A 14 MeV polarized neutron facility

NUCLEAR

INSTRUMENTS

AND

METHODS

164 ( 1 9 7 9 )

311-321:

©

NORTH-HOLLAND

PUBLISHING

CO.

A 14 MeV P O L A R I Z E D N E U T R O N FACILITY J.E. BROCK, A. C H I S H O L M , J . C . D U D E R and R. G A R R E T T

Department of Physics, University of Auckland, New Zealand Received 9 February 1979 A facility for performing scattering experiments with 14 MeV polarized neutrons is described. The polarization is produced in an atomic beam source. The neutron polarization is found to a relative accuracy of 3%. Backgrounds are very low.

1. Introduction Scattering experiments with neutron beams on protons and on deuterons provide for two and three nucleon systems information that cannot be obtained from charged particle experiments alone. Further, to investigate the spin dependence of the interactions, it is necessary to use neutron beams which are polarized. Unfortunately, experiments with polarized neutrons present three serious problems: a) difficulty in knowing the beam polarization to a good absolute accuracy; b) poor statistical accuracy, since the low counting rates require long running times; c) an appreciable background rate. We describe a facility that tries to minimise these problems. Our system is based on the use of the 3He(d, n)4He reaction, in which low energy (N 100 keV) vector polarized deuterons produce a good intensity of highly polarized 14 MeV neutrons. At this low deuteron energy, the neutron polarization is determined to a good accuracy by some simple measurements on the deuteron beam. We shall argue that we can determine the neutron polarization absolutely to a relative accuracy of 3%. The polarized deuterons are produced in a conventional atomic beam source; the deuterons are accelerated to the required energy of about 150 keV by a simple Cockroft-Walton voltage multiplier. Since no large accelerator is involved, the polarized deuteron source can be dedicated to the polarized neutron experiments, so that long running times are possible, without competition for machine time from other users. Further, the experiments run automatically, with adequate safeguards against various malfunctions. The background problem is attacked in two ways.

The alpha particles are detected in coincidence with the neutrons and effectively provide "electronic collimation" of the neutron beam. Also, for each neutron scattering event a number of parameters, such as pulse heights, pulse shapes, flight times, scattering angle, polarization state, are recorded for a multidetector system on magnetic tape; the data are processed off-line to give a clean separation of the genuine events from the background. 2. The polarized ion source (P.I.S.) This produces a low energy beam ( - 4 keV) of deuterons whose vector and tensor polarizations are established by the switching on and off of a set of three radio-frequency transition units (RFTU). The P.I.S. is of the atomic beam type, similar to early models produced by ANAC~). We describe it very briefly. For a full discussion of such sources we refer, for example, to the paper of Haeberli2). The deuterium gas is dissociated in a 14 MHz rf discharge in an air cooled glass vessel. The coupling is inductive. The atomic beam leaves the dissociator through a hole of diameter 2-3 mm. The Stern-Gerlach separation of the two states m,= +½ and rnj= 1 of the deuterium atoms is done in a 5 0 c m long tapered sextupole. This is followed by a "compressor sextupole" from ANAC. Its function is essentially to form with the separator sextupole an approximation to an achromatic doublet, to focus the atoms into the ioniser region independently of their velocities. This device increases the ionised beam by a factor of about 1.5. The atoms issuing from the sextupoles are polarized in electron spin, hut the three deuteron spin states are equally populated. These populations are upset, to give non-zero nuclear polarizations, by inducing transitions among the Zeeman-split hyperfine levels of the deuterium atoms. These transi-

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TABLE 1

Deuteron radio frequency transition frequencies and static field strengths. Transition

v(MHz)

B(G)

2~ 6 3- 5

454 330

80 80

1 -- 5

6.5

7

TABLE 2

Maximum deuteron polarizations available with the system described in the text. Polarization state

454 MHz 2~6

330 MHz 3~5

[0] [1] [2] [3] [4] [5] [6] [7]

