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A search for anomalous using plastic nuclear track detectors
Nuclear Tracks and Radiation Measurements, Vol. 8, Nos. 1-4, pp. 563-566, 1984. Printed in Great Britain.
0191-278X/84 $3.00 + .00 ~c3 1984 Pergamon Press Ltd.
A SEARCH FOR ANOMALONS USING PLASTIC N U C L E A R TRACK DETECTORS
H. Drechsel*, W. Trakowski*, J. Dreute*, S. Sonntag*, C. Brechtmann*, R. Rudat*, J. Beer*, W. Heinrich*, E. V. Benton**, R. M. Cassou** *University of Siegen, Physics Department, P. O. Box 101240 D-5900 Siegen, Fed. Rep. of Germany **University of San Francisco, Department of Physics, San Francisco, Cal. 94117, USA
ABSTRACT Enlarged cross sections for projectile fragments of relativistic nuclei have been observed in Bevalac and co~nic ray experiments with nuclear emulsions. We performed an experiment using plastic track detectors to investigate the anomalon effect. A new automatic measuring technique for heavy ion tracks in plastic detectors enables us to analyze large numbers of fragmentation reactions in reasonable time. Thus we get better statistical significance. Results of the experiment are presented.
KEYWORDS An~nalons, relativistic heavy ions, nuclear collisions, automatic track measurements.
INTRODUCTION Observations suggesting the existence of enlarged cross sections for projectile fragments have been reported since 1954 (Milone, 1954). Systematic studies of cosmic ray experiments showed that the mean free path of nuclear fragments is shorter than for nuclei of the primary cosmic radiation (Judek, 1968). But these experiments were limited by statistics. This lack of significance could be overcQme when beams of relativistic heavy nuclei were accelerated at the Berkeley Bevalac. Experiments with relativistic nuclei using nuclear emulsions showed evidence for an admixture of 6% of anomalous fragments having a mean free path of 2.5 cm in nuclear emulsion (Friedlander et al., 1982). In our experiment we wanted to confirm the anomalon effect using a CR 39 plastic target and at the same time to i m p r o v e the statistics by using an automatic scanning and measuring system.
EXPERIMENTAL METHOD A stack of 300 foils of CR 39 (CI2HIRO~) plastic nuclear track detectors each 0.6 mm thick . was exposed at the Bevalac to 4 OAr ions incident with an effective size of 4.5 cm x ~v cm vertically to the surface of the foil. The energy of the nuclei was 1.8 GeV/nucleon and the particle density was 15OO particles/cm 2. By means of etching the plastic foils in NaOH solution at 80°C the particle tracks were developed to cones. Altogether 1.5 x 107 etch cones were measured on the top and rear side of the foils using an automatic measuring system, which works by means of digital picture analysis (Trakowski et al., 1983). The plastic foils are mounted on a microscope stage and a video camera is looking through the microscope. The video pictures are digitized and stored into computer memory. A special software working in the digitized picture detects the etch cones, which appear as dark areas on a brighter background, and records the co-ordinates of these areas and their sizes and shapes. After a calibration the size of a measured track area gives the charge number of the particle. For a single track measurement the charge resolution is ~Z = 0.2 for z = 8 and AZ = 0.7 for Z = 17 (Trakowski et al., 1983, Heinrich et al., 1983). In Fig. I the 563
564
H. D R E C H S E L et al.
d i s t r i b u t i o n of c h a r g e s a v e r a g e d over the t r a j e c t o r i e s is shown, from w h i c h a r e s o l u t i o n of AZ ~ 0. i is deduced. F r o m the p o s i t i o n s of the e t c h cones on every foil the trajectorie:
o o
~-° z
z o
%ioo
,!
J
,
i • 00
12.01
14~00
16~00
CHARGE Fig.
i.
Distribution
of t r a j e c t o r y
averaged
charges
for individual p r o j e c t i l e f r a g m e n t s w e r e r e c o n s t r u c t e d . E a c h t r a j e c t o r y was e x t r a p o l a t e d to the n e x t foil t h r o u g h o u t the stack l o o k i n g for c o i n c i d e n c e s b e t w e e n the e x t r a p o l a t e d positions and the m e a s u r e d etch cones. For these t r a j e c t o r i e s an i n t e r a c t i o n is defined, if the m e a s u r e d charge c h a n g e s or the t r a j e c t o r y ends, w h i c h m e a n s the f r a g m e n t has a charge Z < 8. Fig. 2 shows an example of a r e c o n s t r u c t e d p a r t i c l e track w i t h a c h a i n of four n u c l e a r reactions.
o o
"~
+
"4-
~+--+~
¢Y
Oo °o
o
3" DEPTH
Fig.
