Nuclear Instruments and Methods in Physics Research A240 (1985) 383-385 383 North-Holland, Amsterdam

N E U T R O N S P E C T R O M E T E R FOR DT-PLASMA D I A G N O S T I C S

T H . E L E V A N T

Royal Institute of Technology, Stockholm, Sweden

M. O L S S O N

Chalmers Institute of Technology, Gothenburg, Sweden

Received 28 May 1985

A neutron spectrometer primarily aimed for DT-plasma diagnostics has been investigated. The technique is based on double interaction of the neutrons in two different scintillators and on time-of-flight measurement. Elastic (n, d) backscattering is utilized in the first detector and (n, p) elastic scattering is used in the second detector. For 14.1 MeV neutrons and for a flight path length of 2 m, energy resolution is about 2% and scales reciprocally to the flight path length.

A reference spectrometer is suggested for diagnostics of DT-plasmas in the JET tokamak, It is capable of measuring ion temperatures above 2.5 keV under various plasma conditions and has an efficiency equal to 1 × 10 4 cm 2.

1. Introduction

In the light of the development of fusion research i and utilization of large devices like the tokamaks JET

r " t

and TFTR, neutron measurements become more inter- / ~ " ~ esting for diagnostic purposes (see refs. [1,2]). In this D° ~ 9 q~ =160° paper we describe a technique for measurements of neutron spectra from which DT-ion temperatures can [ be determined. The method is based on a double inter- ~ '~ action of neutrons in two detectors, and measurements ~ \ \ of time-of-flight give the neutron energy. Backscattered ! \~ neutrons from the first scintillator, which is deuterium ~ I ~,LIO0* \ , based, are utilized. Laboratory tests have been per- formed providing energy resolution of 2% with a flight ~on b e ~ ~ _ \ path of 2 m, This enables one to determine ion temper- \ atures of 2.5 keV and above (see refs. [1-3]). ~

2. Experiment

A monoenergetic neutron source (fwhm---2%) was produced with the Studsvik Van de Graaff accelerator through the T(d, n)He 4 reaction in a Ti target, giving neutrons with an average energy of 14.0 MeV. A de- uterium based scintillator, DO (0.5" x 1.5" O, N E 232) served as the scattering detector, and a set of three hydrogen based scintillators, D1 (0.5" X 4" ~ , Pilot U) were situated near the 180 ° direction from the incident neutron beam at a distance of 1-3 m from DO (see fig. 1), Fast photomultiplier tubes (XP 2020 and XP2040) with decay times of a few nanoseconds were used. Pulse amplitude discrimination was employed for all scintilla-

0168-9002/85/$03.30 © Elsevier Science Publishers B.V. (North-Hol land Physics Publishing Division)

L . J i L i i i i i ~ i m

Fig. 1. Experimental geometry is shown with ion beam, target, detectors and shielding.

384 Th. Elevant, M. Olsson / A neutron spectrometer

0 o

['~-Fo-7- Expem___ ri entat halt CF~OO- ~ - --

Delo.y

[Time to Amptitude] / C°nverter | ~P ST |

Mut t i chonne l Analyser

a 1

IPhiliP iS563][Philips~l~"'''

I

Fig. 2. Detector systems and electronics are shown. Anode pulses from photomultiplier tubes are used.

[L~ E ~2 E ] "I04

15

12 .

1() 9 g 7 6 5 4

3 2 1 0 i i t t D- O0 0.1 02 0.3 0.2 zO.5 t'(m 41

Fig. 3. Experimental results are shown in accordance with eq. (1). Intersection with the ordinates gives the source width equal to 300 keV.

resolution was amended to 0.87 ns. In fig. 3 the fwhm for different flight path lengths is

shown, i.e. ( A E / E ) 2 as function of 1-2. Thus the individual contributing terms in eq. (1) can be studied separately and, e.g., the source term (equal to 2.1%) is obtained from the intersection with the ordinates. The energy resolution taken from fig. 3 is equal to 2.2% for ! = 2m and scales approximately as l-1. These values put into eq. (1) give dl = 1.5 cm.

tors through constant fraction differential discrimina- tions (CFDD) which provide standard timing pulses suitable for the time to amplitude converter (TAC) unit (see fig. 2). Timing spectra were recorded for several flight path lenghts and the energy resolution of this type of spectrometer was investigated.

