A Physical Design of a Neutron Irradiation Spectromter at Csns Facility* Q

A PHYSICAL DESIGN OF A NEUTRON IRRADIATION SPECTROMTER AT CSNS FACILITY* Q. Z. Yu †, W. Yin and T. J. Liang#

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China

Abstract technology is trending towards smaller size, higher We have physically designed a full spectrum neutron density and larger number of memory bits. These factors irradiation spectrometer, which is proposed by China increase the potentially disastrous impact on memory and Spallation Neutron Source (CSNS) facility to single-event chip functions. Rapid testing the susceptibility of each effect (SEE) test in semiconductor device and to other new generation of electron devices to the neutron related researches. Based on the overall layout of the radiation effects is highly needed in many fields such as target station and the neutronic performance, CSNS aviation, automotive, electronic medical device, high neutron irradiation spectrometer lies in the forward reliability computer users and so on. direction with 41 degrees to the proton beam, facing the Accelerator based neutron sources are favorite because front part of the W-Ta spallation target directly. The key the spallation neutron spectrum is similar to the components in the target station have been designed and atmosphere neutron spectrum produced by cosmic rays, manufactured to meet the special requirements of this but usually have more than five to six orders of spectrometer. At high energy region (>10 MeV), the magnifications of neutron flux. It implies that the SEE spectrum of the CSNS irradiation spectrometer calculated effects with 1 hour at test facilities equals to tens or by Monte Carlo code MCNPX2.5, is very similar to hundreds years in the real atmospheric environments. JEDEC standard at ground level with 3×109 magnification. Such kinds of rapid test can greatly satisfy the The integrated neutron flux at high energy region is requirements of government and semiconductor industries. 2.3×106 n/cm2/s at 14 m from the centre of the target There are several neutron irradiation facilities in the station. This spectrometer can also provide broad region worldwide, such as ICE 1 & 2 (Irradiation of Chips and of thermal neutrons in addition to high region of neutrons, Electronics) at Weapons Neutron Research (WNR) [5], covering neutron energy from meV to 1.6 GeV for NIF (Neutron Irradiation Facility) at TRIUMF [6], TSL different kinds of irradiation test and research. The single- ANITA at The Svedberg Laboratory [7], and CHIPIR event upset (SEU) in a static random access memory (Chip Irradiation) at ISIS-TS2 [8]. They provide the good (SRAM) cell is calculated by Monte Carlo code neutron spectra with high neutron flux to perform the SEE PHITS2.24. Results confirm that with neutron spectrum test of electronic devices and integrate circuits all over the similar to the atmosphere environment while high neutron world. flux, CSNS neutron irradiation spectrometer can give a China Spallation Neutron Source (CSNS) Facility [9] reliable SEE test for electronic devices and systems at provides an excellent capability of neutron irradiation greatly accelerated rate. environment for time-saving cosmic ray neutron filed testing. A new generation of neutron irradiation INTRONDUCTION spectrometer was recently proposed by the government. The main goals of this spectrometer are to predict the When high energy cosmic rays reach the earth’s electronic experiences in different irradiation atmosphere, they collide with the oxygen and nitrogen atoms, producing secondary cascade particles including environments, and find ways to overcome the irradiation protons, neutrons and pions. Neutrons have been affects caused by cosmic ray induced neutrons. This paper recognized as the main particles that reach to the aircraft will present the physical design of the CSNS neutron irradiation spectrometer. The structures of this altitudes and below [1,2]. When high energy neutrons spectrometer at the target station are presented. The interact with the silicon nuclei in semiconductor device, charged particles are produced and deposit their energy by neutron spectrum and flux calculated by the Monte Carlo ionizing radiation, inducing a great deal of electron-hole code MCNPX2.5 [10], are analyzed and compared with pairs. If the total number of the induced charge collected other accelerator based neutron facilities. The single- event upset (SEU) in a static random access memory in a sensitive region exceeds its critical value of the (SRAM) cell is calculated by Monte Carlo code device, a single-event effect (SEE) is occurred [3,4]. As PHITS2.24 to assess the test ability of CSNS neutron the fast developments of the electronic systems, irradiation spectrometer. ______

*Work supported by the National Science Foundation of China (Grant Nos. 11075203, 91026009 and 11174358). †[email protected], #[email protected]