OFF OFF OFF OFF ON ON ON ON

OFF OFF ON ON OFF OFF ON ON

6.5 MHz mr~m

Pz

Pzz

r

OFF ON OFF ON OFF ON OFF ON

0 0 2/3 0 + 1/3 - 1 1/3 + 1 + 1/3 + 1 - 1/3 1 + 2/3 0 0 0

magnitude of the vector polarization is obtained for the polarization states [1] and [6] and that switching between these states reverses this polarization. The atomic beam, now with the nuclei polarized, travels into an ioniser, which is in a strong axial magnetic field whose functions include decoupling the electron and deuteron spins and providing a quantization axis. The ionisation is by bombardm e n t with electrons from a heated filament. A system of electrodes extracts the deuterons at an energy of about 4 keV. Since the longitudinal magnetic field in the ioniser provides the final reference direction for the deuteron spins, reversal of this field provides a m e t h o d of reversing the vector polarization; in practice we do not use this m e t h o d , because it introduces the possibility of changing the spatial density of deuterons arriving at the tritium target and hence the possibility of causing spurious asymmetries in the scattering experiments. We reverse our vector polarizations by switching between the polarization states [1] and

[6]. An overall schematic diagram of the P.I.S. and neutron generation is shown in fig. 1.

tions are produced by applying radio-frequency fields of suitable frequencies. W e use the usual labels2), 1 - 6 , for the energy levels in order of decreasing energy. The transitions used, and the frequencies and magnetic fields for them, are shown in table 1. The m a x i m u m possible vector polarization P and tensor polarizations P:: for the various "polarization states" [0] to [7] are shown in table 2; each polarization state corresponds to a particular combination of the rf transitions being " O N " or " O F F " . It is seen that the m a x i m u m

3.

Neutron

beam

COMPRESSOR SEXTUPOLE

j

O~L

OIFFL,~ON ~PS

I

NEUTRON

SOURCE

I

I

IONISER

i

I I

i

t

RF I TRANSITWDNS I

production

The 4 keV deuterons from the P.I.S. are accelerated through 30 keV, passed through a 15 ° analysing magnet and then accelerated through a further 150 keV, provided by a Cockroft-Walton accelerator, on to a tritiated titanium target where the reaction 3H(cl, fi)aHe, Q = 17.6 MeV, generates neutrons of energy about 14 MeV. A substantial proportion of the vector polarization of the deuterons is transferred to the neutrons (see section 4). The neutrons used in the analysing power experiments are those

ATOMIC

BEAM

SOURCE

Fig. 1. Schematic diagram of polarized ion source and polarized neutron generator.

POLARIZED

NEUTRON

at about 90 ° to the deuteron beam direction, where the neutrons have their m a x i m u m transverse polarization. The alpha-particles at 180 ° (centre of mass) to these neutrons are detected in a plastic scintillator at ground potential and effectively define a cone of neutrons electronically. 4. Determination of the neutron beam polarization The 3H(d, n y H e reaction has a strong broad resonance at 107 keV, in which region the reaction proceeds mainly through the spin channel J = ~, initiated by s-wave deuterons. If this were the only channel for the reaction then the neutron polarization would be completely determined by the deuteron vector polarization and geometrical factors, independently of reaction parameters. From the properties of the P.I.S., the deuteron vector and tensor polarizations are related in a simple manner. Unfortunately, there is a small contribution from other reaction channels, resulting in an effect of about 5% on the polarization properties being considered. In the following, we consider first the situation where the reaction goes purely through the J - ~ , / = 0 channel, and then the situation where another channel contributes also. Finally, we look at some measurements that enable us to make a decision on how to determine our neutron polarization in practice. The co-ordinate system used in describing the reaction is shown in fig. 2. If the deuteron beam has vector and tensor polarizations of respectively P. and P.: only, in the reference frame x, y, z (fig. 2), then the neutron beam polarization components in the x', y ' and z' direc-

Lx y M2J

z ~

DEUTERONS _k~n \

\

E:~.n ×Eoo,

kou\"t~tNEUTRONS

Z

Fig. 2. Coordinate systems for describing deuteron polarization and outgoing neutron polarization.

