TLU 9/73

NEUTRON TIMÉ-OF-FLIGNT FACILITY AT THE

UPPSALA TANDEM ACCELERATOR

M Condi, L-E Persson and L-G Sirömbsrg

A Lindholm Mdt G Lodin

L Nilsson and C Nordborg

Tandam

BOR 533

Laboratory

S-701 21 Uppsata, 8w#<Unf Is i 01B/1004 ?#

TLU 9/73August 1973

THE NEUTRON TIME-OF-FLIGHT FACILITY AT THE UPPSALA TANDEM ACCELERATOR

H Condé, L-E Persson and L-G Strömberg:, Research Institute of NationalDefence, Stockholm, Sweden

A Lindholm.and G Lodin, Department of Physics, University of Uppsala,Uppsala, Sweden

L Nilsson and C Nordborg, Tandem Accelerator Laboratory, Uppsala, Sweden

ABSTRACT

The main features of the neutron time-of-fliftfit facility at the Uppsala

Tandem Accelerator are described in some detail. Special construction de-

tails alonf the beam tube as well as target and detector arrangements are

accounted for. The scientific program maintained at and planned for the

facility is presented together with some typical data obtained up to now.

Tandem Accelerator Laboratory

CONTENTS

1 INTRODUCTION

2 ION SOURCE, ACCELERATOR AND BEAM TUBE

3 TARGET ARRANGEMENTS

k DETECTOR ARRANGEMENTS

5 APPLICATIONS

5.1 (d,n) and (d,nY) reactions

5.2 Neutron capture

5.3 Neutron interactions in lipht nuclei

5.1* Neutron-induced fission

REFERENCES

FIGURE CAPTIONS

FIGURES

3

5

7

9

12

12

13

13

11»

15

16

17

INTRODUCTION

The decision to furnish the Uppsala Tandem Accelerator with a pulsed ion

source implied an important extension of the capabilities and usefulness of

the accelerator for a variety of nuclear physics experiments. The availp.bi-

lity of a pulsed ion beam is especially valuable in neutron physics experi-

ments, since the superior technique to determine neutron energies is the

tiirte-of-flight method. The new facility thus acted as a strong encouragement

for research groups involved in neutron physics to maintain and extend the

experimental work earlier performed at AB Atomenergi, Studsvik and at the

Research Institute of Nations''. Defence, Ursvik, Stockholm. Especially, the

Tandem Accelerator can provide monoenerpetie neutron beams of energies up

to 11 MeV. Many interesting phenomena can be elucidated by the use of

neutrons in the 10 MeV region. However, very few experiments in this energy

region have been performed, probably mainly due to the technical difficulties

associated with the production and use of pulsed beams at the high particle

energies required. The inconsistency in placing a pulsed accelerator (well

suited for neutron physics research) several meters below ground with heavy

concrete walls surrounding the target area might at first sight seem dis-

couraging from the neutron physics point of view. However, test measurements

have shown that very careful designs of the detector shielding could help

to overcome the most severe drawbacks associated with neutron scattering

from the concrete walls. The possibility to place the neutron-producing target

close to the centre of the experimental area probably also helped in this

connection.

Experiments with incident neutrons, for example (n,y) and (n,n'y) require

heavy shielding around the detector. Furthermore, it is often desirable to

measure angular distributions of the outgoing radiation. Thus, a detector

arm capable of holding heavy shielding turnable around a vertical axis through

the sample center should be one of the basic components of the time-of-flight

equipment. These experiments also put special demands on the target construc-

tion, i.e. targets giving high yields of clean monoenergetic neutron beans

are needed. Reactions used to produce monoenergetic neutrons in the MeV

region are the ^(p,n)Tle and H(d,n)Tte reactions. To get high fluxes of

clean neutron beams gas targets are preferred. The use of tritium gas targets

requires, however, special radiation safety precautions.

Other types of experiments, where neutrons are emitted in the nuclear reac-

tion put other requirements on the construction of the detecting equipment.

