The n@BTF project

B. Buonomo, R. Bedogni (Art. 23), A. Esposito, G. Mazzitelli (Resp.), L. Quintieri (Art.23)

The n@BTF technical Staff:M. De Giorgi, M. Chiti, A. Gentile

1 Description of the project

At the Beam Test Facility of the DAΦNE collider, we have designed and realized a photo-neutronsource, by exposing an heavy nuclei target to a high energy electron beam. The n@BTF projectarrived at its end, the main tasks of the experiment being successfully achieved as planned. Thisexperiment has been funded by the CSN5 of INFN for 3 years.

In what follows the main results of n@BTF will be presented and discussed.

1.1 Physics background: the photo-neutron production

The photonuclear physic is well known in accelerator field, mainly because related to shielding issuesfor neutrons, that are produced as a consequence of electron and photon interaction with matter(especially in dumps and shields). The most effective way to shield electromagnetic radiation is touse high atomic number (Z) materials (Lead, Tungsten and so on). On the other side, high Z nucleiexhibit higher cross sections for photonuclear reactions than medium and low Z materials, thatmeans a related production of neutrons and protons that has to be properly taken into accountfor dose calculation and so far in the shielding design.

In this way, what is normally an issue from the radioprotection point of view, has beensuccessfully exploited as main source for our task.

More than 80% of electrons, passing through the target, loose energy by bremsstrahlung: theensuing photon shower interacts with the nuclei exciting them (essentially on the base of GiantDipole Resonance mechanism for photon energy lower than 30 MeV and on the base of QuasiDeuteron Mechanism for higher photon energy). In going back to the fundamental state, theexcited nuclei boil-off a nucleon, typically a neutron (in fact, the emission of protons, that is apossible channel, is strongly repressed in heavy nuclei due to the high Colombian barrier). Theseneutrons are emitted almost isotropically all over the space, with a continuos energy spectrum thatis peaked around the equilibrium nuclear temperature characteristic of the target material.

To estimate the neutron rates and energy spectra, all around the target, we used both FLUKA(20083d release) 1) and MCNPX 2) (MCNPX2.5.0) codes. The detailed description of the feasi-bility study and the related Monte Carlo simulations can be found in the previous edition of theLNF annual report 3) and in ref 4), 5), 6).

2 Final experimental set-up: the feasibility test

The photo-neutron source has been located in the BTF experimental hall, at the end of the transferline and since it is not allowed that the produced neutrons could be scattered everywhere, it wasnecessary to shield closely the whole solid angle around the target, with the exception of welldefined extraction lines, along which we should be able to properly collect neutrons.

The shield is a multilayered structure made of lead and polyethylene. Boron Carbide sheetsare also foreseen to be used in next future. These are already available as inventory of the newneutron facility.

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In Fig. 1 the experimental set-up, installed in April 2010, is shown, while Fig. 2 shows theTungsten target placed inside the shielding apparatus.

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Figure 1: Experimental set-up (for feasibility test).

Figure 2: Target located inside the shielding apparatus.

The measurement campaign took about 15 days (up to 7th of May) and has been done incollaboration with the LNF-FISMEL group, using the Bonner Spheres 7) neutron detectors.

The set of Bonner Spheres (LNF-ERBSS) used for the BTF measurements included:

• 7 polyethylene spheres (density 0.95 g/cm3). The following spheres were exposed: 2”, 3”,5”, 7”, 8”, 10”, 12” diameter spheres.

• Dysprosium activation foils (12 mm diameter and 0.1 mm thickness). These are detector ofthermal neutrons located at the centre of each sphere. The exploited isotope is 164Dy (28.2%abundance in natural Dysprosium)1.

1165Dy is a γ-β emitter with Emax = 1.3 MeV and T1/2 = 2.334 h.

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Figure 3: Extraction lines (with lead caps) are designed in the plane perpendicular respect to theimpinging electron direction. The beam dump is also visible (the larger lead cap in the middle ofthe shield).

For our feasibility test we used a Dysprosium activation foil since it appears to be moresuitable for working in a high photon background.

The Dy-BSS response matrix was calculated with MCNPX on the basis of a 68-groups energyequi-lethargy structure from 1.5E-9 MeV to 1.16E+3 MeV 8).

A validation experiment performed in the ENEA-Bologna 252Cf reference field, allowed esti-mating the response matrix overall uncertainty in ±2.3%. This figure refers to the energy intervalcovered by the 252Cf spectrum, i.e. from 0.1 to 15 MeV.

