dx detector for heavy mass nuclear particles

NUCLEAR INSTRUMENTS AND METHODS 95

A V E R S A T I L E dE/dx D E T E C T O R

(I97I)

349-359; © N O R T H - H O L L A N D PUBLISHING CO.

FOR HEAVY MASS NUCLEAR PARTICLES* M. L. MUGA

Department of Chemistry and Department of Physics, The University of Florida, Gainesville, Florida, U.S.A. Received 21 April 1971 We report herein a significant development in the art of heavy fragment (heavy nucleus) particle detection and identification. Recent improvements in the thin film detector have made it possible to achieve an output pulse responding to the energy loss, ,dE, experienced by the heavy fragment in traversing the scintillator film. Typically, for z52Cf fission fragment energy losses of about 10%, a clear separation between the light and heavy fragment AE signals is observed. In recording the AE spectrum, peak-to-valley ratios greater than 5:1 are obtained in the best cases. Exploratory experiments on light fragments

clearly show the different energy losses experienced by ZHe and 4He ions of comparable energies. The signal response (when thicker scintillator films are used) to total energy loss of alpha particles and fission fragments is reported. A preliminary view of the response of the new detector to accelerated 127I ions is also shown. The possible use of these detectors as particle identifiers of heavy mass nuclei is discussed and an attendant wide range of experiments is outlined to which these dE detectors might profitably be applied.

1. Introduction

is then placed between a n d in contact with the parallel faces o f two p h o t o m u l t i p l i e r tubes h o u s e d in a v a c u u m tight chamber. W i t h an experimental a r r a n g e m e n t as shown in fig. 2 for which a b e a m o f ZS2cf fission fragments traverse the film a n d impinge u p o n a solid state detector (SSD), each o f the p h o t o m u l t i p l i e r ( P M ) amplifier circuits is adjusted for equal gain a n d shape distribution. The

The thin film detector ( T F D ) has been s h o w n ' ) to be a useful device for registering the passage o f heavy charged fragments. The T F D is ideally suited for use in time-of-flight ( T O F ) m e a s u r e m e n t s because o f its unexpected characteristics such as 1) g o o d signal-tonoise ratio, 2) fast time response, 3) small energy loss to the transient heavy f r a g m e n t a n d 4) simplicity o f fabrication. Recent i m p r o v e m e n t s 2) in the design o f the thin film detector have resulted in an even m o r e surprising feature, viz. the possibility o f achieving an o u t p u t pulse p r o p o r t i o n a l to the energy loss A E as experienced by the heavy f r a g m e n t in t r a v e r s i n g the scintillator film. Exploited to its fullest measure, this novel detector m a y open a new d i m e n s i o n in the detection a n d identific a t i o n o f very heavy ions.

PHOTOMULTIPLIER TUBE

a)

b) L

0

2. Description

PHOTOMULTIPLIER TUBE

J

J

l ~

I

2

3

4

SCALE-CENTIMETERS

A d r a w i n g o f the new detector a p p e a r s in fig. 1. It consists o f 1) two hemi-cylindrical lucite halves with large: d i a m e t e r holes b o r e d p e r p e n d i c u l a r to the large flat surface in o r d e r to pass the c h a r g e d particles a n d 2) a lucite (or teflon) sleeve or bushing into which the halves m a y be inserted. All external curved surfaces o f the assembled unit are c o a t e d with optical white p a i n t in o r d e r to i m p r o v e light gathering characteristics. A thin scintillator film is p l a c e d across the large fiat surface, the two half cylinders are faced to each other a n d together are inserted in the tight fitting cylindrical sleeve with m a t c h i n g b o r e d holes. Care m u s t be t a k e n in this o p e r a t i o n such t h a t wrinkling o r tearing o f the thin film is averted. The c o m p l e t e cylindrical assembly * Supported in part by the U. S. Atomic Energy Commission through contract AT (40-1)-2843. This report is ORO-2843-19. 349

./~"

-- THIN SCINTILLATOR FILM PLACED HERE

Fig. 1. Scale drawing of improved design of thin film dE/dx detector. It consists of two cylindrical halves c) and d) which are inserted into bushing e) after a thin scintillator film is placed on flat upper surface of one. The assembled unit is shown from end view a) and side (cut-away) view b). The completed assembly is housed in a vacuum tight chamber.

350

M.L. MUGA

I ~"l~ r~l.__i AmP' I

= l ~

t. I

l~~

SignoI(SSD)

Signal (Film)

[~Gate) J

J

PM = Photomultiplier SCA= Single Channel Analyzer Amp= Amplifier

Fig. 2. Diagram of experimental set up for testing thin film detector (TFD) response to fission fragments as a function of residual energy as measured by solid state detector (SSD). The SSD could be placed so as to intercept fission fragments before or behind the TFD.

two amplified PM channels are added (mixed) and a pulse height distribution such as appears in fig. 3b is obtained when gated by the fission fragment distribution (fig. 3a) from the SSD. When gated by the 6.1 MeV alpha particles as well as the fission fragments from the 252Cf source, the corresponding spectrum (shown in fig. 3c) proves the o without TFD in place 1600]'- • with TFD in place 1200

-

U.l Z Z

800]400

< fl"

u) l-Z o

[_

•

"

1600

..... : . . .•

•" o ;

t

o

°o-°' .

