national Bureau 01 btauuTM

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NBS TECHNICAL NOTE 562

National Bureau of Standards

Library, E-01 Admin. Bldg.

OCT 6 1981

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A Right Angle 3 He Cryostat

Incorporating a High Field

Superconducting Solenoid

10NAL BUREAU OF STANSARTS

UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

y S, NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

NBS

-t TECHNICAL NOTE 562

ISSUED DECEMBER 1970

Nat. Bur. Stand. (U.S.), Tech. Note 562, 22 pages (Dec. 1970) CODEN: NBTNA

A Right Angle 3He Cryostat

Incorporating a High Field Superconducting Solenoid

H. Marshak and R. B. Dove

Heat Division

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

/ V \

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

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Contents

Page

1. Introduction 1

2. Description of Apparatus 3

3. Operation of the 3 He Refrigerator 11

k. Conclusions 18

$. References 20
A Right Angle 3 He Cryostat Incorporating a High

Field Superconducting Solenoid

H. Marshak and R. B. Dove

Construction and operation of a novel a He cryostat,

incorporating a large superconducting solenoid mounted

at right angles to the cryostat ' s vertical axis, is described. This new cryostat which is part of the transportable National Bureau of Standards 3

He refrigerator,

has been used successfully for nuclear orientation

studies at the Atomic Energy Research Establishment,

Harwell, England.

Key words: He cryostat} 3 He refrigerator; nuclear

orientation; superconducting solenoid.

1. INTRODUCTION

3 1/ In designing a He refrigerator, — for nuclear orientation studies

a problem often encountered is incorporating a high field superconducting solenoid when nuclear polarization is required. The usual solution

for thin samples ( < 0.5 in.) is to use a split solenoid where one then

has vertical access for the cryostat tails (which contain the sample)

and horizontal access (with a minimum of material) for the beam. In the

case where one needs a thick polarized sample, for example, in some types

of neutron experiments, the sample is of the order of 5 in. thick, the

bore of the superconducting split solenoid has to be very large and the

field is reduced considerably. Although the field can be increased by

adding more turns, this becomes very difficult and expensive when high

1/ The terminology which we will use in this paper is to call the entire

system (cryostat, pumps, vacuum gauges, etc.) a refrigerator, whereas

the part which contains the cryogenic liquids and where the actual cooling takes place is called the cryostat.
2/ fields (>~U.O MA/m)— are required. A solution to this problem is to use a straight superconducting solenoid mounted horizontally. This

3/ approach, although simplifying the magnet requirements— as well as the

sample mounting and window geometry, does add to the complexity of con- struction as the 3 He cryostat must then have right angle tails.

The purpose of this paper is to describe the construction and operation of such a

3 He cryostat. In addition to incorporating a high field

superconducting solenoid mounted horizontally, the following design

criteria were imposed:

1. The new

3 He cryostat should be compatible with the existing

3 He refrigerator [1]" which is of the recirculating type. This

refrigerator was designed to be easily transportable and

self-contained as well as cope with a large heat flow.

2. It should have large enough cryogenic baths so that it can

be operated continuously at temperatures as low as 0,3 K

for periods of 2k - U8 hours with a minimum of attention.

This condition is dictated by the fact that most particle

accelerators are operated 2k hours a day and it is difficult and "expensive" to get into the target area once the

machine is running.

3. Finally, the design should be such that the "working end"

( 3 He cooled region) of the cryostat can be easily modified

for other types of experiments in this temperature region.

The 3 He cryostat described below was initially designed to accommodate a thick (~ 5 in. ) holmium-165 sample and a relatively high field

(U. 38 MA/m) superconducting solenoid.

Figures in brackets indicate the literature references at the end

of this paper.

2/ 1 MA/m = 12.57 kOe.