3. Results

The theoretical energy resolution can be determined from

( ~ _ ) 2 = 4 [ ( d l ) 2 + ( d l . v n ) 2 ] l + ( / AEs z

where A E s / E s is the relative source width, d r is the timing resolution, v, is the velocity of the scattered neutron, dl is the thickness of the scintillators, and l is the length of the flight path. The contribution from the finite scattering angle is neglected in eq. (1). It can be shown to be less than 1% in this particular case. Using an annihilation gamma source 22Na) the system timing

4. Discussion and application

Performance of the spectrometer is in accordance with eq. (1). Calculation gives the efficiency of ~ 1 × 10-5cm 2 with a threshold in detector D1, so that 50% of the proton recoils are accepted.

For the purpose of fusion plasma diagnostics a spec- trometer like this would enable measurements of ion temperatures of 2.5 keV and above. However, due to saturation effects in the DO detector, one needs a large scintillator area for the D1 detector for useful system count rates (200 cps, ref. [1]). Using e.g. six long scintil- lators (50 x 10 × 1.25 cm 3) each one with two light guides and fast photomultiplier tubes, and a meantim- ing system, one would be able to record one spectrum per second.

As an example we assume that the spectrometer is located approximately 20 m from the JET torus viewing one of the ports through a collimator. Table 1 shows the scintillator sizes, estimated count rates, background re- quirements and resolutions, together with neutron fluxes under different plasma conditions for a reference spec-

Th. Elevant, M. Olsson / A neutron spectrometer 385

Table 1 Operation range for the 14-MeV TOF reference spectrometer. Detectors: DO 3.75~ × 1.25 cm2; D1 50× 10X 1.25 cm3; total area 6 X 50 × 10 = 3 × 103 cm 2. Detector efficiency = 1 x 10-4 cm 2, flight path length = 200 cm, minimum system count rate = 2 × 102 cps, average ion density ~ = 5 × 1013 cm -3, maximum tolerable background (n+ y ) = 102 cm 2 s 1 spectrometer resolution A E / E (fwhm) = 2.2%.

Peak ion Density (n) Temperature (T) Total neutron Collimated neutron Active DO Calculated temperature profile profile yield flux detector area background n + y (keV) (s t ) (cm- 2 s - 1 ) (cm 2 ) (cm - 2 s - i )

1.5 flat flat 1016 2 × 10 6 10 10-1 2.0 flat peak 2.5 flat flat 1017 2 X' 10 7 1 1 4.0 flat peak 4.0 flat flat 1018 2 X 108 0.1 10 7.0 fiat peak 8 flat flat 1019 2 × 109 0.0l 102

17 flat peak 30 flat flat 10 20 2 × 101° 0.001 103

trometer . Apparent ly , the spect rometer is useful under b road ranges of p lasma condit ions, provided the count rate in the first detector can be kept on a nearly cons tan t level. This can be ob ta ined either by changing the effective volume of the DO detector or by modifying the effective col l imator cross section area.

One d isadvantage with this k ind of spect rometer is the high sensitivity to background radia t ion causing r a n d o m coincidences, part icular ly in the second set of detectors. A t the highest neu t ron emission level in JET (10 20 s -1) the neu t ron and g a m m a background are es t imated to 10 3 cm -2 s -1 outside the main radia t ion shielding (ref. [4]). F rom table 1 it can be seen that addi t ional shielding of the detectors will be required dur ing such opera t ion condit ions.

We are greatful to Dr. N. Jarvis, Dr. G. Sadler, Dr. J. K~tllne and Mr. P. van Belle of the JET neu t ron di- agnostic group for encouraging discussions, and also to Mr. L. Norel l at Studsvik Van de Graa f f labora tory for provid ing reliable accelerator condit ions.

References

[1] O.N. Jarvis, Neutron Detection Techniques for Plasma Diagnostics, International School of Physics, Varenna Italy (Sept. 1982) vol. 1.

[2] H.W. Hendel, Neutron Yields and Energy Spectra from Fusion Plasmas, ibid.

[3] W.R. Faust and E.G. Hai'ris, Nucl. Fusion 1 (1960) 62; G. Lehner and F. Pohl, Z. Physik 207 (1967) 83; G. Lehner, Z. Physik 23 (1970) 174; G. Lehner and F. Pohl, IPP 1/107 (1970); M.M.R. Williams, J. Nucl. Energy 25 (1971) 489; S.P. Bogdanov and V.I. Volosov, Recent Advances in Plasma Diagnostics, vol. 3 (Consultants Bureau, New York, 1971) p. 1; H. Brysk, Plasma Physics 15 (1973) 611; D.W. Muir, Proc. 1st Topical Meeting on Technology of Contr. Therm. Fusion, vol. 2, San Diego (1974) p. 166; D.H. Lessor, Batelle BNWL-B-409 (1975); H. Liskien, Nucl. Sci. Eng. 71 (1979) 57.

[4] N. Jarvis, private communication (October 1984).