•

•

,;"

I

°

I

4

.] • "~.,

/

c)~

r

4 1,2oo

....

,o,oooL:: .-"" "" . . . . . , . , . " • ~,k

I

o

°"

4o.oool- .,,7 3°'°°0t,.: [ F •

o

•%'*e

°%

]

4

o

b)TFD el

•" ..,'~ ........................

20,O00k

a) SSD

%,.. %,0,i

,.N.,,.

•

120Cf

800 400

oo

• e• ooo o • • o• '* * o •o . o o • o

| ........ ":'. ,°". ...........

ILl + n

•

•

q 800

"--.

•l,.~e +.l,,,%e . . . . . . . . . . . . . . . . . . . . . . . . %.I

I+oo

CHANNEL NUMBER# PULSE HEIGHT Fig. 3. Response to 252Cffission fragments of a) solid state detector (SSD) and b) thin film detector (TFD) when gated by SSD. When gated, in addition, by alpha particles from 2s2Cf, TFD distribution as shown in c) is obtained.

surprising ability of the TFD to respond distinguishingly to the smaller dE/dx signals generated by these lighter ions. An indication of the fission fragment energy loss is given by the SSD spectrum peak positions (fig. 3a) with and without the thin film detector intercepting the fission fragment beam. For this particular case the thin film is of such a thickness that the typical light and heavy fission fragments lose respectively, 16% (17 MeV) and 19% (15 MeV) of their initial kinetic energy. This loss corresponds to a film thickness of the order of 0.4 mg/cm 2 as measured by alpha particle energy loss methods. Although the best results to date have been obtained with the detector as described above, almost equivalent response can be obtained using only a single hemicylindrical lucite piece (fig. ld) with all curved surfaces painted optically white. The thin film can be made to adhere readily by smearing an almost non-existent film of optical grease across the large fiat surface onto which the thin scintillator film is placed. Because of its ease of fabrication, most of the following results were obtained with this more simple version.

3. Investigations In separate exploratory experiments we have examined the dE/dx characteristics of the T F D in response to the passage of 252Cf fission fragments, accelerated 127I ions, and 3He and 4He particles.

dE/dx D E T E C T O R

VERSATILE

FOR HEAVY MASS N U C L E A R P A R T I C L E S

3.1. CORRELATION BETWEEN T F D PULSE HEIGHT SPEC]-RUM AND FISSION FRAGMENT KINETIC ENERGIES

252Cf fission fragments were allowed to pass through a T F D and impinge upon a solid state detector (SSD) as shown in fig. 2. The spectrum of the T F D is recorded, gated by various portions of the fragment residual kinetic energy distributions as viewed by the SSD. The energy degradation experienced by the fragments in the T F D amounts to approximately 12%. The results are shown in figs. 4 and 5. We note that when gated by sequentially increasing energy sections of the SSD low energy (heavy fragment) peak (gates a, b, c, d of fig. 2), the T F D spectrum is reflected in correspondingly higher values under the left peak. When gated by sequentially higher values of the high energy (light fragment) peak (gates f, g, h, i of fig. 3), the T F D spectrum exhibits also correspondingly higher values under the right peak. Of special interest is the T F D spectrum when gated by the middle (valley) portion (gate e) of the fragment kinetic energy. For

2

i i

I

] 300~

(/) I.-. z o (J LL 0 r,,., I..iJ m ~E D Z I,i >

; I/

a:a

/

,

°

./"-'N

1 , e

,

i i

,

,

i

1 ,

',

i i

~

I i

IOC GATED BY f) 60C . °

40C

i,,r, . . . . . . . . L~4~/P-U ~'r a~

o.

1200L

,

,

,

,

,

r.pt.,

,

,

J

.

l,

,

i

i

,,

, . , , , . , ,

GATED BY b)

| 8001 400[---

.i,

•

•

.•

| GATED

BY c)

I 2De %°o r •

_J w

80(3 40C

• i ii1" I i

.o I

i

I i

I

I

i

ftl.i

i

i

i

i

i

i

i

,

i

i

I

i

i

I i

[

GATED BY d) 60C

." ",. •

40C

".

*,.o

20( .....

•

,°

I.•.1•~"~

•

,

i

,

i

i

I

i

.

, ~ i

GATED BY g) ""

400 r

.•

200t-

%

.•

l

, ......

,':'..

"•

, .........

•]'"

•

•

i:

20C

iOC

"i I

,

i

. . . .

I

•

30C

,.,-J "

l l ~ !111

II I

600I

~_ z= ~,

i

,

s

,

,

.

,

. . . .