3/ Large bore-high field ( >~ 8.0 MA/m) superconducting solenoids

are readily available.
2. DESCRIPTION OF APPARATUS

A schematic drawing of the cryostat is shown in figure 1. The

overall length is 79 in. and its outer diameter is 11.5 in. It is con- structed mainly from Type-3U7 stainless steel except for those parts

where high conductivity is needed; these are constructed from HPOF

(high-purity oxygen-free) copper. The construction philosophy was to

helium-arc weld whenever possible, if this was not possible then hardsolder was used, and finally if this was not possible soft-solder was

used. For the few low temperature joints which have to be opened frequently we used a low melting soft-solder rather than soft-metal O-rings.

Various components of the cryostat were helium leak detected as they

were constructed, and whenever possible, at their operating temperature

(e.g. 77 K, U. 2 K, etc.). Thus, the time required for leak testing and

"debugging" the assembled cryostat was reduced considerably.—

The cryostat has two separate liquid nitrogen baths, a large one having a capacity of 16 liters and a small one with a capacity of 3.5

liters. Whereas the large bath and its extended copper radiation shield

serve the usual purpose, the small bath serves as both a radiation trap

and a thermal ground at 77 K for all the tubes going into and making up

the main 4 He bath. The latter, when normally filled to a distance of

k in. from the bottom of the small nitrogen bath, contains 18 liters.

A small He bath having a capacity of 1. 2 liters houses the superconducting solenoid. This bath is connected to the main li.2 K bath by

three 0.5 in. i.d. copper "solenoid bath feed tubes." The entire U. 2 K

liquid helium system is closed so that the gas, which normally passes

into the atmosphere through a blow off valve, can be recovered if necessary. The pumped He bath has a capacity of 1. 6 liters and serves both

to liquefy the returning He gas and as a 0.9 K radiation shield sur- rounding the He tail and holmium sample. Gas from the exhaust side

of the 4 He pumps passes through a gas meter so that the evaporation

h/ The 3 He refrigerator operated successfully the first time it was

tested.

3
5 He RETURN LINE

4 He PUMPING LINE

PRESSURE LINE

He PUMPING LINE

SMALL LIQ N

2 BATH

LARGE LIQ No BATH

2 K 'He BATH

0.9 K

q He BATH

S.S.FRIT

32 K

3 He BATH

HOLMIUM SAMPLE

4.38 MA /m

SUPERCONOUCTING

SOLENOID M ill

5 10 in.

Fig. 1. Schematic view of the 3 He cryostat. Vx

, main vacuum space;

V2

, sample vacuum space; S1

, main 4

He bath transfer tube;

S25 pumped 4 He bath transfer tube; R, radiation shields.

k
rate can be monitored continuously. This gas can also be recovered if

necessary. The He system which is, of course , closed, contains 3

liters at NTP. Under ordinary operating conditions, that is when the

sample is at 0.32 K and the 3 He gas is being continuously circulated,

there is approximately 3 cc of liquid in the 3He bath. The He and 3 He

pumping tubes, the 3

He return tube, and the He vapor pressure tube are labelled at the top of the cryostat in figure 1. The latter tube is

used to determine the temperature of the 3 He liquid. In the 3 He return

tube there is a condenser packed with a very fine copper wool located

in the 0.9 K bath. The sintered stainless steel frit is used to re- strict the flow of 3 He liquid. Some of the radiation shields, R, in

these main tubes are also shown. The transfer syphon S 1 for the main

4 He bath extends (although not shown in this figure) to the very bottom

of the superconducting solenoid bath by means of a 0.125 in. i.d.

teflon tube. This extension serves a dual purpose — first, to remove

all of the liquid nitrogen which is used in precooling; and second, to

yield a greater transfer efficiency in filling the magnet and main He

bath with liquid helium. The small syphon S2 is U-shaped and connects

the pumped 4 He bath with the main 4 He bath. There is a valve composed

of a 0.125 in. diameter sapphire sphere with a gold seat, located at

the end of S2 in the h, 2 K bath. This small syphon enables us to transfer liquid helium during the main transfer without using a second storage dewar, as well as re transferring (if necessary) from the main bath

at a later time.

