Journal of the Korean Physical Society, Vol. 59, No. 2, August 2011, pp. 2022∼2025

Estimates of SEU for Semiconductors Using MC50 Cyclotron and GEANT4Simulation

J. W. Shin

Department of Physics, Sungkyunkwan University, Suwon 440-746, Korea

T. -S. Park and S. W. Hong∗

Department of Physics and Department of Energy Science,Sungkyunkwan University, Suwon 440-746, Korea

J. K. Park and J. T. Kim

School of Information and Communication Engineering,Sungkyunkwan University, Suwon 440-746, Korea

J. -S. Chai

Department of Energy Science and School of Information and Communication Engineering,Sungkyunkwan University, Suwon 440-746, Korea

(Received 26 April 2010)

The Single Event Upset (SEU) on semiconductor RAM memories due to terrestrial neutrons ismeasured by using the MC50 Cyclotron at KIRAMS. Proton beams impinging on a thick Be targetproduce neutrons, which then bombard the semiconductors to cause SEU. Due to the thickness ofthe Be target used in this experiment, the neutron beam has a broad energy spectrum. Thus, theSEU could not be obtained directly as a function of neutron energies. To solve this problem, wepropose an approximate method of estimating the SEU as a function of the neutron energy whenthe neutron beams have a broad range of energy. In this work, three different energies of protonbeams of 30, 35, and 40 MeV were used. By extracting the difference of the neutron yields at theseenergies, we can make a rough estimate of SEU as a function of neutron energies. Monte Carlosimulations are performed to obtain the neutron flux from the MC50 cyclotron. Our results for theSEU turn out to be comparable to the previous results.

PACS numbers: 61.80.Hg, 29.20.HmKeywords: SEU, MC-50, Neutron, Monte Carlo simulation, GEANT4DOI: 10.3938/jkps.59.2022

I. INTRODUCTION

Random access memories (RAMs) are sensitive tohigh-energy particles such as energetic neutrons in theatmosphere. The excessive charges generated by highenergy particles incident on reverse-biased PN junctionscan induce an instant current flow which may disruptnormal system function [1]. This single-event effect(SEE) can change the content of the memory elements,which is called Single Event Upset (SEU). In our pre-vious work, we proposed an approximate subtractionmethod to take into account the non-mono energeticnature of the neutron beam from MC-50 cyclotron atKIRAMS (Korea Institute of Radiological Medical Sci-ences) [2], where we calculated neutron spectra withGEANT4 (v.9.1) with G4binary cascade model and ob-

∗E-mail: swhong@skku.ac.kr

tained good results. In this work, we further developour approach by first confirming our previous results inRef. 2 with an updated version of GEANT4 (v.9.3) andby checking the dependency of the results with threedifferent models; Binary Cascade, Bertini Cascade andLEProtonInelastic models.

II. METHOD

1. SEU cross section

The SEU cross section σSEU is defined as

σSEU (E) =1

Nbits· dNSEU (E)dFLn(E)

, (1)

where NSEU (E) denotes the number of SEUs, Nbits thenumber of total memory bits, and FLn (E) the neutronfluence for a given testing time in units of n/cm2.

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Estimates of SEU for Semiconductors Using MC50 Cyclotron and GEANT4 Simulation – J. W. Shin et al. -2023-

Fig. 1. (Color online) The exterior view of the neutronproduction target area of MC50 cyclotron.

Fig. 2. (Color online) Schematic drawing of the neutronproduction target area including collimators.

2. Neutron production target of MC-50

Figures 1 and 2 show the target area for producingneutron beams of the MC-50 cyclotron. The MC-50 cy-clotron can provide neutron beams up to 40 MeV gen-erated by the proton beams impinging on the Berylliumtarget. The MC-50 cyclotron uses a thick Be target of10.5 mm in thickness. Due to the thickness of the tar-get, the produced neutrons do not have a narrow peak inenergy but have a broad energy spectrum from 0 to theincident proton energy. Thus from these neutron beamswe cannot extract the SEU cross section as a function ofthe neutron energy. In Ref. 2 we proposed an approxi-mate method of extracting the SEU cross section whenneutrons are produced with a broad energy spectrum bya thick target. The detailed approximation procedurecan be found in Ref. 2.

To extract the SEU cross section of Eq. (1) one needsto know the neutron flux that causes the SEU. However,the experimental data of the neutron beam spectrum forthe present experimental set-up are not available. Thuswe perform GEANT4 Monte Carlo simulation to extractthe neutron beam profile by using the geometry shownin Fig. 1.

3. GEANT4

GEANT4 (GEometry ANd Tracking 4) [3] is anobject-oriented Monte Carlo simulation toolkit written

in C++ language, which enables the simulation of prop-agation of particles that interact with materials and/orother particles. It is widely used in many different fieldssuch as high energy and nuclear physics [4-5], medicalphysics [6-7] and space science [7-8].