•

i] "

*, °° •

[GATED o

40

80

°

e"

i:::

120

160

200

NUMBER ~

PULSE

HEIGHT

o

240

Fig. 5. Pulse height distributions from TFD when gated by indicated portions (see previous figure) of solid state detector distribution. Incident particles are fission fragments from z52Cf decay.

THIN FILM DETECTOR

,

,°

20C

i

I00 i , /

%,

,1

CHANNEL

t

I

" •°•'%•

|

, 1

i

l °''•°

20C

'°srAr'i s° :f DETECTOR

THIN FILM DETECTOR

GATED BY e)

~

50[

, c

GATED BY o - i ) ~ /

300

351

• ~', . . . . . . . . :'I., . . . . . . . . . . . . . . ! 40 80 120 160 200 240 C H A N N E L NUMBER ~ PULSE H E I G H T

Fig. 4. Pulse height distributions from TFD when gated (in coincidence with) by indicated portions of solid state detector distribution. Incident particles are fission fragments from 259Cf decay.

this case the associated T F D spectrum is characterized by a much broader peak centered in the valley of the complete T F D spectrum. This broad peak is possibly the sum of two peaks, each representing the T F D spectrum from "typical" light and heavy fragments having nearly equivalent kinetic energies. It should be noted, however, that these "typical" heavy and light fragment masses are not identical to the average binary fission heavy and light masses (with a mass ratio of 1.35), but rather are closer together in mass, having a mass ratio of ~ 1.2 and their associated distributions are broader3). Generally, we see the more energetic light fission fragment as creating a larger T F D signal than the typical heavy fragment. Furthermore, within each group (light or heavy), the more energetic fragment generates the larger signal. The overall shape of the distributions seems to mirror the SSD residual kinetic energy spectrum with the heavy fragment portion of the TFD Spectrum peaking higher and having the lesser width.

352

M.L.

MUGA

peaks of fig. 7 to be made up o f roughly parallel (to the 127[ curve) curved sections reflecting the varied mass composition within each peak.

3.2.

I ~

~"~

RESOLUTION

STUDIES

In the absence of an absolute means of calibration in the early phases of detector development, an arbitrary standard for describing the resolution of a T F D was chosen in terms o f the peak-to-valley ratio (P/V) as obtained from z52Cf fission fragments. Here the ratio of the height of the left peak (corresponding to the heavy fragment energy loss) to the m i n i m u m is used. This P/V ratio appears to increase slightly with thicker scintillating films and typically we can achieve a value of 4-5 for a film which degrades fission fragments

" " " $ $

Energy

Fig. 6. Isometric view of a correlated two parameter (response from SSD and TFD) experiment on '2s'->Cffission fragments. The results of a correlated two parameter ( T F D response vs residual energy measured by a solid state detector) experiment are shown in figs. 6 and 7, wherein the E,AE dependence for light and heavy fragments is more clearly depicted in terms of isometric and contour plots, respectively. The response of the T F D - S S D system to accelerated 1 2 7 [ ions is shown for comparison in fig. 7. We suppose the two contour

P HOTOMU

LT I PL I E RS

/ Cf

252

COLLIMATOR

1

s0<,0 STATE

SOURCE

THIN F I LM DETECTOR

~FISSION J FRAGMENT rRAJECTORY

\ / PHOTOMULTIPLIERS

Fig. 8. Schematic diagram of experimental assembly for testing response of stacked dE/dx detectors• Such a system can be used to measure the Bragg ionization curve of a single heavy ion.

sso s . E c T . u .

8 ~ k2ooo ~,)tv, ¸

o

o ,o

o

o o

C~UJ b O O 0 o Za. o o I~ ~ o °

~

o

o o o o °

o

o

o Qn

D

_oO

I r

;o

°oo

L k

I

,.'n

' /l, Lt;SY/ -

I

I

,-4,

I

I/,/,o"

¢.1

.,.

o

1-

o °

??

o

~

t

NO.COUNTS PER CHANNEL

~

,

r

i

z~,-s6

"

i

SSD CHANNEL NO (,~, E R )

Fig. 7. Contour plot of a correlated two parameter experiment on 2s2Cf fission fragments showing also SSD-TFD response (dashed curve) to accelerated l~Vl ions. Inserts show the summed spectra, over TFD and SSD contours, respectively•

by 15°'o of the total kinetic energy; i.e., for film thicknesses of about 400/xg/cm z. In an effort to improve the effective resolution of T F D ' s for fission fragment energy loss measurements, we have experimented with stacks of these detectors. For this purpose, three T F D ' s were arranged as shown in the schematic o f fig. 8. The signals from all six PM amplifier circuits were adjusted in gain to give equal distribution and then mixed (added) as indicated in the caption of fig. 7 to give the various distributions. It was found that the effective resolution, (as measured by the peak-to-valley ratio P/V when 252Cr fission fragments traversed the T F D stack) improved significantly despite the fact that degraded fission fragment AE peaks are closer together (see fig. 11), For the cases shown in fig. 9, a single detector resuited in P/V= 4.5, two stacked together gave P/V= 5.5 and three stacked T F D ' s produced P/V= 7.5. The sum of the fragment energy losses in the three detectors was about 40% of the total fission fragment energy• A single T F D with a film thick enough to stop all

VERSATILE

dE/dx D E T E C T O R F O R H E A V Y MASS N U C L E A R P A R T I C L E S

PM I

2000

1000

J

• •••°•••°

D

•'•

•°•,• a

le

1500

B

PM 1,2 .o

Z Z

•t

°°Ii lO00

a.