The cryostat 's main vacuum space Vx is common to the liquid nitrogen baths and the I4. 2 and 0.9 K liquid helium baths (there are baffled

holes in the radiation shields). There is a layer of activated charcoal over most of the top of the 0.9 K bath which acts as a cryopump

for Vx once this bath is cold. A second independent vacuum space V2 is common only to the bottom of the 0.9 K bath, the 3 He tail and holmium sample. 3 He exchange gas is used in it to cool the 3 He tail and

holmium sample to ~ 1 K. This exchange gas is, of course, pumped away

before the 3 He system is started. The valves and common pumping system
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for Vl and V2 are not shown in figure 1. During actual operation of

the refrigerator the valves to Vx and V2 are closed. The pumping systems for the 3 He and 4 He baths have been described previously [2] and

will not be described here.

Since the novel feature of the cryostat is the lower part containing the horizontally-mounted superconducting solenoid, it is worthwhile

to describe the construction and assembling procedure for this part in

greater detail, see figure 2. The bottom 3.25 in. of the 3

He tail is

machined from one piece of copper and terminates in a threaded hole

(0.875 in. - 16) to accept the sample holder. There are two resistors

mounted in the 3He tail above the threaded hole. One of these is a

lj.70 P Speer carbon resistor (grade 1002) which is used as a secondary

thermometer. The other one (not shown in the figure) is a 10 k£2 -1 ¥

metal film resistor which is used as a heater. The bottom surface of

the inside of the 3 He tail has a thin copper ribbon (wound in the form

of a spiral) soldered to it in order to increase the contact area of

the 3 He liquid — thereby reducing the "Kapitza resistance. " The holmium sample in the form of a cylinder (U. 7 in. long and 0.875 in. in

5/ diameter) is soldered — into the tubular copper sample holder. This

is threaded into the 3 He tail using Apeizon-N grease to improve the

thermal contact.— A second U70 O Speer resistor is mounted in the

front face of the sample (out of the neutron beam) and is used to monitor the sample's temperature.

In order to get the horizontally mounted superconducting solenoid

as close to the 3

He tail as possible, we used zero radius of curvature

elbows (square elbows). For this reason the 0.9 K right-angle radiation shield (from A on down) and the solenoid bath (from B on down),

— Using Indalloy #2 solder (indium Corporation of America) without

any flux. — Apiezon-N grease was used on all the mechanical joints of the

radiation shields to improve the thermal contact.
although at different temperatures and thus cannot be in contact when

operational , are not independent and represent a single unit in assembling. In addition the 0.9 K radiation shield must be leak tight as it

encloses the vacuum space V2

. It is constructed from 1.2^0 in. o. d.

by 0.0625 in. wall copper tubing which has been cut at U5° and then

welded to form a square elbow. However , before it is welded, the

machining of the solder cup joint at A, the machining at C to accept

the thin soldered window, and the "thinning" of the tube to 0.020 in.

for the rear window have all been done.— After it is welded, two 1.62^

in. o.d. by 0.062^ in. wall, stainless steel tubes — also cut at h$° —

are slipped over it and welded, again, all machining of the stainless

steel tubes having been done prior to the welding. From this point

on in the construction, the 0,9 K radiation shield cannot be removed

from the solenoid bath housing. Next the back plate of the solenoid

bath is slipped on and welded on the inside. The outer solenoid bath

tube is then welded on. At this point the superconducting solenoid is

secured to the back plate. With the three feed tubes on the solenoid

bath temporarily sealed off, it is immersed in water with the face

plate exposed, the latter is then welded to the inner and outer bath

tubes. This last proceedure is necessary to insure that the heating

from the helium-arc welding does not damage the solenoid. Thus, the

solenoid is completely sealed in the stainless steel bath by helium-arc

welding. Although this seems like a very permanent arrangement, the

face plate can still be removed if necessary by machining away the

welds. Since the superconducting solenoid underwent an extensive testing program prior to its mounting in the closed bath, this technique

was considered quite safe. In addition, the resulting assembly — once leak tight — would prove to be trouble free.