For example, the detector ana must be as long as the target area permits to

give optimum neutron energy resolution. Furthermore, in the design of the

neutron detector care must be taken that its time response is fast compared

to the width of the beam bursts delivered by the accelerator. In an extension

of this kind of experiment, it is of interest to study, in coincidence with

the emitted neutrons, the gamma-ray decay of levels populated by the (charged

particle, n) reaction. For this purpose another detector arm capable of

rotating a Ge(Li) detector around a vertical axis through the target center

should be included in the system.

It was considered -worth-while to start this report by a short account of the

basic features of the ion source, accelerator and beam tube relevant for

pulsed operation (section 2). The next section (section 3) describes various

types of target arrangements, and section U is devoted to the description of

detectors for various kinds of nuclear radiation (gamma rays, neutrons,

fission fragments, etc). Finally, section 5 gives a rough overall view of

the research program maintained at and planned for the neutron time-of-flipht

facility. In some cases representative data obtained by means of the equip-

ment are used to illustrate its performance in different types of experiments.

ION SOURCE, ACCELERATOR AND BEAM TUBE

The accelerator (Fig 1) is an HVEC EN tandem Van de Oraaff accelerator.

Negative ions emitted from an ion source are accelerated to the positive

high-voltage terminal where electrons are stripped off in a gas stripping

canal. The positive ions Produced in this way are accelerated to ground

potential at the high-energy end of the accelerator. The ion energy is de-

fined by means of a 90 analyzing magnet. The analyzed ion beam is switched

to the various experimental aspemblies by means of i switching magnet.

The accelerator is supplied with two different ion sources, an HVEC duoplas-

matron source for DC beams and an ORTEC duoplasmatron injector with pulsing1) 1 2

possibilites. From the ORTEC injector negative ions of H and H are

directly extracted without external electron adding and preaccelerated by

means of a 150 kV high-voltage power supply. To get pulsed beams the negative

ions pass between two sets of sweeping plates perpendicular to each other.

The first of these is fed from a 2 MHz FIF source. One plate of the second

set is fed with a positive DC voltage variable from 0 to 150 V and the other

with negative square pulses, count down pulses, the repetition rate of which

is variable in factors of 2 from 62.5 kHz to 2 MHz. The phase of these pulses

relative to the preceeding 2 !1Hz field is continuously variable. The width

of the ion pulses can be varied in the range 15 - 75 ns by means of the RF

voltage amplitude. The negative ions are accelerated to ground potential and

then pass through «. 20° deflecting magnet which prevents unwanted particles,

e.g. molecule ions and electrons from reaching the accelerator. Before reach-

ing the accelerator the beam passes a two-gap clystron bunching system with

a calculated bunching factor of 35:1 and a bunching frequency of 6 MHz. With

this system a time resolution of 1.U ns FWH1^ for protons and 1.8 ns FWHM

for deuterons has been obtained at the target position. These values were

obtained in time-of-flight experiments at 6 MeV incident particle energy

using the H(p,n) He end H(d,n) He reactions at an average current of 2

for proton? and 1 pA for deuterons The resolution figures represent the

total system resolution, i.e. contributions from the neutron detector and

electronics are included.

Fig 2 shows the bean transport syste-s from the switching magnet to the tar-

get which is at an angle of 12.5 relative to the beam direction before the

switching magnet. A quadrupole lens placed between the analyzing and switching

magnets is adjusted to give minimum current on the tantalum slit preceeding

the quadrupole lens of the 12.5° beam tube (see Fig 2). This latter lens is

used to focus the been on the texget. Beam steering through narrow slit

systems is performed by means of two separate electro-magnetic steering

systems placed between the quadrupole lens and the target. To check the po-

sition of the beam there is a mechanical beam profile monitor and an optical

viewer system consisting of a quartz disc which can be turned into beam posi-

tion (Fip 3). Close to the target the beam passes through a pick-up tube,

from which time pulses for the time-of-flight electronics are obtained. The

last part of the beam transport system is interchangeable to fit different

experimental demands. Turbo-molecular vacuum pumps are used in the whole beam

tube. A cold finger (Fig 3), kept at the boiling point of nitrogen, is placed

closed to the target to reduce the vapour pressure of possible contaminants

in the target region.