313

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tions are given by3): a(0)

Px,(O) = ao(O) P, sin 0,

a(0) Pr(O) = 0, a(0) P=,(0) = Go(0) P= cos0, and the differential cross-section from

a(0) is found

O'(0) = GO(0 ) [.l -- ~ P2(COSO) P : : ] ,

where ao(O) is the differential cross-section for an unpolarized beam and P2(cos 0) is the second order Legendre polynomial. The analysing power measurements in the scattering of neutrons from nuclei are sensitive only to the component of neutron polarization transverse to the neutron m o m e n t u m ; we are therefore interested in p,,, which we shall call P,, and which is given by P, sin 0 P.(O) =- Px,(O) =

1 -- ~ P2 (cos 0) P ~ "

where by ~ we indicate the magnitude of any tensor polarization that the deuteron beam may have when it is nominally vector polarized by operating the rf transition units of the P.I.S. in the states [1] and [6]. Clearly, the maximum neutron polarization is at 0 = 90 °. Although the neutron and deuteron polarizations appear to have the same sign from the above equation, we see from fig. 2 that physically their spins are reversed. Note that the 0 above is the centre of mass neutron emission angle with respect to the deuteron spin quantization axis. It has been supposed that this axis coincides with the direction of the deuteron m o m e n t u m . In the real situation, there is a small correction factor because the two directions differ by a few degrees because of the presence of the 15 ° analysing magnet in the deuteron beam. The deuteron vector polarization P produced by states [1] or [6] is related to the tensor polarization P:: produced by any one of the states [2], [3], [4] or [5] by P: = 3e-P==. This result is found by counting the populations of the hyperfine-structure levels and assuming that the rf transitions between these levels all have equal efficiencies. Thus, we can estimate the neutron polarization by measurements on the tensor polarization of the deuteron beam in states [2], [31, [4] and [5]. We find P_ from measurements on the

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anisotropy in the cross-section a(O) when the beam is tensor polarized. Note that a(O) is unaffected by any vector polarization in the beam. By detecting the neutrons emitted at two different angles 01 and 02 we measure simultaneously the counting rates P?(01) and Nr(02) with the deuterons polarized and then the rates N(01) and N(02) with the deuterons unpolarized. We form the quantity

With the counting rate ratio r defined as before the expressions for P~z and P. are modified to become

r -- N(01) N(02) N(O1) -/V(02) '

and

a(O) =

ao 1 +

1 -- ~

P2 (cos O) P=

.

2(1 - r )

1

Pe (cos 01) -

rP2 (cos 02)'

,_

which is independent of detector efficiencies and is self-monitoring. We then find P~z from

1

\ 1 + ½ . ] "P~(c°sO) p(v) zz

2(1 - r ) P== = P2 (c°sO1) " rP2 (COS02)"

As detectors we use the liquid scintillator NE213 coupled to 56AVP photomultipliers. The gamma ray counting rate is appreciable, typically 20% of that due to neutrons. We therefore use neutrong a m m a discrimination on these detectors. The biases are set at levels corresponding to 1-2 MeV electron energy (3-5 MeV proton recoil energy). Of the eight polarization states possible from the rf transition units in the P.I.S., four of them, [2], [3], [4], [5], produce tensor polarization, ideally with P~z of unity. Our measurements find these four values of P:: to be in substantial agreement, but less than unity. The states [1] and [6] that produce m a x i m u m neutron polarization should have P:: of zero. Measurements on these states show this to be very close to correct. (See later for numerical data.) For energies near to its 107 keV broad resonance the 3H(d,n)4He reaction involves predominantly only s-waves in the incident state. There are two possibilities for the channel spin of the reaction, J =32 or J = ~ . The analysis above assumed a pure J = ~ mechanism. We now summarize the effects of a J = ½ contribution. Following Ohlsen4), we define a

=

I

e,~l 2 ~levi 2 ,

b = R~R*~+R*~R~ levi ~

where R: and R, are the matrix elements for Y = ½ and Y - ' respecUvely. The polarization and cross-section equations now become

cr(O) Pn(O)=a°( l a

2

b) P~ sinO'