The spheres with Dy activation foils in their centre were sequentially exposed for an irradiationtime of around 0.5 h for each sphere in a well defined reference point: 149 cm from the target centrealong the 90 direction at 124 cm from the floor.

The foils were counted in a portable beta counter, their specific activity was corrected forthe discrete activation function, the decay from the exposure and the counting, the decay duringcounting, and finally normalized to the number of 510 MeV electrons delivered to the target. Thesevalues were unfolded using the FRUIT code 9).

The energy distribution of the incident electrons was measured by a spectrometer consistingof a pulsed magnet, which deviates the beam from the transfer line, and a 60 bending magnet,which focuses the beam on a system of secondary emission metallic strips 10). The FHWM of thisdistribution is lower than 1%.

The delivered charge per pulse was about 50 pC. To measure the charge and transverse profileof the beam delivered on the tungsten target, a Bergoz ICT (high sensitivity, beam charge to outputcharge ratio 5:1) and two fluorescent screens (beryllium oxide and YAG:CE) were mounted at theexit of the BTF line. The Integrating Current Transformer (ICT) is composed of a capacitivelyshorted transformer and a fast read out transformer with a common magnetic circuit designed tomeasure the charge in a very short pulse with high accuracy.

Variations in the neutron fields due to small instabilities in the electron beam were minimized

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Figure 4: Experimental and MC computed neutron spectra, 149 cm apart from the target at 90o wrto the impinging electron beam.

by keeping the beam position and beam spot dimensions as constant as possible. These parameterscan be easily checked and controlled using wall current monitors and fluorescent targets suitablylocated along the BTF transfer line 11).

Taking into account that the optimized target is a cylindrical bulk (R=35mm and L=60mm), since the maximum electron beam spot size on the target is enclosed in a circle of 1cm radiusand the accuracy by which we can set the transversal beam size and its centre is much betterthan few mm, we can be confident that, even in the most disadvantageous case, all the energy ofthe primary electrons will be deposited in the target. Because the photo-neutron yield dependsessentially on the value of the deposited energy, the estimated neutron yield sensitivity with respectto the beam geometrical parameters (beam spot size and radial location of the beam center) isactually negligible (less than 3%).

3 Final test: Comparison between Monte Carlo Predictions and Experimental data

In Fig. 4 the lethargic (EdΦ/dE) spectra, normalized to the total neutron flux, estimated byMonte Carlo codes are shown together with the normalized lethargic spectrum, obtained withBonner Spheres.

As we can see, there is a good agreement between measurements and simulation resultsconcerning the shape of the neutron spectrum. Moreover statistical tests have been performed inorder to quantify properly the accordance of Monte Carlo predictions with experimental values.In particular we used the χ2 Goodness of Fit test in order to asses quantitatively the MonteCarlo accuracy around the Giant Dipole Resonance. The p-values that we found, respectively forMCNPX and FLUKA, are 0.996 and 0.967. This allows to conclude that MCNPX and FLUKAprovide an accurate reconstruction of the experimental resonance, both in energy position andamplitude.

As predicted for, the majority of produced neutrons belongs to the energy range from 10KeV to 20 MeV: the calculated neutron spectrum has a Maxwellian shape with average around0.7 MeV.

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The experimental and calculated neutron fluxes, collected over a sphere of 10 cm, whosecenter is 1.49 m far away from the target along one of the two extraction lines, are reported inTable 1, confirming again the good agreement between experimental and predicted values.

Table 1: Total neutron flux per primary: comparison between experimental measurements andpredictions.

Measurements [BSS] MCNPX Fluka

1/[cm2/pr] 1/[cm2/pr] 1/[cm2/pr]

8.04E-7±3%. 8.06E-7± 4% 8.12E-7±5%

We can conclude that at present, because we are authorized to deposit only a small fraction ofthe total Linac power (40-50 W), we are able to have, at about 1.5 m from the target, a maximumneutron rate of 4E+5 n/cm2/s, that (≥80%) is mainly composed of neutrons with energy around1 MeV, as correctly predicted by Monte Carlo simulations.

4 Conclusion and future plan

A photo-neutron source has been successfully realized in BTF, concluding the n@BTF project asscheduled.

In fact the feasibility test was successfully carried out in May 2010, demonstrating definitivelythe BTF ability of providing not only electrons, positrons and tagged photons, but also neutronbeam of well defined and reconstructed energy spectrum and fluences.