In coincidence experiments using pulsed beams the beam current must be suffi-

ciently low to permit resonable signal-to-backpround ratios. The beam pick-

up tube mentioned in the preceeding paragraph does not operate at these low

currents. Pick-up pulses are then obtained from a pick-up tube placed between

the analyzing and switching magnets where the current is kept at a normal

value. The current is reduced by a mechanical current reducer placed be-

tween the stabilizing slits and the switching magnet. Thus the accelerator

with its stabilizing system is run under normal conditions. The two identical

pick-up tubes consist of brass cylinders, 20 mm in diameter and 20 mm long,

coupled directly to a fast emitter follower via a vacuum feed-through.

TARGET ARRANGEMENTS

Fig 3 shovs the last part of the beam tube with a gas-target cell. After the

last valve the beam tube can easily be changed to hold adsorbed or evaporated

targets, 2.5 cm in diameter. Beryllium and calcium fluoride evaporated onto

gold or tantalum discs as well as tritium and deuterium adsorbed in titanium

layers onto gold discs were used as test targets. The beryllium and calcium

fluoride targets were used to study the time resolution with pulsed beams of

deuterons and protons ( Be(d,n) B, F(p,orv) 0). Absorbed "TI and H targets

are convenient to use in test measurements but to getclean- nonoenergetic neu-

tron benms gas targets are preferable.

The deuterium gas cell is 1 cm in diameter and 3 cm long with an entranceo

window consisting of 2.5 mg/cm Ni foil. Background measurements performed

with various backing materials showed that tantalum gave low background with

deuterons in the energy region 3-5 MeV. Thus a pas cell, with the mentioned

dimensions was made out of one piece of tantalum. This target is used at

deuteron energies of 3-5 MeV, i.e. neutron energies of 6r8 MeV.

For production of neutrons in the energy region 8-11 MeV the "Tl(p,n)"Tte reac-

tion (Fig k) gives a cleaner monoenergetic neutron spectrum than the

H(d,n)"Tte reaction. This is mainly due to the fact that (d,n) cross sec-

tions in contaminant low-Z materials such as carbon and oxygen are fairly

large at the required deuteron energies. Furthermore, to keep the number of

background neutrons small when using the Tl(p,n) He reaction, target con-

struction materials with high (p,n) threshold energies, e.r. Ni, are chosen.

The tritium cell, with the same dimensions as above, is made of brass. The

inside lateral surface is covered with 0.1 mm of gold and a 0.5 mm thick

Ni plate is used as a beam stop. The entrance foil consists of Ni with

a thickness of 5 mg/cm . The system is designed to be run at gas pressures3)

up to 3 atm. The cell is connected to the gas-filling system with a uranium

oven (Fig 5), containing about 10 g of uranium. The uranium powder was re-

peatedly activated with natural hydrogen gas before an amount of 80 Ci tri-

8

tiun gas was absorbed. Heating of the uranium oven releases the tritium gas

sad •?. gas pressure in the ce-fcl of 1.5 fttra can presently be obtained. At room

temper at ure the pas is reabsorbeö in the uranium. For safety there are two

tritium monitors available. One monitor, continuously measuring the tritium

activity around the target system, is supplied with a recorder and two alarms,

one in the target room and one in the control roore. The other monitor is

porteble and mainly used for checks on the system. It can also be used for

measuring the activity in other parts of the target room in case of an acci-

dent. A gas pressure gauge placed on the beam tube close to the target system

is electrically connected with a vacuum valve situated close to the switching

magnet. If the tritium-gas-target foil punctures, the valve closes and pre-

vents tritium fron entering the accelerator. The outlets of the two vacuum

pumps on the bean tube are connected to a fume-hood.

Different types of target chambers designed for (d,n), (d,ny) and (d,pY)

experiments can also be connected to the system. They are all constructed

with movable target holders with several target positions allowing target

changes without breaking of the vacuum. When usinp self-supporting targets

the ion current is collected in an electrically isolated beam stop with pro-

vision for secondary electron supression. The distance between target and *.•

beam stop varies from 30 cm to 300 cm depending on the type of experiment.

The target chambers also contain various types of tantalum collimator systems

defining the size and direction of the incident beam.