STRUCTURE OF CSNS NEUTRON during evaporation process. Based on the overall layout of IRRADIATION SPECTROMETER the target station, and the neutronic performance of CSNS neutron irradiation spectrometer, the beam port is CSNS is an accelerator based neutron facility being designed in the forward direction with 41 degrees to the constructed in south of China. It is intended to start proton beam, viewing the front part of the spallation operation in 2018. An accelerator delivers a 1.6 GeV target directly. The distance from the target window to the proton beam at 25 Hz frequency to a spallation target. The nearest beam port is 13.3 cm. In the Be/Fe reflectors with initial design proton beam power is 100 kW and will be radius of 50 cm, the beam port is 8×8 cm2. It is expanded upgraded to 500 kW after several years operation. The 2 to 10×10 cm from the helium vessel, passing through the key components at the target station are the W-Ta target, shielding to the outside of the target station. The three hydrogen-rich moderators, beryllium plus iron irradiated samples are positioned at 14 m from the centre (Be/Fe) reflectors and biological shielding. Detailed of the target station to experience the irradiation test. The description of the target station was once presented at geometry of the neutron irradiation spectrometer at the elsewhere [11,12]. There are 20 neutron spectrometers to target station is presented in Fig. 1. be designed in total. 19 thermal neutrons extracting from three different moderators are used for neutron scattering Beam shutter of CSNS neutron irradiation scientific research, while one fast neutron spectrometer in spectrometer at target station addition with thermal neutrons facing the front part of the spallation target directly, is used for SEE test and other There are 20 neutron beam shutters corresponding to irradiation researches. Three neutron scattering each spectrometer at CSNS Facility. Neutron beam spectrometers are being instructed at Stage-One, which shutters, which begin from 2.25 m to 4.25 m relative to are the General Purpose Powder Diffractometer (GPPD), the center of the target station, are used to ensure the the Multi-Purpose Reflectometer (MR) and the Small safety environment during sample replacements. Because Angle Neutron Scattering spectrometer (SANS). The CSNS neutron irradiation spectrometer supplies high CSNS neutron irradiation spectrometer is proposed to be energy neutrons and thermal neutrons with high constructed at Stage-Two. Since this neutron irradiation integrated fluxes, material and structure of the beam stop spectrometer is extracted from the target instead of the used in the neutron shutter is greatly different to the other moderators, its structure is greatly different from other 19 beam stops. Figure 2 shows the structure of the beam neutron scattering spectrometers. stop used in the beam shutter, together with the main structures of the beamline at the target station. The beam Beam port of CSNS neutron irradiation stop consists a 1.4 m stainless steel (SS316) block and a spectrometer in target station 60 cm W block. Heavy concrete used as the biological shielding, origins from 4.8 m to 6 m, while ordinate A 1.6 GeV proton beam bombards the W-Ta target, spallation reactions take place, producing high energy neutrons via cascade process and low energy neutrons

Neutron Irradiation Spectrometer

Biological shielding W-Ta target

Fe/ Be reflectors Proton beam Fig. 2 Structure of neutron beam shutter in CSNS neutron Fig.1 The schematic of CSNS neutron irradiation irradiation spectrometer. It begins from 2.25 m to 4.25 m spectrometer at the target station. It is extracted from relative to the center of the target station, with 1.4 m the front part of the W-Ta target directly, at 41 SS316 plus 60 cm W. There are 1.8 m heavy concrete degrees in the forward direction of the proton beam. shielding and 3 m ordinate concrete pre-shielding.

concrete from 6 m to 9 m is used as the pre-shielding. involves simulating the differential neutron spectrum Figure 3 shows the dose map corresponding to the including both thermal and high energy neutrons. The structure in Fig.2. One can see that near the spallation Monte Carlo code MCNPX2.5 is used to simulate the target, the dose rate is at 109 µSv/h. Since the beam stop proton beam interacting with the W-Ta target. Bertini of SS316&W can greatly shield high energy neutrons, the intranuclear cascade model coupled with RAL dose rate decrease sharply to several µSv/h at the end of evaporation model is used to simulate the productions of the beam stop, ensuring the safety environment in the high energy neutrons, while the nuclear data library experimental hall. ENDF-B/VI is used to calculate the productions of low energy neutrons. For the neutronic performance of the neutron irradiation spectrometer, the calculations are based on a 1.6 GeV, 100 kW (3.9×1014 p/s) proton beam with Gaussian distributions of 9.42 cm and 3.53 cm full width at half maximum, irradiating the 17×7×60 cm3 W- Ta target. While for the design of the shutter beam stop and the related shielding of this spectrometer, the proton beam power is 500 kW (1.95×1015 p/s) considering the update of CSNS facility. Figure 4 shows the differential neutron flux at high- energy region (>1 MeV) of CSNS neutron irradiation spectrometer, at 14 m away from the center of the target station. Other accelerator based neutron facilities TRIUMF, ANTIA, ISIS-TS2, LANCE and JEDEC standard neutron spectrum [13] are also presented for comparison. One can see that the neutron energy of CSNS Fig. 3 Dose map in µSv/h unit, corresponding to the neutron irradiation spectrometer reaches up to 1.6 GeV as structure in Fig. 2. The dose rate near the spallation the incident proton beam. This is benefitted both from the target is very high while it decreases rapidly to high energy of the incident proton beam and from its several 2.5 µSv/h behind the SS316&W beam stop. arrangement in the forward direction. Calculation shows that the neutron energy spectrum at CSNS is very similar SPECTRUM AND FLUX OF CSNS to JEDEC standard spectrum at ground with 3×109 NEUTORN IRRADIATION magnification, especially in the high energy region from SPECTROMETER 200 MeV to 1.6 GeV. It indicates that CSNS neutron irradiation spectrometer can provide more exact The main design criterion of the neutron irradiation atmosphere environment at high energy region. CSNS spectrometer is that the spallation neutron spectra should neutron irradiation spectrometer can also provide high match the atmosphere spectra that the electronics intensity of thermal and epithermal neutrons in addition to experience at aircraft attitudes or on the ground. This