The measured neutron rate and spectrum were in good agreement with the simulations, takeninto account the limitations due to the modest energy resolution of the used BS spectrometer.Further investigations are foreseen, in the direction of developing a photon-insensitive high-energyspectrometer to measure with much higher accuracy also the high-energy component of the neutronspectrum (10 MeV and above).

The BTF neutron source will be made available to the whole international scientific commu-nity for its scopes.

The complete characterization of the new neutron facility is still in progress. In fact othermeasurements of both neutron and photon field in several points of the BTF experimental hallhave to be done, but they have been already scheduled in short time and their results will besoon available. Moreover a new project “NESCOFI” has started as direct spinoff of the neutronfeasibility study in BTF: one of the main goals of this project is to develop neutron diagnosticinstrumentations for the BTF neutron facility.

In spite of this, we have already received requests for running BTF with neutrons by sev-eral groups of University of Roma area, involved in studying inelastic scattering of neutrons anddevelopment and characterization of neutron detectors.

5 Conference Talks

1. L. Quintieri et al, “N@BTF: Feasibility study of a photo neutron source at the DAΦNEBeam Test Facility”, Invited Seminar 25 Febbraio 2010, INFN Trieste, Area di Ricerca diPadriciano.

2. G. Mazzitelli et al., “Neutron Source at the DAΦNE Beam Test Facility (BTF)”, PosterSession, IPAC, Kyoto, 24 May 2010.

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3. G. Mazzitelli et al., “New Neutron Source at the Beam Test Facility (BTF) of Frascati:design and first experimental results”, Invited Seminar at Channeling 2010, Ferrara 3-8October 2010.

4. L. Quintieri et al., “Photo-Neutron Source by High Energy Electrons on Target: ComparisonBetween Monte Carlo Predictions and Experimental Measurements”, Oral Presentation (N31-6) at NSS-IEEE, Scientific Simulation and Computation: Simulation R&D, Knoxville, 3November 2010.

6 Publications

• G. Mazzitelli, et al., “Neutron Source at the DAΦNE Beam Test facility”, Proceedings ofIPAC-2010 (MOPEB063), 23-28 Maggio 2010, Kyoto,Japan

• L. Quintieri et al.,“ Photo-Neutron Source by High Energy Electrons on Target: ComparisonBetween Monte Carlo Predictions and Experimental Measurements”, Submitted article forIEEE-NSS 2010 proceedings, publication in progress

References

1. A. Fasso et al., “A multi-particle transport code”, CERN-2005-10 (2005), INFN/TC-05/11.

2. D. B. Pelowitz (Ed.), MCNPX Users Manual Version 2.6. Report LA-CP-07-1473 (2008).

3. Annual Report 2009, LNF - 10 / 15(IR).

4. L. Quintieri et al., “Feasibility Study of a neutron source at the DAΦNE Beam test Facility”,using Monte Carlo codes”. 2009 IEEE Nuclear Science Symposium Conference Record (N33-6).

5. G. Mazzitelli, et al., “Neutron Source at the DAΦNE Beam Test facility”, Proceedings ofIPAC-2010 (MOPEB063), 23-28 Maggio 2010, Kyoto,Japan.

6. L. Quintieri, et al., “Photo-Neutron Source by High Energy Electrons on Target: Compari-son between Monte Carlo Predicitons and Experimental Measurements”, 2010 IEEE NuclearScience Symposium Conference Record (N31-6), October 30 to November 6, 2010, Knoxville,Tennessee.

7. R. Bedogni, et al., “Comparing active and passive Bonner Sphere Spectrometers in the 2.5MeV quasi mono-energetic neutron field of the ENEA Frascati Neutron Generator (FNG)”.Radiat. Meas., submitted manuscript.

8. A. Esposito et al., Nucl. Instr. & Meth. A 580, 1301 (2007).

9. R. Bedogni, C. Domingo, A. Esposito, F. Fernndez, Nucl. Instr. and Meth. A 580 (2007) 1301.

10. R. Boni, et al., “DAFNE Linac Operational Performances”, Proceed. European Particle Accel-erator Conference, Stockholm (Sweden), 22-26/6/1998, p. 764; LNF-98/023(P), 27/07/1998.

11. B Buonomo, et al., “ Beam Diagnostic for a wide Range Beam Test Facility”, Proceedings ofBeam Instrumentation Workshop-08 (THPC143) Lake Thaoe, California.

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