A simulation of the electromagnetic processes is wellstandardized, since the underlying theories (i.e., Classi-cal and Quantum Electrodynamics) are well established.On the contrary, there is a huge diversity for the hadronicprocesses, depending on the particles under considera-tion and on their energies. To accommodate this di-versity, many packages (or physics models) are built-in in GEANT4. Users should then invoke the relevantphysics models suitable for the process [9-10], which wewill briefly discuss in the next subsection. In this work,GEANT4 9.3 version is used.

4. Hadronic Models in GEANT4

In GEANT4, users must choose a proper process forsimulations. In this work we choose three differenthadronic models: Binary Cascade, Bertini Cascade andLEProtonInelastic.

GEANT4 Binary Cascade is a data driven intra-nuclear cascade model propagating primary and sec-ondary particles in a nucleus. This model generates thefinal state for hadron inelastic scattering by simulatingthe intra-nuclear cascade. The target nucleus is modeledby a 3-D collection of nucleons, as opposed to a smoothnuclear medium.

The Bertini model also generates the final state forhadron inelastic scattering by simulating the intra-nuclear cascade. The final state of each collision is sam-pled according to free-particle cross section data. Thetarget nucleus is treated as an average nuclear mediumto which excitons (particle-hole states) are added aftereach collision.

LEProtonInelastic model generates the final state forproton inelastic scattering. It is one of the LEP modelsderived from the low energy part of GEISHA [11], whichis parametrisation driven model.

In this work, we used these hadronic models to com-pare the prediction capabilities of the proton-inducedneutron spallation process.

III. RESULTS

1. Neutron energy spectra

Neutron energy spectra produced by protons of 2 µAfrom MC-50 cyclotron are calculated with GEANT4 andare plotted in Fig. 3. Three different hadronic modelsare used. In order to obtain the whole neutron spec-tra, neutron tracking cut was not used. Figure 3 showsdFLn/dEn which is the number of neutrons per cm2 asa function of neutron energy. The dFLn/dEn is plottedfor three different hadronic models for three incident pro-ton energies. Ep and En denote the incident proton and

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0 5 10 15 20 25 30 35 40 45104

105

106

En (MeV)

G4LE G4Binary G4Berti

104

105

106 Ep = 35 MeV

dFL n/dE

n ( n /

cm2 / M

eV )

104

105

106

Ep = 40 MeV

Ep = 30 MeV

Fig. 3. (Color online) Neutron energy spectra produced by2 µA protons of MC-50 cyclotron are simulated for Ep = 30,35 and 40 MeV.

5 10 15 20 25 30 35 40 450

3

6

9 (b)

En (MeV)

G4LE G4Binary G4Berti

0

3

6

9x 104

(a)

diffe

renc

e spe

ctru

m

Fig. 4. (Color online) Difference spectra be-tween two neutron fluencies are plotted. (a) repre-sents the difference spectra of (dFLn/dEn)Ep = 35 MeV -(dFLn/dEn)Ep = 30 MeV and (b) represents the differencespectra of (dFLn/dEn)Ep = 40 MeV - (dFLn/dEn)Ep = 35 MeV.

the produced neutron energies, respectively. For neutronenergies higher than about 10 MeV the neutron spectraobtained from three different models are more or less thesame. However, at neutron energies below 1 MeV, theLEProtonInelastic (“LE”) model predicted more yieldsthan the other two models. On the contrary, neutronfluences calculated by three different models are almostthe same near the maximum neutron energies. In Eq.(1) the SEU is defined as a function of neutron energy.Therefore, to obtain the experimental values of SEU, oneneeds the neutron beams in mono energetic form. How-ever, the MC-50 cyclotron produces the neutrons of abroad range of energy as shown in Fig. 3. With thesebroad energy spectra, it is impossible to obtain the SEUas a function of En. Thus we propose a new method toextract the SEU.

2. Subtraction method

In Ref. 2 we showed that two neutron spectradFLn/dEn with broad energy distributions could be sub-tracted from each other to extract a virtual quasi-monoenergetic neutron beam. Figure 4 represents the differ-

Table 1. SEU cross sections (× 10−14) [cm2/bit].