7

) 10 CSNS@14m 13 ) 10 6 TRIMF CSNS@14m 10 ANTIA

/MeV/s ISIS-TS2 11 2 5 LANCE 10

10 /MeV/s

JEDEC*3E9 2 9

n/cm 4 10

10 High energy n/cm

7 3 neutrons 10 10

5 Thermal energy 2 10 10 neutrons

1 3 Neutron fluxNeutron ( 10 10 1 10 100 1000 Energy (MeV) fluxNeutron ( 10-8 10-6 10-4 10-2 100 102 Fig.4 The differential neutron spectrum (>1 MeV) of Energy (MeV) CSNS neutron irradiation spectrometer at 14 m Fig. 5 Thermal and high energy neutron spectra of distance from the center of the target station. CSNS neutron irradiation spectrometer at 14 m from TRIUMF, ANTIA, ISIS-TS2, LANCE and JEDEC the center of the target station. (×3E9) are also presented for comparison.

Table 1 Accelerator based neutron facilities for atmosphere neutron radiation test FACILITY NEUTRON FLUX (>10 MeV) PROTON ENERGY TRIUMF 2.6×105 n/cm2/s 500 MeV TSL (ANITA) 3.7×104/3.6×105/9.5×104n/cm2/s 22/109/174 MeV ISIS TS1 (Vesusio) >5.8x104 n/cm2/s 800 MeV

ISIS TS2 (ChipIR) >1x106 n/cm2/s 800 MeV 5 2 LANSCE (ICE I& II) 4.6x10 n/cm /s 800 MeV 6 2 CSNS (FuNIS) 2.3×10 n/cm /s 1600 MeV high energy neutrons, covering broad energy neutrons neutron irradiation spectrometer. from meV to 1.6 GeV, as shown in Fig. 5. The geometry of the SRAM cell is 40 µm × 40 µm × Except for the broad neutron spectra, the other 40 µm, with sensitive layer of 5.6 µm × 5.6 µm × 5.6 µm. important design criterion of the neutron irradiation The high energy neutrons (10 MeV-1600 GeV) of CSNS spectrometer is the neutron flux. CSNS neutron neutron irradiation spectrometer vertically incident the irradiation spectrometer aims to provide a world leading SRAM cell with flux of 2.3×106 n/cm2/s. The Monte integrated fast neutron flux. The beam port is designed Carlo code PHITS2.24 with the event generator mode is viewing the front part of the target, where the intense used to simulate the neutron induced reactions with Si spallation reaction takes place and more neutrons can be atoms and the transports of nucleus and heavy ions. extracted directly. The integrated neutron flux in the fast PHITS outputs deposition energy distributions in the 6 2 neutron regime (>10 MeV) is about 2.3×10 n/cm /s. specified sensitive region, including energy deposit Table 1 shows the neutron flux at energy > 10 MeV at distributions caused by specific particles, as shown in Fig. other accelerator based neutron facilities in the world for 7. Here only the main contributors to the total energy comparisons. deposition are presented. It can be seen that the recoil Si ions dominate above the deposition energy greater than SEU CALCULATION IN A SRAM CELL 0.2 MeV, alpha ions dominate greater than 1 keV, while Here we present the SEU calculations in a SRAM cell protons are the critical source for low energy deposition to predict the test ability of CSNS neutron irradiation due to its long range. spectrometer, which can be used to detect the neutronic performance and used for further optimization. 10-9 When high energy neutrons interact with silicon atoms Total in an electronic chip, spallation reactions take place, Si producing protons, deuterium, tritium, α, 3He, heavy ions Proton -10 Alpha and recoil nucleons. These charged particles deposit their 10 energy by ionizing radiation, inducing a great deal of