• •o,p. •.m."

o•

• •*

° °t •

0

o•

. •

*o

•o

3.4. RESPONSE TO

•e ot *,o

e•

PM 1 - 4

1000 • Go . ~ .

•

,.

500

•

i • oe* * • o •

°. • *•

o"

°°e

"." *e , • % . . .

Z

D ~000

500

4 *••

*.'*'.

•"

PM 1-6

ee

o • o* °o e . . l

CHANNEL

NUMBER ~

.eel

II

• •

*

ENERGY

peak positions were plotted in figs. l l, 12 and 13, respectively, as a function of a) nickel foil thickness producing the energy degradation, b) average energy of the fragments as determined from the SSD and c) average velocity of the passing fragments. The general features expected for an energy loss measurement as a function of these parameters was obtained. The heavy and light fragment curves in fig. 9 may be compared, for example, with similar data by Lassen 4) and curves derived from Fulmer's 5) investigations.

•

•

u~ Z uJ buJ

353

LOSS

Fig. 9. Response of various photomultiplier (PM) output combinations from a three TFD stacked assembly as diagramed in preceding figure. All signals are gated by pulses from the solid

state detector placed behind stacked detectors. Much improved response, as determined by peak-to-valley ratios, is obtained when multiple AE determinations are summed. fission fragments (100% energy loss) typically gives a P/V ratio of about 6-7. This result demonstrates a possible method by which adequate effective resolution can be achieved for heavy ion particle identification. More recently, 5% AE resolution (fwhm) for 85 MeV 127[ ions has been achieved with a single T F D for which a P/V value of 3.3 for 251Cf fission fragments was obtained. For a given T F D the resolution decidedly improves with increasing ion energy. 3.3. RESPONSE TO DEGRADED FISSIONFRAGMENTS A beam of 252Cf fission fragments was degraded by passage through selected thicknesses of nickel foil as shown in fig. 10. After traversing a T F D the fragments were stopped in a SSD which was used to gate the signals from the TFD. Pulse height distributions of the T F D were collected and the heavy and light fragment

3 H e AND 4 H e PARTICLES

In an effort to demonstrate the usefulness of these detectors as means of distinguishing particles (particle identifiers) the thin film detector response (a function of AE loss) to the passage of 3He and 4He particles of comparable initial kinetic energies was studied. The 3He ions were accelerated in the University of Florida Van de Graaff and were elastically scattered from a thin boron target giving an almost monoenergetic beam at about 5.8 MeV which then passed into a T F D (for AE measurement) and SSD (for residual energy measurement) assembly. The experimental setup is shown in fig. 14. The ~He particles originated from a commercially available alpha particle source which could be inserted in front of the T F D and SSD assembly. The alpha particle source had monoenergetic groups of approximately equal intensities at 8.78, 6.77, 6.28, 5.68 and 5.42 MeV with a weaker group at 6.04 MeV. The thin film scintillator was about 900 pg/cm 2 in thickness and degraded the 8.78 MeV alphas by 0.5 MeV, i.e., 8.78 MeV alphas lost about 0.5 MeV energy in passing through the T F D as determined by an alpha particle energy loss measurement. The response of the SSD (of rather poor resolution) to the residual energy 4He and 3He particles is plotted in figs. 15b and 15c, respectively. The two fragments are seen to be of comparable energies with the 4He groups weighted a little to higher energies (and hence lower dE/dx values). Fig. 15a demonstrates that the energy loss experienced by the two fragment types are NICKEL FOILS

COLLIMATOR

PHOTOMULTIPLIER l

1..

Cf-252

SOURCE

SOLID S T A T E

~_~TECTOR

n THIN

FILM

DETECTOR

PHOTOMULTIPUER

Fig. 10. Schematic diagram of experimental assembly for testing response of thin film detector to degraded fission fragments from 252Cf decay.

354

M.L. M U G A

140 tI £9

uJ 03 .J n:)lO0

l.iJ l-w

oo

60 _I h Z

40

I-.-

20

I 0.2

I 0.4

I 0.6

I 0.8

I 1.0

NICKEL

FOIL

I 1.2

I 1.4

THICKNESS

I 1.6

I 1.8

I 2.0

(rng/crn 2)

Fig. 11. TFD response to passage of degraded fission fragments showing distribution peaks as function of thickness of nickel degrading foil.