The relation between P~ and P= is, of course, unchanged. Thus, the overall result is that in order to take 3 into account the J = ½contribution, the simple J = expression for P~: is to be divided by a factor g -

l+b 1+½a'

and that for Pn is to be multiplied by a factor

F - 1-(a+¼b) l+b l+½a (if we neglect the extremely small effect of any ffzV~ in states [11 and [6I). Ohlsen 4) lists four experiments that can give information on the values of a and b. Only one of these has been done (by two groups) and gives the quantity g. Brown et al. 6) found g = 0.92_+0.03, and g = We g =

Ohlsen et al. 5) found 0.950+0.004. use 0.94

in calculating the P.= of our beam. The correction factor F for P, needs a and b to be known separately, but there exist no data from which these can be determined. W e can, perhaps, get some feeling for the magnitude of F by taking some pairs of reasonable values of a and b that reproduce g correctly, and calculate the corresponding values of F, as in table 3. Thus, we can get no conclusive result and can only say that F does not differ from unity by more than several percent, but it is not even possible to say whether it is greater or less than unity.

POLARIZED

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TABLE 3 Relationship between the quantities a and b (see text) and the polarization correction factor F.

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and when we put back the c o m m o n 3% relative error in A we have

e/A ~g p,.

a b F

0 0.06 1.08

0.2 0.05 1.06

0.04 - 0.04 1.03

0.06 0.03 1.01

0.08 0.02 0.99

0.10 -0.01 0.96

0.12 0.00 0.94

Some further information on the neutron polarization in the 3 H ( d , fi)4He reaction can, however, be extracted from a paper on the m e a s u r e m e n t of the n e u t r o n - d e u t e r o n analysing power at 14 MeV by a Basel group 7) using a polarized neutron source similar to ours. They made s o m e direct m e a s u r e m e n t s of the neutron polarization by measuring the asymmetry e in the scattering of the neutrons from 4He, for which the analysing power, A, can be calculated to an accuracy of about 3% from the n-4He phaseshifts of S t a m m b a c h and Walter8). They also estimated the neutron polarization from P=, using the simple model in which the 3H(d, n)4He reaction is assumed to proceed wholly through the J = ~ channel. In table 4 we list their values of neutron polarization measured in the n-4He scattering (e/A) and estimated from the tensor polarization measurem e n t (~ ~:). In their paper all four values of e/A have included a c o m m o n relative 3% error for the uncertainty in the calculation of A from the n-4He phase-shifts; in our table 4 we have removed this c o m m o n error leaving only the independent statistical ones arising from the actual experiments. C o l u m n 4 gives the ratio of the neutron polarization found from the two methods. The average of these is

c/A - ~vg = 1.005+0.017, 3P:TABLE 4 Measured neutron polarization compared with the deuteron source tensor polarization resulting from different radio frequency transitions. These results from Preiswerk et al. 7) are used to determine a neutron polarization correction factor F. Scattering angle (lab.)

62 ° 63 ° 64 ° 113 °

e/A 2/3P~z

(e/A)

Calculated neutron polarization (2/3Pzz)

0.450+0.014 0.502_+0.015 0.486-+0.029 0.481 +_0.013

0.459+_0.006 0.475_+0.007 0.481_+0.004 0.486_+0.003

0.980_+0.033 1.057-+0.035 1.010+0.061 0.990_+0.027

Measured neutron polarization

315

= 1.005 __+0.034.

To the extent that this ratio is unity, it appears that F = 1, with the d e n o m i n a t o r ' s known departure from unity being balanced out by the numerator. Using the value of g by Ohlsen et al. 5) and the value of F = 1.005_+_0.034 we calculate Ohlsen's 4) parameters a and b to be a =