2.
IN
Chain of four charge
STACK changing
[CM] nuclear
reactions
In total we r e c o n s t r u c t e d 14501 f r a g m e n t t r a j e c t o r i e s w i t h c h a r g e s b e t w e e n Z = 9 and Z = 15 and 7 1 2 0 d e t e c t e d interactions. F r a g m e n t c h a r g e s of Z = 16 and Z = 17 w e r e n o t i n c l u d e d in the a n a l y s i s due to the p o o r e r c h a r g e r e s o l u t i o n and the u n c e r t a i n t y of d e t e c t i n g an intera c t i o n w i t h AZ ~ 2. To d e t e r m i n e the i n t e r a c t i o n m e a n free p a t h the total n u m b e r n Z of i n t e r a c t i o n s of n u c l e i w i t h c h a r g e Z was counted.
A SEARCH FOR ANOMALONS
565
T h e n for each p a r t i c l e i w i t h charge Z the p a t h l e n g t h li Z was d e t e r m i n e d , that is the l e n g t h m e a s u r e d from the p o i n t of omission of the f r a g m e n t to the p o i n t of i n t e r a c t i o n or the point, w h e r e the p a r t i c l e left the detector. The i n t e r a c t i o n m e a n free p a t h for one c h a r g e is t h e n c a l c u l a t e d from Iz
= ~ £iZ/nz i
The i n f o r m a t i o n from the d i f f e r e n t f r a g m e n t c h a r g e s were pooled by n o r m a l i z i n g the IZ for each charge to a c a l c u l a t e d i n t e r a c t i o n m e a n free p a t h Iz c, w h i c h was d e t e r m i n e d based on g e o m e t r i c cross sections for charge c h a n g i n g c o l l i s i o n s (Heinrich et al., 1983). Thus we got a n o r m a l i z e d i n t e r a c t i o n m e a n free p a t h i* = ~ (ziZ/IzC)/E n Z i z
RESULTS To i n v e s t i g a t e the a n o m a l o u s m e a n free p a t h effect I* was d e t e r m i n e d for d i f f e r e n t interv a l s of d i s t a n c e x from the p o i n t of e m i s s i o n of the fragment. Fig. 3 shows the r e s u l t of this a n a l y s i s involving 6444 charge c h a n g i n g interactions. A reduced m e a n free p a t h for the i n t e r a c t i o n of f r a g m e n t s can be seen for short d i s t a n c e s from the p o i n t of emission (x < 1.2 cm) and s u r p r i s i n g l y above x > 4.2 cmo
N
°i o
%.oo
'
6.00
9.00
2.oo
, .oo
,,:oo
X [ CM] Fig. 3.
The n o r m a l i z e d i n t e r a c t i o n m e a n free p a t h as a f u n c t i o n of d i s t a n c e x from the p o i n t of e m i s s i o n
O
~ oo C3-W N
(21 '7 o
%.00
3.00
B.O0
9.00
12 O0
15 O0
%B O0
DEPTH IN STACK [CM] Fig. 4
The n o r m a l i z e d i n t e r a c t i o n mean f r e e path as a f u n c t i o n o f depth D i n the stack
566
H . D R E C H S E L et aL
To rule out systematic effects of the d e t e c t o r the same a n a l y s i s was p e r f o r m e d for the d e p t h D in the stack instead of d i s t a n c e x f r o m p o i n t of emission. The result in Fig. 4. gives no e v i d e n c e for a s i g n i f i c a n t d e v i a t i o n of I*(D) from the average. A systematic error in our e x p e r i m e n t arises from a small f r a c t i o n of short p a r t i c l e t r a j e c t o r i e s w h i c h are a r t e f a c t s c o n s t r u c t e d by the c o m p u t e r program. M e a n w h i l e this p r o b l e m is solved by an improved p i c t u r e a n a l y s i s software~ But for the e x p e r i m e n t d e s c r i b e d here the only way out was a r e a n a l y i s by eyeball of m o r e than 2000 f r a g m e n t t r a j e c t o r i e s w i t h a p a t h length of x < 1.2 cm~ A r t e f a c t s c o n s t r u c t e d by the c o m p u t e r p r o g r a m were thus d e t e c t e d and eliminated. T h e d a t a in Fig. 3. are free of this systematic error. A r e e x a m i n a t i o n of the e x p e r i m e n t a l c o n d i t i o n s for the d a t a of x > 4.2 cm is still in progress, but no e x p l a n a t i o n for the r e d u c e d m e a n free p a t h in this d o m a i n has yet been found.