160 l-r

140

I

I w co j

~ -r W "l-

120

140

120

w u') j

,

(3- IO0 n." 0 I..o 8c

LIGHT FRAGMENT

w I-w

~

I00

n,0 I-

80

C3

60

o m iw -

6C :E _j

HEAVY

:E

FRAGMENT

-

LL z

4C

HEAVY FRAGMENT

~ z

-r

I-

LIGHT FRAGMENT

40

-~

2C

I-

50

60 AVERAGE

70

80

90

ENERGY

(MeV)

I00

Fig. 12. TFD response to degraded fission fragments showing pulse height distribution peaks as a function of average fragment kinetic energy.

20

o18

o.'9

I

,.o

,'.,

I

,2

,13

AVERAGE VELOOTY V (cm/nsec) Fig. 13. TFD response to degraded fission fragments showing pulse height distribution peaks as a function of average fragment velocity.

dE/dx DETECTOR

VERSATILE

the two particles had exactly equal kinetic energies for comparison.

ACCELERATED

•

I

He BEAM FROM

•

UF VAN DER GRAAF

IMPROVED TIME-OF-FLIGHT RESOLUTION

3.5.

/

/

\c..,. /

"SOLID STATE

%. ~

DETECTOR

Fig. 1.4. Schematic diagram of experimental assembly for testing response of thin film detector to light charged particles.

clearly distinguished. We note that the T F D had a resol ution of P/V -- 2 in these exploratory experiments; considerably better TFD's are now available. Further, the separation shown in fig. 15a would be enhanced if

-•

355

FOR HEAVY MASS NUCLEAR PARTICLES

Since experiments using c o m b i n a t i o n AE, E a n d V (velocity) m e a s u r e m e n t s are expected to be m o r e definitive in establishing particle identity, we have tested the new design versions of the T F D along with i m p r o v e d electronic units (e.g. c o n s t a n t fraction discriminators) in attempts to i m p r o v e time-of-flight ( T O F ) resolution measurements. The results are shown in figs. 16a a n d 16b for T O F distributions o f 252Cf fission fragments with flight distances o f l0 c m a n d 20 cm, respectively. Fig. 16c depicts similar measurem e n t t a k e n with the older system ~) with 70 c m flight path. O p e r a t i n g with such short flight p a t h s (while still m a i n t a i n i n g a d e q u a t e time resolution) should m a k e feasible simultaneous E, AE a n d I / m e a s u r e m e n t s on heavy particle fragments o f low p r o d u c t i o n cross sections.

3.6. RESPONSE OF T F D TO TOTAL ENERGY LOSS OF CHARGED PARTICLES Thicker scintillator films were fabricated by laminao•

• He 4

•

• e

o

• He 3

•

o •

•

500 o•

•

Hd

•

500

60q o

o Q

O°o

o• o

z z

50~

o

6~ o

< "7U

o_

5.60MeV I

~

300

,~ w

%

400

•

• o

o

oe eee=

t

e•

o• o•

•

o.

200

o

.

• •e

I00

-.eo

.-*

o o

ooo o

% o

"..

o

•

i

i

°

140

o

"" -.,

o

u•

160

500

O u

500

O

180

He 3

400 o

:300 o

z~ E

100

° o

o

-~ 26 ' 2o '~b- ' ~o ',do' 1~o' i,lo'

%o o• ° °9°°°°

~o

~ z

200

0

oo°°O

%

o

o

o

\

NUMBER ~

100 •B ° e ? °°'~

o

oo

o• o 0

CHANNEL

t

coo • =

o

O

Z

1½0

I

•

5.40 MeV t

.':."

•••CO

:

100

•

0 •

i

_ _

• o

©

o

300

,

80

z

u~

3, Z < I U

200

o

t~

0 U

400

8.3 0 MeV

•

'

CHANNEL

,6o

I~o

oooooo,~

14o

N U M B E R ~-- RESIDUAL

,~o

i~o

ENERGY

Fig. 15. a) 3He and 4He energy loss response from thin film detector; signals,are ungated. Residual energy spectra of particles are shown in b) for 4He and c) for aHe particles.

356

M.L. MUGA 30 or 20 + with respect to the collimated charged particle beam. Pulse height distributions for known alpha decay groups are recorded in figs. 17a and 17b for thin films of 100 and 200 laminations, respectively. For the 100 lamination film we observe a linear response to alpha particle energies of about 6.8 MeV and below. Alpha particles of energy 8.78 MeV are clearly beyond the linear response range. Resolution (fwhm) is calculated as 6% for 6.11 MeV alpha particles from 252Cf. Using the scintillator film composed of 200 laminations, the 8.78 MeV alpha group "moves out" to its proper location to give a linear response of pulse height vs energy; the resolution is perhaps slightly better for 6.11 MeV alpha particles. Efforts to obtain equally good alpha energy resolution using single films (i.e. non-laminated) of equivalent thickness were uniformly unsuccessful. Placing a laminated film directly on the flat surface of a PM face with incident alpha particles entering perpendicular to the face resulted in even poorer energy response. Surely the laminated structure of the scintillator film (or its unusually smooth surface) and its orientation to the photomultiplier tube face (and perhaps clean living) play a critical role in photon production and or

A) 10 CM F L I G H T PATH g ",

..5

600

%

400 • •,, •e

20(1

B)20

~3 I--

CM F L I G H T

PATH

z 0

1500

U

;

100(

uJ

;

50£

i

z

i

,

•

•

~°•

C) 70 CM FLIGHT PATH--OLDER SYSTEM 3000

2000

o•

I

•

6,04

6.28

1-o.,2 +.66 .

