale-up

Prototype development led to the 0.4 m³ CYGNO04 demonstrator.

PREPRINT · 2026

First Optical Observation of Negative Ion Drift at Surface Pressure

F. D. Amaro et al. (CYGNO/INITIUM Collaboration)

MANGO operated at 900 ± 7 mbar with He:CF₄:SF₆ and provided optical and timing signatures of negative-ion transport. Measurements at 650 mbar also resolved a faster minority carrier population alongside the dominant SF₆⁻ species.

Preprint Submitted to Physical Review Letters

Read on arXiv

Raw MANGO camera images comparing broader alpha tracks in electron drift with sharper tracks in negative-ion drift — Raw MANGO sCMOS images at the same pixel scale: electron drift in He:CF₄ (left) and negative-ion drift in He:CF₄:SF₆ (right).

After the ERC project

Integration with the CYGNO programme

The ERC project ended in 2025. Its negative-ion gas studies, amplification work and detector development continue within CYGNO, with CYGNO04 as the next integration step.