CONCLUSION Comparing our r e s u l t s w i t h that one of F r i e d l ~ n d e r et al. (1982) and r e s t r i c t i n g the analysis on the d a t a for x < 4.2 cm we d e d u c e d an a d m i x t u r e of 1.2% of a n o m a l o u s f r a g m e n t s having an i n t e r a c t i o n m e a n free p a t h of 0.65 cm. Due to this small p o r t i o n of a n o m a l o n s the s t a t i s t i c a l s i g n i f i c a n c e is still b e l o w 3 standard d e v i a t i o n s a l t h o u g h based on our e x p e r i m e n t a l t e c h n i q u e we were able to a n a l y z e a large number of interactions. This w o r k is supported by the B u n d e s m i n i s t e r
f~ir F o r s c h u n g and Technologie,
No. 06 SI 159.
REFERENCES Milone, A. (1954). Nuovo Cimento Suppl. 12, 353. Judek, B (1968). Can. J. Phys. 46, 343. Friedlander, E. M., R. W. Gimpel, H. H. Heckmann, B. J u d e k and E. G a u s s a u g e (1982). Phys. Rev. Lett. 45, 305 Trakowski, W., J. Dreute, S. Sonntag, C. Brechtmann, J. Beer and W. Heinrich, this conference. Heinrich, W., H. Drechsel, W. Trakowski, J. Beer, C. Brechtmann, J. Dreute and 2. Sonntag (1983). Univ. of Siegen Rep. Si-83-ii, p r e s e n t e d at the 6th High E n e r g y H e a v y - I o n Study and W o r k s h o p on Anomalons, LBL Univ. of C a l i f o r n i a .
0191-278X/84 $3.00 + .00 ~c3 1984 Pergamon Press Ltd.
A SEARCH FOR ANOMALONS USING PLASTIC N U C L E A R TRACK DETECTORS
H. Drechsel*, W. Trakowski*, J. Dreute*, S. Sonntag*, C. Brechtmann*, R. Rudat*, J. Beer*, W. Heinrich*, E. V. Benton**, R. M. Cassou** *University of Siegen, Physics Department, P. O. Box 101240 D-5900 Siegen, Fed. Rep. of Germany **University of San Francisco, Department of Physics, San Francisco, Cal. 94117, USA
ABSTRACT Enlarged cross sections for projectile fragments of relativistic nuclei have been observed in Bevalac and co~nic ray experiments with nuclear emulsions. We performed an experiment using plastic track detectors to investigate the anomalon effect. A new automatic measuring technique for heavy ion tracks in plastic detectors enables us to analyze large numbers of fragmentation reactions in reasonable time. Thus we get better statistical significance. Results of the experiment are presented.
KEYWORDS An~nalons, relativistic heavy ions, nuclear collisions, automatic track measurements.
INTRODUCTION Observations suggesting the existence of enlarged cross sections for projectile fragments have been reported since 1954 (Milone, 1954). Systematic studies of cosmic ray experiments showed that the mean free path of nuclear fragments is shorter than for nuclei of the primary cosmic radiation (Judek, 1968). But these experiments were limited by statistics. This lack of significance could be overcQme when beams of relativistic heavy nuclei were accelerated at the Berkeley Bevalac. Experiments with relativistic nuclei using nuclear emulsions showed evidence for an admixture of 6% of anomalous fragments having a mean free path of 2.5 cm in nuclear emulsion (Friedlander et al., 1982). In our experiment we wanted to confirm the anomalon effect using a CR 39 plastic target and at the same time to i m p r o v e the statistics by using an automatic scanning and measuring system.