To facilitate the description of the assembling procedure, imagine

that the RT (room temperature), 77, k» 2 and 0.9 K right angle radiation

7/— Although one could use extremely thin windows with this geometry, no effort was made to do so since the experimental conditions did not

warrant it.

8
shields are all removed at their vertical decoupling joint (in the

region above the solenoid bath), and that the only item completely

assembled is the 3 He tail (without the holmium sample). The solenoid

bath assembly is then put in place and supported by the "solenoid support rod. " At this point the stainless steel sleeve B is pushed up out

of the way and the cup joint A is soldered. During this assembly a jig

is used to accurately position the solenoid bath, so that when the

holmium sample is in place, it is located in the center of the solenoid.

A bakelite sleeve is used to hold the horizontal part of the 0.9 K

radiation shield in place during the soldering of the cup joint A. The

sleeve B is then moved down into position and secured. The solenoid

bath feed tubes from the magnet bath are connected to the ones above

using solder sleeve joints. This is done, of course, after the current

and persistent switch leads as well as the teflon extension for S1 are hooked up. The teflon extension for Sa actually terminates in a 0.125

in. o.d. by 0.010 in. wall stainless steel tube which is welded to the

inside bottom of the solenoid bath. All three solenoid bath feed tubes

are off -centered so as not to be in the way of the neutron beam. The

sample is put in next and the 0. 010 in. window for the front of the

0.9 K radiation shield is soldered in at C and the vacuum space V2 is

checked for leaks. A photograph of the lower part of the cryostat at

this point in the assembly is shown in figure 3.

Since the k. 2 and 77 K baths of the cryostat share a common vacuum,

their radiation shields and windows can all be fastened by mechanical

means rather than solder joints. Thus, the 0.010 in. thick front window for the solenoid bath, which is put on next, is done so using screws. The 77 K right angle radiation shield is made in two pieces with the

front piece being attached about midway down the solenoid bath, see figure 2. With the back of the 77 K radiation shield just hanging on

the solenoid bath, the right angle RT tail (with its large front window

off) is put over it and brought to a few inches of its final position.

The 77 K radiation shield is then manipulated into place and made fast.

Then the RT tail is fastened. The front of the 77 K radiation shield

(along with its 0.010 in. thick window) and the large RT front window
Fig. 3. Photograph of the lower part of the 3 He cryostat showing the

superconducting solenoid bath and 0.9 K radiation shield.

10
are put on. Finally, the two small outside 0.030 in. stainless steel

windows are put on completing the assembly.—

The superconducting solenoid has an i. d. of 1. 75 in. , an o. d. of

5.0 in. and is 5.0 in. long. It could be operated as high as k. 38 MA/m

(center field) in the persistent mode without going normal. At this

field the stored energy is 5.1 kJ. The power and persistent switch

leads from the solenoid terminate in a connector located in the bottom

of the main U. 2 K bath. Removable leads are used from this connector

to the top of the cryostat in order to reduce the heat leak into the

U. 2 K bath when the solenoid is operated in the persistent mode. A

1 CI - 10 W resistor which shunts the power leads is located near the

connector in the main k. 2 K bath to help dissipate some of the energy

away from the small solenoid bath if the solenoid should accidently

go normal.

A photograph of the entire refrigerator with the cryostat fully

assembled is shown in figure h.

3. OPERATION OF THE 3 He REFRIGERATOR

After the cryostat is assembled the vacuum spaces Vx and V2 are

pumpedc During this time heater tapes are wrapped around the outside

walls of the cryostat to speed-up the out-gassing. A heater is also

used to out-gas the charcoal located on the 0.9 K bath. It normally

— It is relatively easy to change the sample after the cryostat is

assembled as it only involves removing the large RT window, the front

part of the 77 K radiation shield, the U. 2 K window, and the 0.9 K soldered window. Lining up the sample with the neutron beam is also fairly

straightforward for this type of cryostat as we can use standard visual

techniques. The procedure is to remove all the inside windows in front

of the sample and replace the outside window with a transparent one —

thus the sample can be viewed directly. The cryostat can then be cooled

to 77 K, at which temperature 97$ of the vertical shrinkage ( 0.180 in.)

has taken place, and then the sample can be lined up with the neutron

collimator.

