The Neutron Spectrometer



The DESPEC neutron Time Of Flight Spectrometer will be used for measuring the neutron emission probabilities and energies emitted in the decay of neutron rich isotopes with relevance to basic nuclear physics and nuclear technology. The measurements will be performed alone or in combination with the DESPEC Ge array.


Requirements for the TOF spectrometer (conclusions of the Madrid meeting pointed out by N. Orr - LPC Caen):


n-g discrimination

mandatory for reducing backgrounds, enable b-2n,3n ... detection
Liquid scintillators (or "new" solid scintillators) NE213 + EJ309 (only for short detectors), EJ325 (?, inexpensive).

Longer bars, higher threshold!

Plastics/crystals like Stylbene.

The best n/g discrimination comes from a granular array. Test NE213 with various PMT, EJ309 and EJ325 pushing the thresholds down as much as possible.


Energy resolution

Improve DEn/En and eliminate "tails" in TOF spectra visible in TONERRE (BC400 plastics)

Thin, small volume detectors with increased dflight~3,4 m (space for both n and g setups)

Neutron energy range

0 < En < 10 - 15 MeV with a reasonable efficiency.

To have the lowest possible threshold in En requires thin, small volume detectors (no bars) + digital electronics (30 keV En)



The cross-talk rejection will enable b-2n, 3n... detection.
A modular, highly granular array (>100 detectors) and with variable geometry (cm alignment precision) will be necessary. 2 options: DEMON-like or TONERRE-like (curved or straight bars). The cross talk rejection will be performed by time and energy deposited in the detectors.


Neutron detection efficiency

The detection efficiency should be as high as possible over the entire energy range.

Solid angle to be covered should be as high as possible: 1p DWn 2p depending on the type of experiment.


g-ray detection efficiency

Should be also as high as possible for the identification of neutron decay to excited states

Large volume high efficiency Ge detectors will be necessary, which is in conflict with the need for high DWn.


Digital electronics

Will be used for a better control of systematic uncertainties. Flash ADCs with 1 GSample/s and resolution larger or equal than 10 bits are requested. Pulse shape analysis algorithms will be programmed on a PC farm or FPGAs.


Other issues

  • b-trigger, investigate the efficiency of the b-trigger with the DSSSD and/or need of additional detectors at SPIRAL (plastics)

  • size of the beam spot defined most probably by the Ge needs

  • have enough space for both neutron and Ge arrays

  • minimise the influence of magnetic fields!

  • minimise the amount of dead material in and around the implantation setup: no B, no Li, no Br, avoid plastics (or materials rich in hydrogen). Al is a \93tentatively good material\94 (low cross sections).



Fresnel law's



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