0.05 _+0.029,

b = -0.026+0.014. We conclude that the polarization of the neutron b e a m can be found to an accuracy of about 3% from a m e a s u r e m e n t of P::, using a model of the 3H(d, n)4He reaction assuming a pure J = ~ channel spin mechanism. This uncertainty is rather smaller than the statistical errors in our analysing power measurements. 5. Neutron polarization monitor The P: m e a s u r e m e n t required for the estimation of the neutron polarization was normally done twice daily. It was felt desirable to have also some continuous direct monitoring of the neutron polarization throughout the analysing power experiments. The use of n-4He for such a monitor would be very good because of its well-known analysingpower, but the practical problems associated with running a high-pressure gas scintillator or liquid scintillator of helium reliably over the long times required by our experiments m a k e its use unsuitable. We have used instead neutron--carbon scattering; this has a large analysing power at some angles; (e.g. A - 0 . 6 near 50°). In order to keep the background rate low in this monitor, we wish to obtain a signal from the recoiling carbon nucleus, and put this into a triple coincidence with the alpha-particle from the 3H(d, n y H e reaction and the neutron scattered to left or right, just as for the main analysing power experiment (see fig. 6). W e therefore used the plastic scintillator NE102A as the carbon scatterer. The problem of the small light output from the low energy carbon recoil being down at the level of the photomultiplier noise was solved by viewing the scintillator with two photomultipliers put into a fast coincidence. The effective singles rate in this detector was further reduced by the use of an upper level

316

J.E.

B R O C K et al.

N

rO

N

10

ECAR B FLG

q2

~]M]

LINEA" FAN IN rO;Q F N ~-LG

TO 2 - FOLD Ol~

I ~"°~

I j TO 10 - FOL [h OF~ & THENCE TC/ TrM E 'STOp"

POLARISATION MONITOR Lo GATED E N C O D E R (CH 9 )

T

10 F O L D OR &~2 ~cHgl

ro

Fig. 3. Block diagram of electronics for n - 12C polarization monitor.

discriminator providing an anticoincidence pulse for the large signals from recoiling protons or from gamma-ray detection. (The light output from the carbon recoils corresponds to that from electrons of energy of 20 keV or less.) A block diagram of the electronics is shown in fig. 3; this system was coupled in with the electronics of the main scattering experiments for the data recording on magnetic tape (see section 6). The performance of the system as a polarimeter was tested by making some analysing power measurements at a few scattering angles. The pulseheight spectra of the carbon recoils were quite clean of background, as seen in fig. 4. The analysing

8o

>

oc

0,6

r

I

I

I

PARRS 0.4 A nc

BASEL AUCKLAND

0.2

5O 40 3C 2O 10 z

power results are compared with other results in fig. 5 and are seen to be in satisfactory agreement. In use with another analysing power experiment, this monitor was placed downstream from the scatterer at a distance of 2 m from the neutron source. Neutrons scattered at 45 ° (lab.) were detected in scintillators centred at 45 cm from the carbon scatterer. With this geometry the inelastically scattered neutrons are not separable from the elastically scat-

0

-0.2

w

-0.4 ~

0

~

~°°1

D 150

-O.~

Z

lO0 5O 0

-O.& O5

10

15

20 5 10 CHANNEL NUMBER

15

20

Fig. 4. Carbon recoil spectra for various laboratory neutron scattering angles.

210

40

I 60

I 80

100

BLab

Fig. 5. Comparison of measured n - 12C analyzing powers with those of Basel group 7) and a Paris groupl2).

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NEUTRON

317

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N 5 '¸,

/

N7

,

N3

n

/

5"

N9

N2

N8

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~N6' L POLARISEO I+ - - NEUTRON ---~1 SOURCE

I,

-

-

,

THE E X P E R I M E N T

)

I I.,

POLARISATION MONITOR

)

I

,

Fig. 6. Orientation of neutron source, scattering experiment and polarization monitor.

tered ones, but this is unimportant since we require a check on the constancy of the neutron beam polarization, rather than a measurement of it. 6. Analysing power experiments 6.1.

T H E DETECTORS

The general layout of the detector system for an analysing power experiment is indicated in fig. 6. The associated alpha particle detector and the carbon polarimeter have already been discussed. The scatterer S is a liquid scintillator, NE213 for n-p experiments or NE230 for n - d experiments. These have diameters 2 5 r a m or 5 0 r a m and lengths 75 m m or 100 ram. The scatterer is coupled to a 56 AVP photomultiplier. The presence of carbon in the scatterer could lead to the presence of a systematic error caused by the inelastic scattering of neutrons from carbon in which the de-excitation gamma-ray is detected in the scatterer and the inelastic neutron in a side counter. As shown in fig. 7, a time-of-flight measurement is unable to separate these neutrons from those scattered from protons, for the laboratory scattering angles forward of 60 ° . Pulse height considerations are also unable to separate clearly such events over this same angular range. We have therefore used pulse-shape discrimination between electrons and protons in NE213 (or between electrons and deuterons in NE230). In the early work we used a simple but effective device obtained from the C . E . N . - G r e n o b l e 9) and more recently the Model 7113 rise-time amplitude converter of SAIP-Schlumberger.