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takes about 2k hours to reduce the pressure low enough (~ 10-5 mm of Hg)

to start the precooling. However , before this is done 3

He exchange

gas is vented into the vacuum space V2

. The cryostat is then precooled

to 77 K by filling both liquid nitrogen baths, the large liquid helium

bath, and the small pumped liquid helium bath with liquid nitrogen.

During this time the small syphon S2 is removed. Once the sample and

the system are at 77 K (this is determined by the two carbon resistors

mentioned previously, plus a similar carbon resistor mounted on top of

the 0.9 K bath and a copper wire resistance thermometer wrapped around

the 0,9 K bath)— , the liquid nitrogen is removed by pressurizing the

U. 2 K bath and having it re transferred out of the large syphon S

1

. The

liquid nitrogen is removed in a similar manner from the pumped 4 He bath

•by inserting a long tube to the very bottom. After this is done the

small syphon is replaced. The entire precooling to 77 K takes about

one hour and consumes approximately 80 liters of liquid nitrogen.

Liquid helium is then transferred into the k. 2 K bath. This transfer

is done rather slowly at first (~ 5g liter/minute) so that maximum use

of the cold helium gas is obtained. Once the solenoid is superconducting, the transfer rate is increased to about 1 liter/minute. When there

is sufficient liquid in the main helium bath (~ 5 - 6 inches above the

bottom plate) the small syphon is opened and the 0.9 K bath is partially

pumped. In this way the sample, 3 He tail and 0.9 K bath are cooled from

77 to k*2 K. The pumped 4 He bath is then filled. During all this time

the main helium transfer is still going on. When the 1|,2 K bath has

approximately 18 liters in it, the transfer is stopped. The time re- quired for the complete helium transfer is also about one hour and

about 50 liters are used.

9/— The copper resistor is wound non-inductively from No. k0 wire and

such that its total resistance is about £00 Q, at room temperature. Its

resistance versus temperature relation is almost linear down to about

50 K and then begins to "flatten out. " However, it is still a useful

thermometer to k. 2 K. 13
The next step in the operation of the refrigerator depends upon

what type of data is to be taken with the holmium sample; that is, either

unpolarized or polarized data. Since holmium is ferromagnetic, once it

is magnetized hysteresis effects will show up as a small but measurable

le11/

polarization at 0.3 K, even in zero magnetic field. — Thus, all the zero polarization data must be taken before the magnet is energized. 1 Assuming this has been done the magnet is next energized to its rated

field and put in the persistent mode. At this point the main power

leads can be removed to reduce the heat leak into the lu 2 K bath. The

small 4 He bath is pumped on next. While the temperature of this bath

is being reduced from U. 2 to 0.9 K, the 3 He system is activated. 3 He

gas from the storage tank is vented into both sides of the frit; that is,

the 3 He pump line and the 3 He return line (normal condensing side).

This technique of condensing the gas on both sides of the frit reduces

the start-up time for circulating 3

He. When the temperature of the

small 4 He bath reaches approximately 1.0 K, the 3 He exchange gas in V2 is pumped out. After about 10 minutes the 3

He mercury pump is started

and the 3 He cooling cycle begins. The 3

He gas from the exhaust side

of the mercury pump is returned through a nitrogen- cooled trap to the

condensing line (the storage tank being isolated from the system). When

the temperature of the 3 He tail reaches about 0. h K, the 3 He oil diffusion pump is used with the mercury pump backing it in order to obtain a

— Holmium metal is paramagnetic down to 130 K (N^el temperature) where

it becomes antiferromagnetic with a helical spin structure. At 20 K it

becomes ferromagnetic along the c-axis — still maintaining the antiferromagnetic spin structure in the basal plane. Thus, if the sample

is accidently magnetized it has to be then warmed up above 133 K in

order to be sure it is completely unmagnetized when cooling down. — Data have to be taken at temperatures of h. 2 and 0.32 K for zero nuclear polarization in order to make certain that there are no "nuclear effects" due to just cooling the sample.