electron-hole pairs. If the total number of the induced charge collected in a sensitive region exceeds its critical 10-11 value of the device, a single-event effect (SEE) is occurred. The interactions of charged particle on silicon (1/source) Number (Si) atom in a SRAM cell are showed in Fig. 6. Because 10-12 single-event upsets (SEU) is the most probability of SEE, -5 -4 -3 -2 -1 0 1 2 10 10 10 10 10 10 10 10 the SEU calculations in a simplified SRAM are shown below as an example, to illustrate the test ability of CSNS Deposit energy (MeV) Fig. 7 Deposit energy distributions for specific n particles in the sensitive region of a SRAM cell, Sensitive layer calculated by PHITS2.24 with event generate mode. neutron flux =2.3×106 n/cm2/s By converting the deposition energy into the induced charge with the average required energy to produce an electron-hole pair (3.6eV/(e-h pair)), the probabilities of the events can be calculated where the induced charge is greater than a critical charge. Then we can obtain the SEU Si chip in the SRAM cell as a function of different critical charges, as shown in Fig. 8. The collection efficiency here is set to 0.5 as determined by most experiments. For Fig. 6 Neutron induced nuclear reactions in a example, if the critical charge of the device is 20 fC (1fC=10×10-15 C), the SRAM cell irradiated by CSNS SRAM cell of the Si chip, producing charged 6 2 particles and electron-hole pares. high energy neutrons with flux of 2.3×10 n/cm /s,

produce one error rate per hour. This means that in case of SEE test for semiconductor device at greatly accelerated a device with 1 M cells, the error rate is 106 per hour. rate. Calculation results implies that CSNS irradiation neutron spectrometer has the most rapid SEE test ability in the REFERENCES worldwide, while one should note that events may occur [1] Z. Török and S. P. Platt, IEEE Trans. Nucl. Sci., 53, more rapidly than that can be processed if the thermal 3718 (2006). neutrons are also considered. [2] R. C. Baumann and E. B. Smith, vol. 41, 211 (2001). [3]J. F. Ziegler and W. A. Lanford, Science, 206, 776 (1979). 101 [4] J.F. Ziegler "Terrestrial cosmic rays" IBM J. Res. Develop. 40, (1996). [5] ICE House: 100 http://lansce.lanl.gov/NS/instruments/ICEhouse/index.ht ml [6] E. Blackmore, P. Dodd, and M. Shaneyfelt, in Proc. -1 10 IEEE Radiation Effects Data Workshop, 149 (2003). [7] ieeexplore.ieee.org/iel5/5164157/5173200/05173383.pdf. 10-2 [8] C. Andreani, et al., Appl. Phys. Lett., 92, 114101 (2008). -3 [9] http://csns.ihep.ac.cn. SEU rate (error/hour/cell) rate SEU 10 20 40 60 80 100 120 140 [10] MCNP/MCNPX CCC-730, Monte Carlo N-Particle Critical charge (fC) Transport Code System Including MCNPX 2.5.0 and Data libraries (2006). Fig. 8 SEU rate as a function of the critical [11] Q. Z. Yu, T. J. Liang and W. Yin, Radiat. Prot. charge in a SRAM cell irradiated by the CSNS Dosim. 136, 216 (2009). high energy neutrons (>10MeV). [12] W.Yin, B.Zhang, T.J.Liang, Q.Z.Yu, Shielding design for CSNS target station, The 19th Meeting of the International Collaboration on Advanced Neutron Source SUMMARY (ICANS-XIX), Switzerland, T0071 (2010). A full spectrum neutron irradiation spectrometer has [13] JEDEC Test Specification JESD89, www.jedec.org. been proposed at CSNS facility to satisfy the rapid SEE test requirements by government and electronic industries. Based on the layout of the target station and the neutronic performance of the neutron irradiation spectrometer, the beam port is designed in the forward direction with 41 degrees to the proton beam, facing the front part of the W-Ta spallation target directly. The neutron beam shutter with special beam stop has been designed to ensure the safety environment at the experimental hall. At high energy region (>10 MeV), the spectrum of the CSNS irradiation spectrometer calculated by Monte Carlo code MCNPX2.5, matches well to JEDEC standard at ground level with 3×109 magnification. The integrated neutron flux at high energy region is 2.3×106 n/cm2/s at 14 m away from the centre of the target station. This spectrometer can also provide broad region of thermal neutrons in addition high energy neutrons, meaning that CSNS neutron irradiation spectrometer can cover energy neutrons from meV to 1.6 GeV for different irradiation test and other research. The comparisons of neutron spectrum and flux to other accelerator based neutron irradiation facilities have performed. With the irradiation of CSNS neutron irradiation spectrometer, the SEU rate in a SRAM cell is calculated by PHITS2.24 Monte Carlo code. Results confirm that with neutron spectrum similar to the atmosphere environment while high neutron flux, CSNS neutron irradiation spectrometer can give a reliable