PROCEEDINGS OF THE 31st ICRC, ŁODZ 2009 1

Geant4 simulation of the Solar Neutron Telescope (SNT) at SierraNegra, Mexico

L. X. Gonzalez‡, F. A. Sanchez∗, J. F. Valdes-Galicia‡, G. Medina-Tanco∗

‡Instituto de Geofısica, Universidad Nacional Autonoma de Mexico, Mexico, D.F., C.P. 04510.∗Instituto de Ciencias Nucleares, Universidad Nacional Autonoma de Mexico, Mexico, D.F., C.P. 04510.

Abstract. The SNT at Sierra Negra (19.0◦N, 97.3◦Wand 4580 m.a.s.l) is part of the Solar NeutronTelescope Network [1]. This SNT has an area of 4m2

and it is composed by four 1m×1m×30cm plasticscintillators. The telescope is completely sourroundedby anti-coincidence proportional counters (PRCs). Inorder to discard photon background it is shielded onits sides by 10mm thick iron plates and on its topby 5mm lead plate. It is capable of registering fourdifferent channles corresponding to four energy de-postion thresholds: E>30, >60, >90 and >120 MeV.The arrival direction of neutrons is determined byfour layers of PRCs orthogonally located underneaththe SNT. In this paper we present the simulateddetector response to neutrons, protons, and gammasentering the SNT with a range of energies from 100to 1000 MeV. In particular, we report on the detectorefficiency and on its angular resolution for particlesimpinging the device with different zenith angles.The simulation code was written using the Geant4package taking into account all relevant physicalprocesses [2].

Keywords: Simulation, Geant4, Solar neutron

I. INTRODUCTION

From 2004 June, the SNT at Sierra Negra is takingdata and is ready for the solar cycle 24. This detectoris the newest of the solar neutron telescope network(Fig.1). The detector consist of four 30 cm tick plasticscintillators (PS) surrounded by gondolas of proportionalcounters (PRC) to detect charged particles. These PRC’sare in electronic anti-coincidence with the PS, that candetect charged and neutral particles. Incident particlescollide with nucleus of the PS’s and release protons (p+),by the reaction: n + 12C → p+ + X. These recoil p+ can

Fig. 1. Solar neutron telescope network.

be detected by the orthogonally PRCs underneath theSNT, therefore we can determine the arrival directionof the incident particles. Based on the electronic anti-coincidence we can discriminate between charged andneutral particles. Energy deposited (Edep) by neutronsis measured by pulse height discriminators connected tothe photomultiplier (PMTs) installed above each PS.The pulse height is discriminated and registered in fourdifferent energy deposition thresholds channels, whichcorrespond to E>30, 60, 90 and 120 MeV (Fig.2).This design is enough to estimate the original incomingneutron direction with an accuracy of 15◦, as countingrates are measured for each of the 25 possible hit patternsin the two possible directions (N-S and E-W). To ensurethe detection of solar neutrons, the TNS is shielded ontop, and the four vertical sides, by a layer of lead (5 mmthick) and iron plates (10 mm thick), respectively. In thisway, the signal of γ and α-particles, that could contributeto the neutron signal, are minimized. A schematic viewof the SNT at Sierra Negra is shown in [1].

II. SIMULATION DESCRIPTION

We have simulated all the detector active components:the proportional counters, the iron and lead plates andthe plastic scintillator. In everyone of these materials, thesimulation package takes into account all the physicalprocesses relevant to particles passing through matter[2]. These processes are energy losses, scattering, ra-diatial lenght, density effects and nuclear interactions(n-p+, p+-p+, p+-12C, n-12C).

As an example, we show in Fig.3 the graphical outputof the simulation of a vertical impinging neutron of 0.5

Fig. 2. Electronic discriminator for different channels of SNT. Showthe anti-coincidence for four energy deposition thresholds [3].