DETECTOR ARRANGEMENTS

A lay-out of the experimental area is shown in Fig 2. There are two detector

arms, turnable around a vertical axis through the target centre, one about

1 m long for Ge(Li) detectors and another, 6 m long, for the Nal(Tl) and

neutron detectors. The shorter arm can be placed at any angle with respect

to the incident beam, whereas the angular interval for the longer arm is re-

stricted to -30 - +160 . A monitor detector arm, not included in Fig 2, is

placed above the beam tube at an angle of 135° with respect to the incident

beam.

For neutron time-of-flight measurements there are two NE 213 liquid scin-

tillators, suitable for pulse-shape discrimination between neutrons and gamma

radiation. The smaller detector, 5" in diameter and 1" thick, is optically

coupled to a 2" photomultiplier (RCA 8850) via a truncated cone of polyvinyl-

toluene. It is designed for high-resolution neutron spectroscopy and has a

time resolution (FWHM) over a wide dynamic range of less than 1 ns. The

larger detector, 12" in diameter and 2" thick is viewed by three 5" photo-

multipliers (XP 1OU1). The aim of this detector is to give the high efficien-

cy required in coincidence experiments. A preliminary test measurement gave a

time resolution of about 2.5 ns.

1"An NE 102A plastic scintillator, 1" in diameter and - thick, directly coupled

to a 2" photomultiplier (RCA 8850), is used for monitoring the neutron flux.

This detector is pieced above the beam tube at an angle of 135 with respect

to the incident beam in a strongly collimating shield of lithium paraffin and

lead. The time resolution of the detector is well below 1 ns.

The neutron monitor is calibrated against a neutron telescope (Fig 6) placed

close to the target. The detector arrangement in the telescope consists of

two totally depleted surface barrier detectors placed behind each other and

operated in coincidence. The depletion depths of available detectors are 100,

180, 250 and TOO \m* These detectors will allow the use of the telescope in

10

a vide neutron energy range. Polyethylene foils of various thicknesses,

mounted on a wheel, can be placed in front of the detectors. Recoil protons

from the radiator pass through the first detector (ÄE): 9he pulse-height

distribution from the second (E) detector gated with pulses from the AE

detector gives a proton-recoil spectrum. Background is recorded without ra-

diator. The neutron flux is calculated by means of the geometry of the detec-

tor, the foil thickness and the n-p cross-section.

The main Y""ray spectrometer used in the time-of-flipht work is a Nal(Tl)

crystal, 9" in diameter and 8" long. The crystal is viewed by seven 3" photo-

multipliers (RCA 805*0 connected in parallel. Each multiplier tube is supp-

lied with a separate voltage divider by which the gain can be adjusted to

f re the same pulse-height from each multiplier when the crystal is irradiated

with mono-energetic y rays. The detector assembly is surrounded by 5 cm of

lithium hydride, 10 cm of lead and 20 cm of borated paraffin except for a

10 cm diameter aperture (Fig 7)» The shielding can be extended by a 60 cm

long and 10 cm diameter lead collimator between the sample and the crystal.

Neutrons and y rays from the target are attenuated by boron paraffin and lead

placed between target and detector. Scattered neutrons from the sample are

attenuated by lithium hydride in the lead collimator. The energy resolution

(FWHM) of the detector is about 6 % in the energy range of 2 to 25 MeV. The

time resolution is about 10 ns.

The laboratory is equipped with a number of Ge(Li) detectors with various

efficiencies and energy resolutions. In connection with the neutron time-

of-flight facility these are used or planned to be used in essentially two

types of experiments. In the first type the Ge(Li) detector is used t* re-

cord, in coincidence with emitted particles, the gamma radiation produced by

charged-particle induced reactions. The detector is placed close to the tar-

get on the turntable designed for the purpose. In the second type of experi-

ment the Ge(Li) detector is used to investigate the details of gamma-ray

spectra from neutron-induced reactions, e.g. (n,y) and (n,n'Y) reactions,

in this case it is necessary to place the detector in a heavy shielding to

prevent severe neutron-radiation damnge. The detector Bust also be placed

11

sufficiently far from the samplet to permit time separation of gamma rays

and neutrons (from the sample). In all these experiments Ge(Li) detectors

with large efficiencies arc required, otherwise the running tines will be

prohibitively long. In the (n,y) reaction studies, the energy resolution is

not a critical parameter, since it is difficult to produce intense neutron

beams with energy spread less than a few tens of keV.