/¢,v¢%

,.,,

t.,.,,.v

,oo Z2°o,,o°,

Q)

e

I000

f"

..'.

1500

%

i

i

20

i

40

i

io

8'0

"

tion of individual films formed as previously describedt). These films were then glued to a hemicylindrical light pipe (fig. ld) for support. Up to 300 laminations, equivalent to a thickness of approximately 0.01 cm were used in fabricating thicknesses suitable for recording the total energy loss of alpha particles emitted from a radioactive source. An effectively increased thickness could be obtained when desired by orienting the plane of the scintillator at 45,

o"

• "

,o0i'.." : - ".."'."""

C H A N N E L NUMBER ..~--~ REVERSE FLIGHT TIME

Fig. 16. Time-of-flight (TOF) spectra for 2a2Cf fission fragments using two improved versions of thin film detector (TFD). a) 10 cm and b) 20 cm flight path. Similar spectrum taken with older version of TFD and longer flight path as reported in ref. I is shown for comparison in c).

"o "o'

o

o

".

b)

'" "

8

.oop.-

,oo1-. _ L_T I

: "-

~,2 ~.66 i

.......

.

......... 80

PULSE

160

240

HEIGHT RESPONSE

Fig. 17. Response of thin film detector (TFD) to total energy loss of alpha particles, a) Scintillator film composed of 100 laminations. Open circles show response of 6.ll MeV alphas from 25eCf. See text for explanation of other alpha source energy distribution, b) Scintillator film composed of 200 laminations.

VERSATILE

dE/dx

DETECTOR

F O R H E A V Y MASS N U C L E A R

PARTICLES

357

iopoo

03

I

i-Z d

~

g I

0

8~000

4,000

.°,°.°,° •

,• • o°

Z 0. 2~000

".°

,*

".°

.°"

°.

°,..

"°.° .°

I 80

I 160

CHANNEL NUMBER ~-, TFD PULSE

"'1"" 24.0

HEIGHT

Fig. 18. Response of thin film detector ( T F D ) to total energy loss of 252Cf fission fragments. Scintillator film is c o m p o s e d of 40 l a m i n a t i o n s with a thickness of a b o u t 4 m g / c m 2.

collection. Although it is widely accepted that plastic scintillators are inherently incapable of energy resolution better than 10-20% fwhm, we submit that improved resolution can be achieved. Tlhe T F D response to total stopping of 252Cf fission fragments is shown in fig. 18. The peak-to-valley ratio is typically about 6-7. Clearly, a different response, relative to solid state silicon counters, is experienced by this new detector for total fission fragment energy loss. This non-linear response characteristic is the subject of current investigations. 4. Discussion

In evaluating the possible applications of these detectors, they may be classified in terms of use as a registering device, dE/dx counter or both. 4.1. USE AS REGISTRATION COUNTER

Examples of applications taking advantage of this feature are as follows:

4.1. I. Time-of-flight (i.e. velocity) measurements of individual charged particles For lighter charged particles thicker (laminated) thin films would be used, sufficient to give adequate signalto-noise ratio and yet minimize the energy loss in the film. For heavy ions (e.g. fission fragments) the thin film detector has been shown to be ideal as a time-pickoff signal device. Coupled with residual energy measurements obtainable with solid state detectors, mass determinations of heavy ions produced in a variety of nuclear reactions can be rather easily accomplished. 4.1.2. Mechanical gating device Studies of delayed gamma, neutron or short-lived betel decay fission fragments have been made by impinging the fragments on a movable tape and then

transferring the tape section to a counter area for analysis. By passing the individual fragments through a thin film detector prior to impact on the tape, one can use the signal from the T F D to drive the tape after a single fragment (or any preset number) has been caught. Such arrangements should result in more definitive data from these experiments.

