National Centre for Nuclear Research
Astrophysics Division
Cosmic Ray Laboratory

90-558 Łódź
ul. 28 Pułku Strzelców Kaniowskich 69
tel./fax (+48 +42) 6786431
tel./fax (+48 +42) 6783223

Neutron Research

Participation in the ISOTTA project

The main purpose of the ISOTTA project (ISOTope Trace Analysis) is to create the conditions for the production of a large amount (mass ∼1 ton) of radiopure isotopically enriched material for the performance of a next generation experiments for neutrinoless double beta decay search.

Our task in this project is to construct the shields for germanium counters to reduce neutron background. We are studying processes of neutron transport and interactions comparing results of simulation with GEANT4 package with measurements with the use of the weak neutron source and the germanium counter.

ISOTTA project is carried out by the international consortium in the framework of the European Program ERA-NET ASPERA II.
Polish participation in the ISOTTA project is funded via
The National Centre for Research and Development.

Neutron Measurements in the Underground Laboratories

Slanic (Romania)
Gran Sasso (Italy)

Measurement of the neutron background in the salt mine in Slanic, Romania

Neutrons create hard to eliminate background in such experiments as dark matter search, double beta decay or neutrino studies.

Horia Hulubei National Institute of Physics and Nuclear Engineering (IFIN-HH) in Bucharest maintains a low background laboratory in the publicly available part of Unirea salt mine in Slanic. The site is one of possible localisations of the LAGUNA project. The walls of the mine contain almost exclusively clean salt NaCl, free of potassium, so the expected neutron flux is very low.

From November 19, 2011 till January 10, 2012 we made there measurements of the neutron background. We used a tray of 16 helium counters made by ZDAJ IBJ in the 70s. We have measured thermal neutron flux with naked helium counters and energetic neutrons with helium counters in polyethylene (PE) blocks. Each counter measures thermal neutrons which enter the counter volume and in the nuclear reaction with 3He protons and tritium are produced with total kinetic energy of 764 keV. The helium counters are set in proportional mode and in most of the neutron events this energy is measured. At very low neutron flux density we have a difficult problem with background due to α particles emitted from residual radioactive elements in the detector construction.

For the measurements in Slanic we have used the same set of helium counters and the same acquisition system as we used in measurements made in the underground laboratory in Gran Sasso in 2008. So we can nearly directly compare the neutron background in both underground laboratories.

For thermal neutron measurements only 13 counters were working stably (there are blasts in the mine everyday). As a result we have measured 471 events with energy deposited near to 764 keV, (273.6 events were expected from α background), therefore we have measured 197.4 neutron events in nearly 229 hours of effective measurements. This gives a rate of 1.6 thermal neutron per day per counter and the flux of thermal neutrons was equal to

(1.35±0.32) × 10-7 neutron/cm2/s

(about 4 times lower than in Gran Sasso).

During nonthermal neutron measurements (12 stable helium counters in PE moderator) we have nearly 2295 neutrons (3727 events in 764 keV peak region, and 1432.1 expected as α background) in 35.5 days. This gives a rate of 5.4 neutrons per day per counter which was 1.4 times higher than the corresponding rate in LNGS.

Measurements of neutron background in the Gran Sasso Underground Laboratory.

In april 2008 we have measured thermal neutron background flux in the deep underground Gran Sasso National Laboratory. The measurements were performed in frame of ILIAS-TA-P2007-12-LNGS grant and their goal was to demonstrate the potential sensitivity of our registration method to low fluxes of neutrons.

To register neutrons we used the system of 16 helium counters produced by ZDAJ IPJ over 20 years ago.

Registrations have been performed in two configurations of counters:

Layout "line".

Layout "circles".

We have compared results of our registrations with the predictions of Monte Carlo simulations (GEANT 4) for both configurations of counters
and obtained:

the flux of thermal neutrons

(5.4±1.3) × 10-7 neutron/cm2/s

The result is still preliminary, and the quoted error is only statistical.

More information can be found in the presentation of Karol Jedrzejczak (pdf 1.6 MB) .
Detailed description of registration method and analysis of results can be found in Report for LNGS (pdf 0.6 MB).

We plan further measurements at higher neutron energies at Gran Sasso and in Boulby Underground Laboratory.