The side detectors N are blocks of plastic scintillators, NE102A of size 5 0 x 100× 100 m m 3, with a 5 0 x l 0 0 m m 2 face towards the scatterer and the 5 0 r a m dimension in the polar angle direction. These are coupled to 56 AVP photomultipliers; up to four pairs were operated simultaneously. Neut r o n - g a m m a discrimination is not required on these detectors, since a time of flight measurement allows the removal of gammas from inelastic scattering from carbon in the scatterer and of any other gammas that travel between the scatterer and side detectors. In fact, it is better not to discriminate against these gammas since one gets a small gain in efficiency ( - 1 0 % ) by detection of the gammas from the neutron excitation of carbon.

£ i

3o-

~EumONS~

(np*n~

10 -

( nC ~" n'C4"4)

00%

~

_~

30 o SCATTERING

t

.

GAMMAS -60 ° ANGLE

90 °

Fig. 7. Calculated flight times and spreads from scatterer to side detector as a function o f laboratory scattering angle.

318

J.E. BROCK et al. EXPERIMENTAL HALL

-- ~ANODE

ANODE

E

COUNTING ROOM

1 . . . . .

10

FROM POL/MON

+ L r J

E 1- E I o . . . . . 10

OR (SIDE)

10 FOLD & ~2

TO TrME

TO POt/NON

"START"

COINC S

I GE~ J

TO APPROPRIATE COINCS 1--

10

lllllJllll

[,o-~o,D oRqI (co~)

INTERVAL TO DET CH NO (*e GATED ENCODER )

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PFS ALARM

TL

TO R r ('SWITCH d')

FROM SCATTERER

7v

TO TIME ,,STOp

'`

l ; , t"MVEE TO OM M OF RR YFULL" BUFFERSTORE

~

AND

GATE

,

C

GATE GEN

FLG-EN(GATE) ~ TIMEENCODER( SC-ALE~

CAT T ~ i ~FLG)UTENES G (GAR TE )Y FLG-SCATT (GATED) NP DISC E NCODEF (GAIE) (GATE)

Fig. 8. Block diagram of the logic system.

The scatterer and side detectors are m o u n t e d on a light metal structure, that allows their accurate alignment with respect to the neutron cone defined by the associated alpha particles. 6.2. THE ELECTRONICS In brief, s o m e fast logic electronics picks out events that satisfy fairly loosely the requirements for an event to be due to the scattering being studied. For each such event, a number o f analogue

pulses are passed through linear gates and A D C ' s and, together with s o m e coding signals, stored temporarily in a semiconductor buffer m e m o r y and then written on to magnetic tape to await off-line examination. All the electronics is of the N.I.M. type. We use up to 10 side detector channels (8 for the main scattering experiment and 2 for the carbon polarimeter). In order to keep the a m o u n t of electronics within reasonable bounds we make use of

POLARIZED

NEUTRON

10-fold logic OR-gates and a 10-fold linear fan-in, enabling all 10 channels to be handled by a single c o m m o n chain of linear electronics.