4.1.3. Velocity measurements of groups of low energy ion molecules Mass determinations of molecular ions from gaseous chemical reactions are widely made in physical chemistry research. Typically, bursts of singly charged ions are extracted from a reaction mixture and accelerated to 3 keV per ion and impinge on a suitable detector at known distance some time At later. Although it remains to be shown whether the sensitivity of the T F D is competitive with presently used electron multiplication techniques, special conditions may arise wherein the use of a T F D would be advantageous. 4.2. USE AS dE/dx COUNTER The use of silicon transmission detectors for particle identification (Z,A) of light charged particles 6,v) has received wide attention in recent years. The particle identifier technique employs the semi-empirical rangeenergy relation R = aE h (R = range, E = energy, a and b are adjustable constants). Utilizing signals from a AE and an E counter, this relation may be used to calculate a particle identification pulse I proportional to T/a = (E + AE) b - E b. This identification pulse is unique for each isotope in the low mass range. Unfortunately, these silicon detectors cannot conveniently be made thin enough to transmit heavier ions (in the medium energy range of a few MeV/nucleon or less) and retain dE/dx output characteristics. In contrast, the thin film detector (TFD) has been

358

M.L. MUGA

shown to exhibit a functional dE/dx response to these transient heavy ions, e.g. fission fragments, with only about a 10-20% energy loss. Furthermore, the output signals associated with light and heavy fragment energy losses that differ by less than 15°o are clearly separated as shown in fig. 3b. Undoubtedly, this differential response is a function of mass, charge and ion velocity, but it is precisely this dependence (unknown at this time) which we must exploit in order to achieve particle identification. For fission fragments and other heavy ions in the low energy range, we are dealing, not with completely stripped nuclei, but with ions of some equilibrium charge as they pass through matter. Although charge equilibrium is rapidly attained (in about 25 /~g/cm2) 8) upon entering the film, the difference in AE, E combinations to produce identity patterns may not be easily distinguished for these heavier ions. For example, the Z dependence of the stopping power may be approximated numerically by comparing adjacent isobars of comparable velocities. Using tabulated results 9) we expect stopping power differences (between adjacent isobars) to be about 1°//o in the higher mass range (A > 200 ainu) and perhaps 2% difference in the middle mass range (A ~ 100 ainu) for ions having about 1 MeV/nucleon kinetic energy. Present forms of the T F D are capable of about 5% (or better) resolution fwhm in this energy range using a single detector. However, by resorting to several stacked detectors (see section 3.2) improved differentiation should result. Essentially, this scheme is equivalent to measuring the Bragg ionization curve (dE/dx vs residual range) of a single energetic ion. Moreover, at still higher energies per nucleon, the stopping power difference between adjacent isobars increases (e.g. ~2°'o difference at mass ~ 200 ainu and ~ 3% difference at mass ~ 100 ainu at 5 MeV/nucleon). Combined with time-of-flight (TOF) mass measurement, sequential stopping power measurements should allow confident identification of heavy fragment ions in the energy range 1-10 MeV per nucleon. Some of the more obvious types of experiments to which the thin film detector might profitably be applied are listed below: 4.2. I. Detection and identification of l o w mass fragments

.formed in ternary ,fission decay By measuring relative stopping powers, (combined with velocity-energy measurements) the low mass fragment reported 1°) to arise from ternary fission decay can be determined uniquely, without any concern

as to its half-life and even if it is non-radioactive. 4.2.2. Determination of heavy nuclear fragments formed

in medium and high energy induced nuclear reactions Again, combined E, AE and velocity measurements could be used on single (heavy) fragments emitted. Two advantages come to mind, viz. a) direct and immediate indication of mass and charge is available and b) pulsed accelerator operation is not required to obtain time zero pulses for TOF measurements since a thin film detector can serve this purpose (see fig. 4 of ref. 1). 4.2.3. Particle identification of light fragments of low

energy As shown in section 3.4, distinction between 3He and 4 He ions is possible with AE losses of only 500 keV. Improved T F D versions should be capable of distinguishing these two ions with as little as 250 keV kinetic energy degradation permitting low energy ( < 4 MeV) mixed spectra of these isotopes (and others) to be separated. Thus the T F D has a capability in this respect beyond either gas proportional or silicon dE/dx counters. 4.2.4. Detection and identification of new elements One scheme 11) for producing the superheavy elements ( Z ~ 114) is based on the fusion of two heavy nuclei (e.g. 238u+Z38u) followed by (an asymmetric mass) fission in which one fragment is formed in the superheavy region. Combined mass (by time-of-flight techniques) and rate-of-energy loss measurements might establish the charge without regards to the mode of decay or the half-life; the recoil energy imparted to the products by the fragmentation/fission process itself will cause the predicted superheavy element to emerge with adequate kinetic energy for these measurements. Recent work 12) reports the fission activity of a possible superheavy element congeneric to mercury. By looking at the AE vs residual energy plot of these fission fragments and comparing it with the fragment peak positions established for 252Cf fission (see fig. 7), one should be able to determine whether or not the parent (fissioning) nucleus is significantly more massive than/s/Cf. Thus if the interpretation of superheavy element formation is correct, we expect a) the typical lower mass fragment to be greater than that of MI, = 106 anau for 25Zcf, b) the typical heavy masses to not differ significantly, and c) both fragment kinetic

VERSATILE

dE/dx D E T E C T O R

FOR HEAVY MASS N U C L E A R P A R T I C L E S

energies to be greater than in the case of 252Cf fission. This distinction in expected fission fragment masses should show up on the E vs AE plot of fig. 7 in the following manner: a) The low mass peak position would shift to lower AE values as a consequence of an average higher mass value and would move up in both E and AE along a curve roughly parallel to the 127I curve as a consequence of increased kinetic energy release; a net shift to higher E values with little change in AE response, b) The high mass peak position would be expected to shift upwards in both E and AE (along a curve parallel to ~27[) to accommodate an increased kinetic energy only without a significant change in mass.