2 L. X. GONZALEZ et al. GEANT4 SIMULATION SNT

GeV. As can be seen, the rationale behind the detectiontechnique is very simple: all neutral particle detectorsrequire that the neutral particle have an interaction thatresults in the liberation of a charged particle somewherewhitin its sensitive volume; in our case, neutrons, n, havea very small probability of interacting with the top leadplates and eventually trigger the proportional counterson the top of the telescope; nevertheless, because thescintillator plastics are hydrogen rich compounds, theyhave a relative high chance of producing a recoil proton,p+, or anther charged secondary while crossing theplastic (either by means of elastic scattering or inelasticreactions); in this way, a neutron can be distinghuishfrom other charged particles (such as the proton back-ground) because its pattern in the detector will be ananti-coincidence signal between the top and bottom PRCaccompanied by a signal arising from the deposition ofenergy in the scintillators. Nevertheless there are someissues that push against the above simple argument andcompete with the detection efficiency of the SNT. Firstof all, the neutrons can go through the whole telescopewithout interacting at all or, contrary, interact whitin thelead plate or the PRC gas. Another possibility is that therecoil products of the reaction inside the scintillators willno reach the proportional counters beneath the detectoror, if they are sufficiently energetic to be pulled out ofthe plastics, they do not trigger the 4 PRCs needed tohave an angular reconstruction of the event.

On the other hand, there are some other events thatmay be mistakenly assigned as a nuetron events. That isthe case of high energy photons. In fact, photons cango through the top PRCs and convert to a electron-positron pair in the scintillators leaving a neutron-likesignature. To reduce this effect, the SNT have on itsroof a lead plate of 5mm thick. In this way, most of theγs will convert on the top of the telescope producinga coincident signal (see section III-A). High energyprotons would also have the chance of producing aneutron-like signature. Nevertheless, as we shall see, the

nγ

+p

Scint.

Lead plate

Iron plate

PRCs

(not triggered)

PCRs(triggered)

n

Fig. 3. An example of the simulation output. One impinging verticalneutron interacts in the plastic scintillator and produces a recoil protonthat triggers the underneath PRCs. The anti-coincidence with respectto the top PRCs is the typical signature of a neutral particle crossingthe telescope sensitive volumen.

(GeV)injE0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

(M

eV)

Sci

dep

E

40

60

80

100

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140

160

180

200

220 °neutron 00

°neutron 30

°neutron 60

S1_with_anti

S2_with_anti

S3_with_anti

S4_with_anti

Fig. 4. Energy deposited by the impinging neutrons in the scintillators,ESci

dep, as a function of their incident energies, Einj . The differentchannels threshold allow an estimation of the neutron spectrum (seetext).

probability of this to happen is negligible.

III. SIMULATION RESULTS

We have simulated neutrons, gammas and protonshitting the detector with energies in the range 100MeV to 1 GeV using Geant4 [2]. We studied thecase of 3 different impinging zenith angles: 0◦, 30◦

and 60◦. For each case, 50,000 incident particles werelaunched. We consider only anti-coincidence type ofevents that additionally trigger, at least, 4 proportionalcounters of the bottom layers in order to have alsoangular reconstruction. For the trigger efficiency of thePRC we assumed a 100% of efficiency, this is, theproportional counters were considered to trigger if anyparticle deposits energy in the counter gas (or, EPRC

dep >0MeV). Althoug this is an unrealistic case, will not affectthe general considerations developed in the followingsections. The electronic signal in the PMTs was assumedto be proportional to the energy released by chargeparticles in the scintillators, ESci

dep. In this way, we wereable to emulated the response of each telescope channel.On the other hand, the different detector channels canbe used to estimate the neutron spectrum of the incidentneutrons as can be seen in Fig.4, at the three angle understudy. For example, substracting to the channel namedS3 with anti (sensible to all neutrons which depositmore than 90 MeV in the PSs) the counting of thechannel S2 with anti (sensible to ESci

dep > 60 MeV),incindent neutrons of energy in the range ≈200-300MeV are selected.