EXPERIMENTAL METHOD A stack of 300 foils of CR 39 (CI2HIRO~) plastic nuclear track detectors each 0.6 mm thick . was exposed at the Bevalac to 4 OAr ions incident with an effective size of 4.5 cm x ~v cm vertically to the surface of the foil. The energy of the nuclei was 1.8 GeV/nucleon and the particle density was 15OO particles/cm 2. By means of etching the plastic foils in NaOH solution at 80°C the particle tracks were developed to cones. Altogether 1.5 x 107 etch cones were measured on the top and rear side of the foils using an automatic measuring system, which works by means of digital picture analysis (Trakowski et al., 1983). The plastic foils are mounted on a microscope stage and a video camera is looking through the microscope. The video pictures are digitized and stored into computer memory. A special software working in the digitized picture detects the etch cones, which appear as dark areas on a brighter background, and records the co-ordinates of these areas and their sizes and shapes. After a calibration the size of a measured track area gives the charge number of the particle. For a single track measurement the charge resolution is ~Z = 0.2 for z = 8 and AZ = 0.7 for Z = 17 (Trakowski et al., 1983, Heinrich et al., 1983). In Fig. I the 563
564
H. D R E C H S E L et al.
d i s t r i b u t i o n of c h a r g e s a v e r a g e d over the t r a j e c t o r i e s is shown, from w h i c h a r e s o l u t i o n of AZ ~ 0. i is deduced. F r o m the p o s i t i o n s of the e t c h cones on every foil the trajectorie:
o o
~-° z
z o
%ioo
,!
J
,
i • 00
12.01
14~00
16~00
CHARGE Fig.
i.
Distribution
of t r a j e c t o r y
averaged
charges
for individual p r o j e c t i l e f r a g m e n t s w e r e r e c o n s t r u c t e d . E a c h t r a j e c t o r y was e x t r a p o l a t e d to the n e x t foil t h r o u g h o u t the stack l o o k i n g for c o i n c i d e n c e s b e t w e e n the e x t r a p o l a t e d positions and the m e a s u r e d etch cones. For these t r a j e c t o r i e s an i n t e r a c t i o n is defined, if the m e a s u r e d charge c h a n g e s or the t r a j e c t o r y ends, w h i c h m e a n s the f r a g m e n t has a charge Z < 8. Fig. 2 shows an example of a r e c o n s t r u c t e d p a r t i c l e track w i t h a c h a i n of four n u c l e a r reactions.
o o
"~
+
"4-
~+--+~
¢Y
Oo °o
o
3" DEPTH
Fig.
2.
IN
Chain of four charge
STACK changing
[CM] nuclear
reactions
In total we r e c o n s t r u c t e d 14501 f r a g m e n t t r a j e c t o r i e s w i t h c h a r g e s b e t w e e n Z = 9 and Z = 15 and 7 1 2 0 d e t e c t e d interactions. F r a g m e n t c h a r g e s of Z = 16 and Z = 17 w e r e n o t i n c l u d e d in the a n a l y s i s due to the p o o r e r c h a r g e r e s o l u t i o n and the u n c e r t a i n t y of d e t e c t i n g an intera c t i o n w i t h AZ ~ 2. To d e t e r m i n e the i n t e r a c t i o n m e a n free p a t h the total n u m b e r n Z of i n t e r a c t i o n s of n u c l e i w i t h c h a r g e Z was counted.
A SEARCH FOR ANOMALONS
565
T h e n for each p a r t i c l e i w i t h charge Z the p a t h l e n g t h li Z was d e t e r m i n e d , that is the l e n g t h m e a s u r e d from the p o i n t of omission of the f r a g m e n t to the p o i n t of i n t e r a c t i o n or the point, w h e r e the p a r t i c l e left the detector. The i n t e r a c t i o n m e a n free p a t h for one c h a r g e is t h e n c a l c u l a t e d from Iz
= ~ £iZ/nz i
The i n f o r m a t i o n from the d i f f e r e n t f r a g m e n t c h a r g e s were pooled by n o r m a l i z i n g the IZ for each charge to a c a l c u l a t e d i n t e r a c t i o n m e a n free p a t h Iz c, w h i c h was d e t e r m i n e d based on g e o m e t r i c cross sections for charge c h a n g i n g c o l l i s i o n s (Heinrich et al., 1983). Thus we got a n o r m a l i z e d i n t e r a c t i o n m e a n free p a t h i* = ~ (ziZ/IzC)/E n Z i z
RESULTS To i n v e s t i g a t e the a n o m a l o u s m e a n free p a t h effect I* was d e t e r m i n e d for d i f f e r e n t interv a l s of d i s t a n c e x from the p o i n t of e m i s s i o n of the fragment. Fig. 3 shows the r e s u l t of this a n a l y s i s involving 6444 charge c h a n g i n g interactions. A reduced m e a n free p a t h for the i n t e r a c t i o n of f r a g m e n t s can be seen for short d i s t a n c e s from the p o i n t of emission (x < 1.2 cm) and s u r p r i s i n g l y above x > 4.2 cmo
N
°i o
%.oo
'
6.00
9.00
2.oo
, .oo
,,:oo
X [ CM] Fig. 3.