A cylindrical liquid scintillator tank designed for V and (n,2n) measure-

ments is available. It contains about 100 1 of a gadolinium-loaded scintilla-

tor (NE 313) and is 50 cm in diameter and 50 cm long with a 6 cm axial hole.

The scintillator is viewed by six 5" photomultipliers (Du Mont 636U). For

background reduction the detector is shielded by 10 cm of lead and 30 cm of

borated Daraffin.

Fig 8 shows the principles of a back-to-back ionization fission chamber with

four separate outputs. This chamber is used to measure fission cross-section

ratios. The fissile materials ere evaporated onto thin aluminium plates

mounted in the chamber with their backs to each other as indicated in Fig 8.

The fission chamber is filled with a mixture of argon and carbon dioxide gas

and a DC voltage of about +500 V is applied to the blank aluminium plates.

In an effort to improve the time resolution of the detector methane gas will

be used instead of the argon-carbon dioxide mixture.

12

APPLICATIONS

5.1 (d,n) and (d,nY) reactions

The present aim of the (d,n) and (d,nv) experiments is to study the proper-

ties of proton states in 2s 1d and T-T/O shell nuclei. Of special interest are

the population pud decay of isobaric analogue states.

The Fe(d,n) Co reaction has been studied et energies between 5 and 6 MeV.

Fig 9 shows a time-of-flight spectrum from an experiment at 5.8 MeV deuteron

energy. The target was a 200 lig/cm self-supporting Fe foil. The 5" x 1"

NE 213 neutron detector was placed at s distance of 5.6 m from the target.

With 2 ns time resolution this gives an energy resolution of 250 keV at 8.6

MeV neutron energy, corresponding to the ground state transition, and 80 keV

at 3-9 MeV, corresponding to the transition to the U.76 MeV ground-state

analogue level.

Preliminary (d,nY) coincidence experiments on the Fe tarpet have been per-

formed. Time-of-flight techniques with pulsed ion beam were used to improve

the tine resolution in the neutron tine-of-flight spectrum. The 5" x 1"

NE 213 detector was placed close to the target (flight path ~ 1.5 m) to fet

an acceptable count rate in the neutron channel. The y-ray count is still an

order of magnitude or more higher than the neutron count rate. This situation

was the main argument for furnishing the time-of-flight equipment with the

larger NE 213 scintillator described in section U. In the preliminary experi-

ments the 8" x 9" Nal(Tl) detector was used for y-ray detection. This detec-

tor will be used when the main features of the y-ray spectrum is of interest.

To pet more detailed information a Ge(Li) detector will be required.

The experiment is run using the PDP-15 computer on-line with a two-parameterh)

program . Fig 10 shows a block scheme of the electronic set-up. Coincident

events are stored on magnetic tape and the y-ray spectrum connected with a

selected neutron energy interval, i.e. originating from a selected exitation

energy region in the final nucleus, can be sorted out afterwards.

13

5.2 Neutron capture

The main application of the Nal(Tl) detector is the study of neutron capture

processes in the giant resonance region. Measurements have earlier been per-

formed at Studsvik for neutron energies below 8.5 MeV and at 15 MeV. Using

the tandem accelerator the available neutron energy range extends up to 11

MeV. This energy range covers the peak of the giant resonance in most medium-

weight and heavy nuclei.

In these experiments time-of-flight techniques are used to reduce the neutron-

induced background in the y~ray spectra. A distance of about 1 i is sufficient

to distinguish between y and neutron events in most applications. For data

collection the PDP-15 computer is used with the two-parameter pulse-height

analyzing program OLDAX . The time spectrum is fed into ADC X (32 channels)

and the Y~ray spectrum into ADC Y (256 channels), y-ray spectra from neutron

capture in Y and Ce in the energy range 8-11 MeV have been recorded with this

equipment.

5«3 Neutron interactions in light nuclei

The Nal(Tl) detector is also used in the study of gamma-rays from neutron

interaction with light nuclei. Scattering experiments on 0 in the energy

range 6.5 to 9.5 MeV have been performed using the H gas-target system.