A. Particle detection efficiency

An earlier simulation of the Solar Neutron Telescope[1] demonstrated that for the very energetic solar neu-trons (i.e, with an incident energy ≥ 500 MeV), thedetection efficiency would always be more than 10% ormore. Nevertheless, that estimation did not take into ac-count the proportional counters gondolas. Every neutronwith a recoiling proton in the scintillator was consideredto be detected. Nevertheless this is not alwayas the casein a more realistic situation. It is possible, that the proton

PROCEEDINGS OF THE 31st ICRC, ŁODZ 2009 3

Fig. 5. Detection efficiencies for incident zenith angle at 0◦ (top),30◦ (middle) and 60◦ (bottom). The curves in each panels correspondto one out of the four acquisition channels.

is not pulled out from hte scintillator, or if so, do nottrigger at least 4 PRCs of the bottom gondolas. If theseeffects are taken into account, the efficiency is reduced.Our present results are shown in Fig.5. It can be seenthat the most sensible channel, S1, has ∼8-10% while S4∼3-6% for Einj ≥500 MeV. In both cases, as expected,the efficiency increase with the increasing zenith angle.

In the case of gammas incident, the simulation showsthat summing the converted photon in the top lead plates(which produce a coincidence signal (or a proton-likesignal) with those photons that are transparent to thedetector, more than 65% of the photon background isrejected. This result, in complete agreement with [1], isalmost independent of the incident energy or angle.

Finally, protons in all cases are rejected by its coin-cidence signal by more than 99%.

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2.1%

2.9%

3.4%

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11.9%

6.4%

3.4%

2.8%

4.5%

6.4%

4.5%

2.8%

2.1%

2.9%

3.4%

2.8%

2.2%

Fig. 6. A simulated vertical flux of neutrons with plane energydistribution in the range of 100-1000 MeV (top). A real SNT eventregistered on September 7th 2005 around noon (bottom).

B. Angular resolution

The angular reconstruction of the telescope is pro-vided by proportional counter located underneath thescintillators. These 2 layers of PRCs allow to classifythe direction of the recoiling proton (which is, in turn,correlated to the incoming neutron direction) in a 5×5angular matrix of ∼15◦ resolution. The field of viewis, therefore, ∼60◦ in the North-South and East-Westdirections. The SNT carries out this process on lineby means of specific logic circiuts. The simulationpackage we are presentig, because takes into account thesimulation of all the PRCs, is capable to estimate howthe reconstructed angular directions are correlated withthe true incoming direction. As an example, the top panelof Fig.6 shows the case of a simulated solar neutron fluxarriving vertically to the telescope (or, putting in otherwords, a mid-day incoming flux). In this case, we haveconsidered a plane flux of neutrons with energies from0.1 GeV to 1 GeV. As can be seen, around 11% of the

4 L. X. GONZALEZ et al. GEANT4 SIMULATION SNT

reconstructed angles, fall in the central pixel (the notedas NS-EW) while in its first neighbouring pixels thereis less than 6% of the events. The bottom panel of Fig.6shows a real SNT event register the September 7th 2005around noon. As can be appreciated there is a very goodagreement between real and simulated data ginving usconfidence that we are reproducing quite reasonables theresponse of the telescope.

IV. CONCLUSIONS

We have presented a simulation tool to study theresponse of the Mexican Solar Neutron Telescope lo-cated at Sierra Negra. The core of the code uses Geant4package [2] and takes into account all the sensitivecomponents of the detector. As a simple application,we used the simulations to estimate the efficiency ofthe telescope to solar neutrons and its capability todiscriminate other kind of particles such protons andgammas. We demonstrate that solar neutrons of energies≥500 MeV can be detected with an efficiency around of,and greater than, 10%. On the other hand, photon andproton background are rejected with ∼65% and ∼99%efficieny respectively. The angular resolution obtainedwith our simulations are also in very good agreementwith real data validating the simulation code.

REFERENCES

[1] Valdes-Galicia et al., Nucl. Instrum. Meth. A 535 (2004).[2] S. Agostinelli et al., Nucl. Instrum. Meth. A 506 (2003). http:

//geant4.web.cern.ch/geant4[3] Watanabe, K., Ph.D. thesis, Nagoya Univ., Nagoya, Japan (2005).