It is quite possible that confirmation could be made using a thickened spontaneous fission source, as we would expect the degraded light and heavy mass E vs AE response to produce roughly parallel ridges following the light and heavy mass curves shown in fig. 12. If the reported fission activity indeed arises from a superheavy element, these ridges should be closer (possibly coinciding) together relative to the spacing shown in fig. 12 for 252Cf fission fragments. In either case, an E, A E counter system can be devised with a geometrical efficiency equal to a single E detector; the resulting data would lead to a more confident interpretation. 4.2.5. Study of ultra-heavy fragment component of cos-

359

empirical correlations between T F D response and Z, A and E (or velocity) of the transiting heavy ion. Experiments are now in progress to achieve these goals. Notwithstanding these current limitations, it appears that the thin film detector will be a useful addition to the tools for experimental nuclear studies.

5. Summary Current investigations of the thin film detector indicate that its usefulness can be somewhat extended by operating it as a dE/dx counter for heavy charged particles. Various characteristics of the detector have been presented showing its response to the passage of 252Cf fission fragments, accelerated 127I ions and also, 3He and 4He particles. A variety of experiments in which these novel detectors might be profitably applied is briefly discussed. It is indeed a pleasure to acknowledge the major contributions of D. J. Burnsed and W. E. Steeger toward the design, fabrication and testing of the thin film detectors. We thank Dr. H. Weller of the Physics Department for his assistance in conducting the experiments comparing the T F D response to 3He and 4He ions. The cooperation of Dr. H. W. Schmitt of Oak Ridge National Laboratory and H. E. Taylor of ORTEC, Inc. is acknowledged in connection with the data on accelerated 127[ ions. Full and detailed reports on these exploratory experiments will be forthcoming.

mic rayflux in energy range 1-10 MeV/nucleon The abundances of cosmic ray elements heavier than iron are still poorly known. Recent studies ~3) in this region make use primarily of solid state plastic detectors, sometimes in combination with photographic emulsions, and by their nature are restricted to energies well above 10 MeV/nucleon. Below this energy level (and even above) stacked T F D assemblies (similar to that shown in fig. 8) should be capable of mapping the relative abundances of the ultra heavy mass (Z > 26) component of cosmic rays. Long exposure times aboard space satellites are expected to compensate for the smaller active area of these T F D systems. The method is particularly suited to deep space probes where the perturbation of cosmic rays by the solar wind is negligible and the recorded data can be telemetered back to earth. Other applications will no doubt be found. To be sure, much additional work must be done in characterizing these detectors. In particular, it is necessary to determine a) the ultimate resolution obtainable from the dE/dx signal, b) calibrations of the pulse height response vs energy loss for known isotopes and c)

References 1) M. L. Muga, D. J. Burnsed, W. E. Steeger and H. E. Taylor, Nucl. Instr. and Meth. 83 (1970) 135. 2) M. L. Muga, University of Florida, Nuclear Chemistry Progress Report ORO-2843 - 15 (1969); O RO-2843 - 18 (1970) unpublished. 3) A. Smith, A. Friedman and P. Fields, Phys. Rev. 102 (1956) 813. 4) N. O. Lassen, Dan. Mat.-Fys. Medd. 25, no. 11 (1949). 5) C. B. Fulmer, Phys. Rev. 108 (1957) 1113. 6) F. S. Goulding, D. A. Landis, J. Cerny and R. H. Pehl, Nucl.

Instr. and Meth. 31 (1964) 1. 7) A. M. Poskanzer, S. W. Cosper, E. K. Hyde and J. Cerny, Phys. Rev. Letters 17 (1966) 1271, 8) C. D. Moak, H. O. Lutz, L. B. Bridwell, L. C. Northcliffe and S. Datz, Phys. Rev. Letters 18 (1967) 41. 9) L. C. Northcliffe and R. F. Schilling, Range and stopping power tables for heavy ions, Texas A&M Variable Energy Cyclotron Report (1970) to be published. 10) M. L. Muga, C. R. Rice and W. A. Sedlacek, Phys. Rev. 161 (1967) 1266. 11) For a review of this and other possible approaches, see G. T. Seaborg, Ann. Rev. Nucl. Sci. 18 (1968) 53. rz) A. Marinov, C. J. Batty, A. I. Kilvington, G. W. A. Newton, V. J. Robinson and J. D. Hemingway,Nature 229 (1971) 464. 13) For a review of this work, see M. M. Shapiro and R. Silberberg, Ann. Rev. Nucl. Sci. 20 (1970) 323.