The n o r m a l i z e d i n t e r a c t i o n m e a n free p a t h as a f u n c t i o n of d i s t a n c e x from the p o i n t of e m i s s i o n
O
~ oo C3-W N
(21 '7 o
%.00
3.00
B.O0
9.00
12 O0
15 O0
%B O0
DEPTH IN STACK [CM] Fig. 4
The n o r m a l i z e d i n t e r a c t i o n mean f r e e path as a f u n c t i o n o f depth D i n the stack
566
H . D R E C H S E L et aL
To rule out systematic effects of the d e t e c t o r the same a n a l y s i s was p e r f o r m e d for the d e p t h D in the stack instead of d i s t a n c e x f r o m p o i n t of emission. The result in Fig. 4. gives no e v i d e n c e for a s i g n i f i c a n t d e v i a t i o n of I*(D) from the average. A systematic error in our e x p e r i m e n t arises from a small f r a c t i o n of short p a r t i c l e t r a j e c t o r i e s w h i c h are a r t e f a c t s c o n s t r u c t e d by the c o m p u t e r program. M e a n w h i l e this p r o b l e m is solved by an improved p i c t u r e a n a l y s i s software~ But for the e x p e r i m e n t d e s c r i b e d here the only way out was a r e a n a l y i s by eyeball of m o r e than 2000 f r a g m e n t t r a j e c t o r i e s w i t h a p a t h length of x < 1.2 cm~ A r t e f a c t s c o n s t r u c t e d by the c o m p u t e r p r o g r a m were thus d e t e c t e d and eliminated. T h e d a t a in Fig. 3. are free of this systematic error. A r e e x a m i n a t i o n of the e x p e r i m e n t a l c o n d i t i o n s for the d a t a of x > 4.2 cm is still in progress, but no e x p l a n a t i o n for the r e d u c e d m e a n free p a t h in this d o m a i n has yet been found.
CONCLUSION Comparing our r e s u l t s w i t h that one of F r i e d l ~ n d e r et al. (1982) and r e s t r i c t i n g the analysis on the d a t a for x < 4.2 cm we d e d u c e d an a d m i x t u r e of 1.2% of a n o m a l o u s f r a g m e n t s having an i n t e r a c t i o n m e a n free p a t h of 0.65 cm. Due to this small p o r t i o n of a n o m a l o n s the s t a t i s t i c a l s i g n i f i c a n c e is still b e l o w 3 standard d e v i a t i o n s a l t h o u g h based on our e x p e r i m e n t a l t e c h n i q u e we were able to a n a l y z e a large number of interactions. This w o r k is supported by the B u n d e s m i n i s t e r
f~ir F o r s c h u n g and Technologie,
No. 06 SI 159.
REFERENCES Milone, A. (1954). Nuovo Cimento Suppl. 12, 353. Judek, B (1968). Can. J. Phys. 46, 343. Friedlander, E. M., R. W. Gimpel, H. H. Heckmann, B. J u d e k and E. G a u s s a u g e (1982). Phys. Rev. Lett. 45, 305 Trakowski, W., J. Dreute, S. Sonntag, C. Brechtmann, J. Beer and W. Heinrich, this conference. Heinrich, W., H. Drechsel, W. Trakowski, J. Beer, C. Brechtmann, J. Dreute and 2. Sonntag (1983). Univ. of Siegen Rep. Si-83-ii, p r e s e n t e d at the 6th High E n e r g y H e a v y - I o n Study and W o r k s h o p on Anomalons, LBL Univ. of C a l i f o r n i a .