6.3. THE LOGIC

ELECTRONICS

This is shown in block diagram form in fig. 8. A triple coincidence, alpha-scatterer-side, selects possibly good events. In practice, a fast coincidence with a resolving time of 5 - 1 0 n s was formed between the alpha and scatterer pulses. The output of this was then fanned out so that it could make a coincidence with any of the side detectors, with a resolving time of about 100 ns. This latter width is fast enough because the spectra of flight-times from the scatterer to the side detectors are recorded and from two dimensional spectra involving these and the recoil pulse height in the scatterer, resolving times of a few nanoseconds are obtained after correcting for time variations due to the scattering kinematics. Before one of these possibly good events is accepted for recording, several further conditions must be met at the "8-fold A N D - g a t e " . An event is acceptable if one, and only one, of the side detectors has produced a pulse from its discriminator during the 100 ns coincidence period. If there is more than one such pulse the event is rejected. The function is performed by the "10-fold OR ~ />2 (side)" OR-gate together with a gate generator producing a complementary logic pulse O. This OR-gate, besides producing the usual ORoutput, provides another output (1>2) if two or more inputs are present simultaneously. This system, originally for the purpose of removing events in which two side detectors detect neutrons, is in practice used to remove events triggered by corona from the Cockroft-Walton generator. We require, also, that not more than one of the 10 alpha-scatterer-side coincidences has fired by means of a similar OR-gate system [10-fold OR >/2 (coinc)]. This condition ensures that there is no cross-talk among the 10 channels. A double-discriminator system is used on the side detectors to give a small improvement in the timing resolution with our leading-edge discriminators. The 10 individual discriminators are at fairly low levels (about - 5 0 mV) and provide pulses for the time of flight measurement. A second, highlevel, discriminator is set at - 100 mV to 200 mV and it is required that both discriminators trigger before an event is acceptable. The use of ten separate high-level discriminators is avoided by having a

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319

single discriminator at an output of the linear fanin, into which all ten side detectors are fed. W h e n the buffer memory is full, the system is disabled by a veto pulse from the buffer while the m e m o r y empties itself on to magnetic tape. The master scaler counts the alpha-scatterer coincidences and provides an " i n t e r v a l " level that enables one channel of the 8-fold A N D gate. After a preset number of these events, the scaler stops counting and the interval level changes to disable the A N D gate and stop the accumulation of data. The polarization control unit then changes the deuteron polarization state, resets the scaler, changes the " i n t e r v a l " level and data accumulation resumes. If all the conditions at the 8-fold AND gate are correct, it provides an output that via gate generators and fanouts open the gates of the modules of the "linear electronics". To allow sufficient time for an event to be encoded and stored, a dead time is introduced by feeding the output of the "8-fold A N D gate" back into one of its inputs through a gate generator providing a veto pulse of 100/~s duration. 6.4. THE "'LINEAR" ELECTRONICS The "linear electronics" (fig. 9) makes the final selection of the data to be recorded. It consists essentially of fast linear gates (FLG) and their associated ADCs for the scatterer pulses, the carbon recoil pulses of the polarization monitor, the analogue signal from the neutron-gamma discriminator on the scatterer and for the pulses from the ten side detectors (after passing through a 10-fold linear fanin), of a time encoder for the scatterer to side detector flight-time, of a " d e t e c t o r n u m b e r encoder" which records the number of the side detector that has triggered the event and therefore gives the scattering angle and whether the scattering was to " l e f t " or " r i g h t " , and of an encoder to give the polarization state of the beam. There is an " e v e n t m a r k e r " associated with each event. All these quantities are stored as 6 " p a r a m e t e r s " , each parameter being of 8 bits. 6.5. DATA STORAGE AND TREATMENT The six parameters are stored event by event, first in a small buffer memory ~°) and then on magnetic tape. The data on magnetic tape is processed off-line at the Burrough's B6700 computer of the University of Auckland's Computer Centre, in the following manner.

320

J.E. BROCK et al.

SIART

STOP

ES

ECA~B

E~

N-F S

TO Ew:)

BUSY

,t

IIHHH PARAME~'ER 5

't

IIIIIIII PARAMETER

6

PARAMETER

"TOTAL"

1

~ 3

PARAMETER

10

CONTROL UNIT

IIIII 4

I--

-

-

-

PARAMETER

2

--

-

PARAMETER 1

Fig. 9, Block diagram of the linear electronic system.

