Resistive Plate Chambers as thermal neutron detectors DIAMINE Collaboration WP-2 BARI M. Abbrescia, G. Iaselli, T. Mongelli, A. Ranieri, R. Trentadue, V. Paticchio 8th Topical Seminar on Innovative Particle and Radiation Detectors October 2002 Siena, Italy

2 Outline of the talk Reasons to build RPCs for thermal neutrons and the Gd-choice The method used to build Gd-bakelite RPCs Expected performance and possible options Some experimental results

3 Reasons for new thermal neutron detectors For instance...The humanitarian demining problem Neutron Backscattering Tecnique (NBT) Metal Detectors not effective against anti- personell mines: The signature of the presence of a mine is an increase in the number of thermal neutrons from the ground Neutrons are emitted from a 252 Cf source and are revealed after interaction in the ground

4 Why RPCs for thermal neutron detection? Bakelite electrodes Gap: 2 mm HV electrodes: graphite 100  m Operating pressure: ~ 1 Atm Gas flow: 0.1 vol/ora Al or Cu read-out electrodes bakelite resistivity  cm electrodes treated with linseed oil RPCs are easy to build, mechanically robust, light-weighted, cheap, can cover large surfaces, are adapt for industrial production, etc. particularly suitable for “on-field” applications

5 Neutron Detection Neutrons can be revealed only after the interaction in a suitable material The choice of the converter is crucial for the performance of the detector Production of secondary ionising particles

6 Gaseous detectors with solid converters Compromise between: Disadvantage: the particle produced after the conversion has to escape from the converter and enter the detector active volume to be revealed large conversion probability large thickness large escape probability small thickness Advantage: high density High macroscopic cross section  =   N (N: # cent. of scattering/cm 3 )

7 Natural Gd Nat. Gd has the following isotopic composition: As a consequence of the capture process of a thermal neutron, Gd produces, in the 60% of cases, an electron from internal conversion “interesting” isotopes are about 30% “complex” energy spectrum

8 Reasons for the nat.Gd choice (compared to the “standard candle”) Natural Gd is characterized by a thermal neutron  (  50 kbarn) 12 times larger than 10 B  (3840 barn) Produced electron range (15-30  m) is >than  ’s (3-4  m) Beyond E=100 meV, Gd cross section decreases much more rapidly than the one of 10 B For E  1 eV it is smaller than the one of 10 B. For application concerning only thermal neutron detection we have preferred Gd to 10 B

9 The use of Gd as a converter Gd is a metal, weakly reacting in humid air, where it oxidises. It is cheap, except when required in very thin layers (order of  m). Gadolinium Oxide Gd 2 O 3 (vulg. “Gadolina”) is a white inert powder (easy to handle), with granuli of 1-3  m in diameter, very cheap. It is difficult and expensive to obtain Gd enriched in 157 Gd (material of strategic interest)

10 The layer of converter Mirror surfaces … It is constituted by Gd 2 O 3 mixed with linseed oil; the mixture is sprayed onto the bakelite electrodes, which are used to build standard RPCs. Linseed oil is standardly used on the inner surfaces of RPCs built with bakelite (but it is deposed in a different way). It is used by the future LHC experiments, by ARGO, OPERA, etc. (also by BABAR…) Thanks to A. Valentini

11 The advantages of the method 4.It can be used for industrial-scale applications (as required for practical uses), and factories have a great experience about it: it is the very same method used to paint cars … 1.It is possible to obtain extremely uniform layers, with very constant thickness and density 2.The electric properties (surface resistivity) of bakelite electrodes are not altered 3.It is a method easily appliable to surfaces having large dimensions

12 The performance with Gd-RPCs backward e - Since neutron intensity, in Gd, decreases exponentially, just the “first layer” “takes part” to the conversion process Backward e - have always the same thickness to cross Layer thickness is not important (in the backward configuration) Bakelite RPC sensitivity to thermal neutrons: about 1/1000 RPC with 10 B: ~5% (note that half of  are lost into the bakelite)

13 The chambers 2 with a different concentration of the oil-Gd 2 O 3 mixture 3 RPCs 10x10 cm 2 in dimensions 1 without Gd 2 O 3, used as a reference Gas High Voltage Signal readout

14 How the story goes on… The chambers have been brought to Geel, where we could use GELINA Geel Electron Linear Accelerator An e - beam on an Uranium target produces, for Bremsstrahlung,  which, in turn, produce, via photonuclear emission, neutrons Energy: from a few meV to 20 Mev 12 flightpaths: from 8 to 200 m Peak Yield: 4.5x10 19 n/s Average Yield 3.4x10x 11 n/s

15 How the system works U e-e- RPCRPC CICI TDC1TDC1 TDC2TDC2 t 0 start DAQ t n stop to a multihit TDC CI: two layers of 10 B of 0.35  m each

16 The chambers at Geel “Frame” in plastic material (the RPCs are in plastic material too…) “Backward” configuration Flightpath=15 m (CI = 13.5 m) n e-e- e-e- Gd 2 O 3 RPC

17 Some “raw data” Ionisation Chamber RPC “HighConc” Gd Measured: Time Of Flight (t- t 0 ) Spectra acquired at the same time for RPC and CI comparison between RPC and CI Two regions: thermal n resonances

18 The results (after calibration) Energy resolution for RPC worse than for CI...who cares? Some peaks are present only in RPC the spectrum: peaks of the Gd cross section Ag W Spectra in the resonance zone (few eV) Resonances due to the presence of filters on the beam: Ag, W, Na, S, Co Ionisation Chamber RPC “HighConc” Gd

19 The thermal neutron region Relative efficiency: Conversion efficiency of 10 B: well known “Roughly”  2.5-3

20 The background of the measure How to measure background: use a Cd filter opaque to neutrons with E kin < 0.5 eV (Cd “cutoff”) Advantages: data coming from the same chamber (also for CI) run in the same conditions Noise distributed uniformly in time and not in energy

21 Efficiency Subtracting the background … Integral efficiency Differential efficiency

22 Conclusions We have developed and demonstrated the feasibility of this simple method (useful for practical and industrial applications) but very effective, to make out of RPCs detectors for thermal neutrons RPC-Gd experimental efficiency is > 10 B theoretical maximum eff. >> 10 B-RPC experimental efficiency Coupling two of these detectors together efficiency reaches about eff. CI (analysys in progress) We are still far from max. th. eff. for Gd-RPC... Possible improvements: Gd concentration optmisation, linseed oil polimerisation procedure, more layers,...