NSCL logo National Superconducting Cyclotron Laboratory
NSCL

A new neutron wall for the NSCL

A new large-area neutron detector is proposed to be built at the NSCL. This new detector is planned to be used in connection with the new Sweeper Magnet and the S800 magnetic spectrograph.

The Sweeper Magnet is a 4 Tesla dipole magnet and a new development that will be ready after the cyclotron upgrade is completed. Many experiments involve neutron-rich, weakly bound nuclei, and often the final states will be neutron-unbound. To measure these states it is necessary to detect the decay products, i.e. a charged particle in coincidence with a fast neutron, both typically emitted close to the beam axis. The purpose of the sweeper magnet is to deflect the charged fragments from the axis to a shielded location at larger angles, where the charged particles are stopped or detected. For the detection of the charged particles, a detector array or the S800 spectrograph can be used. The neutrons will be detected under zero degree further down the beam axis. The coupled cyclotron in combination with the A1900 beam analysis system will be able to deliver very neutron rich beams at energies up to 200 MeV per nucleon.

The existing neutron walls have a deficiency in detection efficiency for neutrons of higher energies. A detection efficiency of 20% is only reached for neutrons of 50 MeV, it drops to only 12% for neutrons of 100 MeV or more. This limits their use to low energy beams and low multiplicity experiments.

Design goals

Listed here are the basic features that the detector should have:

Design ideas

The LAND detector at GSI, Germany, gives a good example of a large area neutron detector that utilizes a combination of plastic scintillator and passive iron converter.

The LAND detector covers an area of 2 m by 2 m. It consists of 200 paddles that are 2 m long, and 10 cm by 10 cm in cross section. Each paddle is a sandwich of 5 mm thick iron sheets and 5 mm thick scintillator, alternatingly stacked (see left figure below). One photo-multiplier tube is fitted on each end of each paddle, enabling the determination of the neutron interaction position through the time difference of the signals.

The detection efficiency of the LAND detector (see above figure on the right) is above 80% for energies larger than 400 MeV. Below this energy, the efficiency drops rapidly with energy. To build a detector for the energy range that we are interested in, this design has to be changed, and it has to be shown that the use of passive converters is still advantageous, even for energies below 250 MeV.

To investigate this question extensive simulations using the detector description and simulation tool GEANT are being performed.

The effect of the passive converter

Since neutrons can not be detected directly, nuclear interactions where the neutrons create charged particles are needed. The reason why one would put iron into the detector is the shorter interaction length that iron has. For high energy neutrons, it is roughly 4.7 times shorter than in plastic. This means that there are much more interactions that can produce charged particles in iron than in plastic of the same thickness. The trick is to balance and mix the passive converter and the active scintillator in such a way that there is an enhancement of detection efficiency.

How this could be done even for energies below 250 MeV is illustrated in the following:

Simulations for a 2 m by 2 m large neutron wall were done. In the first case, the neutron wall consisted of 60 cm pure plastic scintillator, arranged in 10 cm by 10cm large blocks (blue line). In a second case, a part of the plastic was exchanged by iron (see figure below). Here, the fourth to sixth layer consisted of 2 cm iron plus 8 cm scintillator each. Although the overall volume of the detector stayed constant, compared to the pure plastic scintillator case, an enhanced detection efficiency for energies above 125 MeV can be seen (dashed red line). In a real detector design it would be simpler to just add the 2 cm iron layers, this is also shown (solid red line).



These simulations show clearly that the passive converter can enhance the detection efficiency for neutrons with energies between 100 and 250 MeV.

Experimental data for the neutron detection efficiency of an iron-plastic combination are scarce, especially regarding neutron energies between 50 and 250 MeV. In order to verify the above mentioned simulations, a test experiment is being undertaken at the RIKEN accelerator facility in Tokyo, Japan. The RIKEN laboratory has a neutron detector consisting of 6 by 6 by 108 cm3 blocks of plastic scintillator. Six of these blocks are being used for a relative measurement of the detection efficiency of a pure plastic detector and a detector equipped with 3 cm of iron converter.

The design of the NSCL detector is not yet finished. In order to reach an efficiency of 70%, roughly 90 cm of plastic scintillator plus the layers of converter material are needed.

Links to other documents

Detailed internal report (pdf file) on the design of the neutron detector:
Design of a neutron detector
This report is a summary of the ongoing work and will be updated frequently.


Last modified: Mon Feb 19 12:24:40 EST 2001 created by Thomas Baumann