After an experimental run the data are printed as sets of the three matrices: 1) TvsE+, 2) P vs Es, 3) T vs En, where T is the scatterer-to-side-detector flight time, E+ is the pulse height from the scatterer, E. is the pulse height from the side detector and P+ is the "particle" pulse from the n e u t r o n - g a m m a discriminator on the scatterer. For each detector there are

(a) N E U ~ ~ GAMMAS

two sets of such matrices, one for spin " u p " and one for spin " d o w n " . The nature of the matrices is shown in fig. 10. An inspection of a set of matrices enables one to pick out on each matrix the "region of interest" for the scattering of interest. The positions of boundaries of these regions are then provided to the computer, which then applies a "coincidence" requirement on the set of three matrices. The final computer output is then a set of matrices free of unwanted events. An overall block diagram of the whole facility, from polarized ion source to off-line processing is shown in fig. 11. 7. Performance of the facility

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Ion currents were measured in a Faraday cup placed after the ioniser. The beam current of polarized deuterons was taken as the difference between readings with the separator sextupole on and with it off. This current is'at best 2/~A. The corresponding neutron flux is about 2 0 0 c m 2s i at 1 m from the tritium target.

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NEUTRONS

7.2. BEAM POLARIZATIONS The tensor polarizations P= measured as in section 4 were typically 0.7-0.8, depending very sensitively on the state of the vacua in the atomic beam source and in the ioniser. This produces neutron polarizations of about 0.5.

En Fig. 10. Schematic representation of the two dimensional computer print outs used for interpreting neutron-proton elastic scattering data. T is .neutron flight time from scatterer to side detector. E s is proton recoil energy in scatterer. Ps is scatterer pulse shape discrimination output and E n is pulse height from side detector due to scattered neutrons.

7.3. THE ANALYSINGPOWEREXPERIMENTS Our system of electronics and off-line computer treatment of the raw data produce remarkably clean spectra. For example, in the case of n-p scattering, the only background remaining could be accounted

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Fig. 11. Block diagram of complete system used for few body polarized neutron induced reactions. for in t e r m s of m u l t i p l e scattering o f n e u t r o n s in the scatterefl ~).

8. Use of the facility and its future W e h a v e a l m o s t c o m p l e t e d a set of a n a l y s i n g power m e a s u r e m e n t s for n e u t r o n s o n p r o t o n s , deuterons a n d carbon. T h e next set of e x p e r i m e n t s u n d e r c o n s i d e r a t i o n include: W o l f e n s t e i n D or D T p a r a m e t e r s in n - p scattering, the D p a r a m e t e r in n - d scattering, the n - p a n a l y s i n g power for l o n g i t u d i n a l l y polarized n e u t r o n s (to look for p a r i t y - v i o l a t i n g effects in the n u c l e o n - n u c l e o n system). Such e x p e r i m e n t s will, of course, require a greatly increased n e u t r o n flux. By a m a j o r m o d i f i c a t i o n o f the ioniser, a n d also by p a y i n g m o r e a t t e n t i o n to the ion optics d u r i n g the acceleration, we expect to gain m o r e t h a n a 10-fold increase.

References l) ANAC Ltd., P.O. Box 16066, Auckland 3, New Zealand. 2) W. Haeberli, Ann. Rev. Nucl. Sci. 17 (1967) 373. 3) F. Seller and E. Baumgartner, Nucl. Phys. AI53 (1970) 193. 4) G.G. Ohlsen, Phys. Rev. 164 (1967) 1268. s) G. G. Ohlsen, J. L. McKibben and G. P. Lawrence, Proc. 3rd Int. Syrup. on Polarization phenomena in nuch,ar reactions, Madison (1970). 6) L. Brown, H. A. Christ and H. Rudin, Nucl. Phys. 79 (1966) 459. 7) M. Preiswerk, R. Casparis, B. Th. Leeman, H. Rudin, R. Wagner and P. Zupranski, Nucl. Phys. A263 (1967) 276. 8) Th. Stammbach and R.L. Walter, Nucl. Phys. AI80 (1972) 225. 9) j. p. Cremet and J. Pouxe, Note PhN68, Lab. de Phys. Nucl. Universit6 de Grenoble. 10) K. R. George, Nucl. Instr. and Meth. 127 (1975) 249. tl) j. E. Brock, A. Chisholm, J. C. Duder and R. Garrett, Nucl. Instr. and Meth. 137 (1976) 537. 12) R. Sene, P. Delpierre, J. Kahane and M. de Billy de Crespin, Proc. 3rd Int. Syrup. on Polarization phenomena in nuclear reactions, Madison (1970).