In the interaction of fast neutrons with 0 very few gamma-ray lines are

produced, predominantly the gamma ray from the 6.13 MeV 3 state to the ground

state. This implies that the poor energy resolution of the Nal(Tl) detector

is sufficient. For other samples, e.p. Nt the pamma-ray spectrum is more

complex and a Ge(Li) detector is needed to resolve the many Y~ray components.

It is intended to cover a wider enerpy region using the H(p,n) He reaction

for neutron production. Gamma-ray and neutron spectra will also be recorded

simultaneously. The PDP-15 computer is used in a vny similar to the proceed-

ing experiment.

The techniques described in this paragraph are being applied to investigate

the nitrogen and oxygen contents in biological matter .

5.U Neutron-induced fission

Measurements of fission cross-section ratios, 0" (X) / a f ( U ) , for various

fissile nuclei have been started, using the back-to-back ionisation fission

chamber described in section U. The neutron energy region 5 to 11 MeV will

be covered, using the H(d,n)%e reaction below E = 8 MeV and the %(p,n)%e

reaction above. The fission cross-section ratio, o, (IT ) / O f (U ) hcs

been measured and the experiment will be extended to cover other nuclei, e.g.2^2U and Th. The PDP-15 computer is used on line for data aquisition with

a program called OLDAfcS . This program permits the simultaneous collection

of four independent singles spectra, each 102U channels.

Alonp with the study of fission cross-section ratios measurements will be

made of fission-fragment angular distributions using a scattering chamber

with macrofol detectors.

15

REFERENCES

1 B Hemryd, TLU 6/72, Tandem Laboratory Peport, Uppsala 1972,

in Swedish

2 G Lodin, TLU 2/71, Tandem Laboratory Report, Uppsala 1971,

in Swedish

3 B Holmqvist and T Wiedlinp, Nukleonik £ (196U) 183

k J Pihl, TLU 8/73s Tandem Laboratory Report, Uppsala 1973,

in manuscript

5 R Bergman, A Lindholm, L Nilsson, C Nordborg and I Bergqvist,

GWI-R 5/72, The Gustaf Werner Institute Report, Uppsala 1972

16

FIGURE CAPTIONS

Fig 1 Lay-out of the Uppsala Tandem Accelerator Laboratory.

Fig 2 The principle components of the time-of-flight facility

(at the +12.5° beam tube).

Fig 3 Lay-out of the torpet part of the beam tube with a pas-target cell.

Fig k Tritium-gas-filling system.

Fig 5 Time-of-flight spectrum from the ^(p^njTte reaction at 10 WeV.

Fig 6 Proton-recoil counter.

Fig 7 Lay-out of target-sample-detector arrangement used in (n,y) and

(n,n*Y) experiments.

Fig 8 Back-to-back ionization fission chamber.

Fig 9 Time-of-flight spectrum from the Fe(d,n) Co reaction a\, the

deuteron energy 5.8 MeV and the laboratory angle 20 .

Fig 10 Block scheme of the electronic equipment used in (d,ny) experiments.

\A \

/

SWITCHINGMAGNET

90° BEAMANALYZINGMAGNET

TARGET ROOM

ENERGY SLITS

Q ##-»CHARGING BELT

CONTROL ROOM

HIGH VOLTAGE TERMINAL6 MILLION VOLTS POSITIVE

POSITIVE IONACCELERATOR TUBE

GAS STRIPPINGCANAL

NEGATIVE IONACCELERATOR TUBE

PRESSURE VESSEL

CHARGE EXCHANGEION SOURCE

IORTEC IONINJECTOR

CIRCULAR RAIL

1 m

DETECTOR

BEAM VIEWER

BEAM STEERING

CONCRETE

COLUMN

VACUUM PUMP

BEAM STEERING

QUADRUPOLE LENSTa SLIT

Pig 2

GAS TARGETTa SLIT

VACUUM VALVE

VACUUM VALVE

COLD FINGER

BEAM VIEWER

PICK-UP TUBE

Ta SLITVACUUM VALVE

BEAM PROFILE

MONITOR

J V

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C. To tritium bulb

D. Vacuum gauge

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G. To target cell

o

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FIG 10