En

(MeV)

G4LEG4BinaryG4Bertini

Previousresults [2]

SRAM032.5 1.83 ± 0.72

1.99 ± 0.811.51 ± 0.79

1.79 ± 0.88

37.5 2.77 ± 1.122.84 ± 1.172.62 ± 1.15

2.48 ± 1.20

SRAM132.5 3.97 ± 0.59

4.24 ± 0.793.50 ± 0.69

4.00 ± 0.76

37.5 4.88 ± 0.915.01 ± 0.954.50 ± 0.82

4.35 ± 1.03

ence spectrum between two neutron fluences from two in-cident proton energies. In our analysis, neutrons havingthe energy lower than 1 MeV are excluded, because lowenergy neutrons cause little contribution to SEU crosssections [12]. By subtracting (dFLn/dEn)Ep = 30 MeV

from (dFLn/dEn)Ep = 35 MeV we obtain the spectrum inFig. 4(a) which looks similar to the quasi-mono energeticspectrum at En = 32.5 MeV with a width of 5 MeV. Thesame can be done at 35 < En < 40 MeV to get the spec-trum in Fig. 4(b). We may take Fig. 4 (a) as a virtualquasi-mono energetic neutron beam at En = 32.5 MeV.Similarly Fig. 4 (b) can be taken as a virtual quasi-monoenergetic neutron beam at En = 37.5 MeV. Figure 4 alsoshows that the difference spectra from different hadronicmodels are more or less the same.

3. SEU cross section

In Figs. 5 and 6 we compare our resultant SEU crosssections from KIRAMS MC-50 with those from TSL[12] for SRAM0 (CY62127BVLL-70BAI) and SRAM1(CY62157CV25LL-70BAI), respectively. TSL and TSL(Raw) represent the SEU cross section with the full neu-tron spectrum and the SEU cross section with only thepeak of neutron spectrum, respectively. Our SEU crosssections for En = 32.5 and 37.5 MeV denoted by theopen circles, triangles, squares and diamonds agree wellwith TSL results. We listed in Table 1 the SEU crosssections from the present work and the previous work[2], which agree with each other well. (Note that thevalues listed in Table 1 as the previous results are some-what different from those in Table 1 of Ref. 2. It isbecause in Ref. 2, neutrons with the energy lower than10 MeV are excluded in calculating the SEU cross sec-tions, whereas in this work neutrons with the energylower than 1 MeV are excluded.) The differences amongthe results using three hadronic models are too small tobe seen, and so the open symbols almost overlap witheach other. The results show that our approximate sub-

Estimates of SEU for Semiconductors Using MC50 Cyclotron and GEANT4 Simulation – J. W. Shin et al. -2025-

0 20 40 60 80 100 120 140 160 180 20010-15

10-14

10-13

SEU cross section (SRAM0)

SEU (c

m2 /b

it)

En (MeV)

TSL TSL (Raw) KIRAMS with G4LE KIRAMS with G4Binary KIRAMS with G4Berti KIRAMS with G4Binary (previous results)

Fig. 5. (Color online) SEU cross sections for SRAM0 de-vice. The filled circles and triangles represent TSL (with thefull neutron spectrum) and TSL (Raw) (with only the peakof neutron spectrum) results, respectively [12]. The opensquares, circles and triangles represent KIRAMS results withLE, Binary and Bertini models, respectively. The open dia-monds denote our previous results [2].

0 20 40 60 80 100 120 140 160 180 20010-15

10-14

10-13

SEU cross section (SRAM1)

SEU (c

m2 /b

it)

En (MeV)

TSL TSL (Raw) KIRAMS with G4LE KIRAMS with G4Binary KIRAMS with G4Berti KIRAMS with G4Binary (previous results)

Fig. 6. (Color online) SEU cross sections for SRAM1 de-vice. The filled circles and triangles represent TSL (with thefull neutron spectrum) and TSL (Raw) (with only the peakof neutron spectrum) results, respectively [12]. The opensquares, circles and triangles represent KIRAMS results withLE, Binary and Bertini models, respectively. The open dia-monds denote our previous results [2].

traction method can be used with a reasonable accuracyto obtain SEU cross sections when the neutron beamshave a broad spectrum.

IV. SUMMARY

An approximate method of estimating neutron SEUcross section is applied to the neutron beams of broad en-ergy spectra produced by MC-50 cyclotron. Three differ-ent hadronic models are used to estimate the neutron flu-ence, but differences among the three different hadronicmodels are not significant. Also the difference between

GEANT4 version 9.1 (our previous results) and 9.3 (thepresent work) are not significant. We find that our re-sults agree with the TSL experimental results within afactor of about 2. When experimental or evaluated dataare available with sufficient coverage, data driven mod-els are most optimal. However, in the energy ranges weconsidered, except for the neutron energies below 1 MeV,the LEProtonInelastic, Binary Cascade and Bertini Cas-cade models can be used with a reasonable accuracy forthe simulation of neutron production. At energies below1 MeV, the results from the LEProtonInelastic modelwhich is a parameterization driven model significantlydeviate from those from Binary model which is a datadriven model.

ACKNOWLEDGMENTS

This work was supported in part by the WCU pro-gram (R31-2008-10029) and by National Nuclear R&DProgram (No. 2011-0006347) and (No. 2011-0006294)through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science andTechnology.

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