Hi
a final temperature of 0.32 K.

If the refrigerator is operated in a "one-shot mode", namely,

returning the 3 He gas from the mercury pump to the storage tank rather

than the condensing line, the temperature can be reduced by 20 to 30 mK.

However, the disadvantage of this method of operation is that after

8-12 hours all the 3

He gas is pumped back into the storage tank and

the sample warms up.

The temperature of the sample is determined by the two carbon resistors mentioned previously; one in the front face of the holmium

sample and the other in the copper part of the 3

He tail. Carbon re- sistors were chosen over germanium because they exhibit a much smaller

magnetore si stive effect. The disadvantage of carbon resistors is that

their calibration can change slightly when cycling between room temperature and liquid helium temperatures. Their temperature dependence

is given by the following logarithmic equation:

_ A log R

T - (log R-B) a >

where the constants A and B are determined from two accurately known

temperatures (usually Iu 2 and 0.9 K). The generated curves for the

resistor mounted in the 3 He tail are shown in figure 5 for the case 12/

of zero magnetic field and also for H = 3.58 MA/m. — Its temperature

dependence below 0.9 K is checked against the vapor pressure of 3 He by

assuming it is in good thermal contact with the liquid. ~•' —

assumption has been verified in previous work with similar resistors —

12/ —• This is ^bhe field at the center of the solenoid, that at the

resistor is ~ 0.5 MA/m. —13/ As pointed out previously the surface area of the bottom of the

inside of the 3 He tail has been increased substantially. — The thermomolecular pressure correction has been taken into account

using the data of T. R. Roberts and S. G. Sydoriak [3], in determining

our low temperature points.

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one mounted directly in the 3

He liquid. Although the resistor mounted

in the front face of the sample has additional thermal boundary resistances (the greased threaded joint and the holmium to copper solder joint)

between it and the resistor in the 3

He tail, their temperatures agree

to within 5 mK when the final temperature is reached. The uncertainty

in the final temperature of the sample is estimated to be ± 10 mK.

The cooling capacity of the 3 He cycle is determined solely by the

pumping characteristics of the system; that is, just the through-put

of the 3 He gas. Although this can be calculated, the error in such calculations may be as large as %0%. Measurements can be made by replacing the sample with a heater. This has been done for a pumping system

similar to ours and the results were that the cooling capacity is

approximately 2 mW down to 0.U K and then falls off to zero at 0.26 K; see figure 3 of reference [2]. The time it takes to cool our holmium

sample (2.5 moles) during the 3 He cycle depends upon its heat capac15/ ity—• , its thermal contact to the 3 He, the heat leak into it, and the

cooling capacity of the refrigerator. If we just consider its heat

capacity and use the cooling curve of reference [2], calculations show

that it should take about 1.5 hours to cool it from 1.0 to 0.32 K. This value agrees fairly well with what we measure (1.5 to 2 hours) —

indicating that we are making good thermal contact with the sample and

that the heat leak into it is rather small.

The total time needed from the beginning of ths 4 He transfer to

when the sample is at 0. 32 K is 3 - h hours. Once the sample is at

0.32 K the refrigerator can maintain this temperature for 2-3 days

without retransferring any liquid helium. The only servicing required

is keeping the liquid nitrogen traps full — this is usually accom- plished by an automatic filling system. Retransferring into the large

k» 2 K bath is done before the liquid level drops below the bottom plate,

lF7 —• The extremely large hyperfine field (~ 700 MA/m) in holmium metal

gives rise to a Schottky type specific heat anomaly which has a maximum of 7 J/mole at — 0. 3 K.

17
thus assuring us that the superconducting solenoid does not go normal.

This retransfer usually goes quite fast and uses a minimum of liquid as the magnet and the bath are at k. 2 K. If data are to be taken at h. 2 K

the sample can be warmed up to this temperature in 20 minutes. The

procedure is to vent 3

He exchange gas into the vacuum space V2 and warm

up the pumped He bath. The latter is accomplished by simply closing

the valve to the pump and retransferring U. 2 K liquid from the large

bath using the small syphon S2

. If the sample has to be changed , the entire system can be warmed

up in a few hours by venting dry nitrogen gas into both vacuum spaces

Vl and V2. The screws holding the outside windows are loosened so that

when Va reaches 1 atmosphere the windows "pop out. " Nitrogen gas is

continuously blown out at a very slow rate to insure that no moisture

gets into the cryostat. Once the cryostat reaches room temperature

the sample can be changed and the "start up" procedure described previously is begun. The total time for changing samples and getting

"cold" again takes about 36 hours.

h. CONCLUSIONS

After the 3 He refrigerator with the new cryostat was tested the

entire system was dismantled and shipped by air freight to the Atomic

Energy Research Establishment, Harwell, England. The first experiment

was a measurement of the effect of nuclear deformation on the total

neutron cross section of 165 Ho over the energy range of 2 to 135 MeV. [U]

This was scheduled at the Harwell synchrocyclotron as it is the only

existing source of neutrons covering such an extensive range in energy.

The experiment was to commence 11 days after the equipment arrived in

England — it was completely assembled and operational 2 days ahead of

schedule. The measurements took 10 days of machine time. Additional

data measuring the effect as a function of temperature were taken 3

weeks later. The entire experiment took less than 2 months. In con- trast previous work done at only four discrete neutron energies using

Van de Graaff accelerators took from 1 to 2 years. [5-8]. Thus, this

experiment demonstrated the feasibility of using such transportable

18
oriented nuclear targets at other laboratories.

The cryostat alone was shipped back to the United States where it was modified to accept a larger bore superconducting solenoid to accom- modate a larger diameter holmium sample. This was used for a second

experiment at Harwell, this time at their linear accelerator.

The right angle geometry of the bottom part of the cryostat can easily be included in the design of a

3

He -

4 He dilution refrigerator.

With such a refrigerator, incorporating a very high field superconducting solenoid, one could obtain quite large polarizations for many

nuclei using the "Brute Force" technique.

!Lt is a pleasure to acknowledge helpful discussions with Mr. R.

S. Kaeser and members of the Cryogenic Physics Section. We would like

to thank the N. B. S. Instrument Shops for their help in fabricating

various components of the cryostat, and in particular, thank Mr. R. R. Orwick for his very fine helium-arc welding. The assistance of Mr. D.

F. Martin in the later modification is deeply appreciated.

19
5. REFERENCES

[1] E. Ambler, R. B. Dove, and R. S. Kaeser, Advances in Cryogenic

Engineering (Plenum Press, Inc., New York, 1963), p. hk3.

[2] E. Ambler and R. B. Dove, Rev. Sci. Inst. 32, 737 (1961).

[3] T. R. Roberts and S. G. Sydoriak, Phys. Rev. 102 , 30J4 (1956).

[k] H. Marshak, A. Langsford, T. Tamura, and C. I. Wong, Phys.

Rev. 2, 1862 (1970).

[5] R. Wagner, P. D. Miller, T. Tamura, and H. Marshak, Phys.

Rev. 139, B29 (1965).

[6] H. Marshak, A. C. B. Richardson, and T. Tamura, Phys. Rev.

150, 996 (1966).

[7] T. R. Fisher, R. S. Safrata, E. G. Shelley, J. S. McCarthy,

S. M. Austin, and R. C. Barrett, Phys. Rev. 157, lli|9 (1967).

[8] J. S. McCarthy, T. R. Fisher, E. G. Shelley, R. S. Safrata,

and D. Healey, Phys. Rev. Letters, 20, 502 (1968).

20
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