ROHACELL SUPPORT FOR THE PHENIX MULTIPLICITY VERTEX DETECTOR

Eric Bosze

University of California at Riverside

April, 1995

Introduction:

The Relativistic Heavy Ion Collider (RHIC) is scheduled to come on line at Brookhaven National Laboratory in April of 1999. The RHIC will accelerate Gold nuclei to relativistic speeds in opposing directions and study their interactions. The PHENIX project is an experiment to be performed at RHIC with the primary goals of the detection of a new phase of matter, the quark-gluon plasma, and the measurement of its properties. The Heavy Ion Group at the University of California at Riverside (UCR) is currently involved in the development of the PHENIX Detector, more specifically, the Multiplicity Vertex Detector (MVD) subsystem. The MVD is the first detector that particles produced by the heavy ion collison will come into contact with. The signifigance of the MVD lies in its ability to measure the multiplicity of the collision.

The MVD is constructed of an array of 300mm thick Silicon detectors. The Silicon wafers are glued to 24 hexagonal open structures made from a rigid, polymethacrylimid foam known commerically as Rohacell. Two characteristics of Rohacell were responsible for its selections, namely, its strength and low density. Rohacell has, however, been found to expand and contract depending on the changing humidity of its surrounding environment. Tests have demonstrated that when the humidity level of the surrounding environment is less than 50%, the Rohacell will contract, and when the humidity is greater than 50%, it will expand. It is thus logical to assume that the expansion and contraction of the Rohacell could result in either the fissure of the Silicon-glue bond to the Rohacell or the cracking of the Silicon wafer. This failure possibility has obvious implications for the MVD. The objective of my current research is focused on finding a coating that will make the Rohacell support less susceptible to moisture changes in its environment. This paper will describe the procedure that I used to test various coatings and offer preliminary findings.

Two methods used for measuring the amount of moisture absorbed into a coating include Moisture Absorption and Permeability Value. Moisture Absorption is defined as percent by weight the material will absorb. The Permeability Value is related to the rate at which moisture can diffuse through a coating. It is possible for a coating to have a low Moisture Absorption and still retain a high Permeability Value. In fact, most coatings have a high Permeability Value due to pinholes in the coating. Thus far, the coatings that have shown the most promise in preventing structural changes in the Rohacell resulting from moisture levels in the environment are the 105 West Systems Epoxy with 209 Hardner and Parylene. Parylene, which is a vapor deposited in a vacuum, adds little mass to the Rohacell and is one of the most moisture resistant coatings on the market. A detailed explanation of my set up and results to date is as follows.

Preparing the Rohacell:

1. In order to make the cages to the right dimensions, two Rohacell blocks were glued together with HEXCEL Eplite 5313 Vesin with hardner. A machinist took the glued block and cut the "C" cage out of it. At times, instead of testing the whole cage, strips that were roughly the same dimensions as the sides of the cages were tested. The testing process was the same for both the strips and the cages.

2. To assure that the coatings would stick, a small, medium bristle paint brush was used to clean the surface.

3. The brushed Rohacell was dried in an oven at about 65 C for 24 hours. The heat expelled the moisture from the Rohacell.

Preparing Coating:

4. Coatings were mixed per directions on the labels. Rubber gloves and eye protection were worn to safely handle the materials under a vented hood. Before application, the 105 epoxy was precured for about 2 hours at room temperature. At this stage, the epoxy can be applied to the surface of the Rohacell to prevent it from running and so as not to cause bubbles to form on the cage edges. The coated Rohacell is then set to complete its cure overnight at room temperature.

If a second coating is needed, the first coat is allowed to dry for about 2 hours. The composite is then allowed to dry overnight. (Note, the second coat is also precured before its application.)

Preparing for Testing:

The epoxy coating was applied to the Rohacell by the following procedure:

5. The dried Rohacell was taken out of the drying oven to be coated. While wearing rubber gloves, the epoxy was spread with a wooden stick in a single motion in order to make the film thickness as uniform as possible.

6. For a measure of variance, test samples were made in triplicates. Several uncoated Rohacell strips were left to act as a control base.

7. All coatings were allowed to dry (cure) for 2 days before testing.

Testing Procedure:

8. Cured strips were measured for their length and mass, then placed into the first testing environment (each strip's length was measured three times so as to calculate an average, a standard deviation, and a range to estimate my measurement errors)

9. The strips were placed in an environment for about 48 hours. During that 48 hours, their length and mass were measured several times to observe changes as they adjusted to the surrounding environment.

10. After the 48 hours, the samples were taken out of the test environment and measured again for their length and mass. They were then exposed to a different environment where they were left for 48 hours and length and mass measurements were again calculated 2 to4 times.

11. The procedure was continued until approximately 400 hours of testing was completed. Using that time frame, the strips were exposed to about 8 to 9 environments

Table 1 contains strip data taken on January 10, 1995 at 3:30 p.m. in an environment of 22 C at 55% R.H. High humidities were achieved in a jar with conditions of 55 C at approximately 100% R.H. The strips are identified as N1, N2, and N3. Their length was measured three times in mils, from which the average, range, and standard deviation were calcualted.

	N1	N2	N3
	4001	4002	4005
	4002	4003	4002
	4000	4002	4003
Average	4001	4003	4003
Range	2	1	3
STD	1.18	0.59	1.77

Jan. 10, 1995 3:30 pm Conditions: Room: 22 C at 55% R.H. Jar: 55 C at ~100% R.H

The Parylene coating was tested following the procedure described above

Equipment Used:

Mitutoyo caliber No. 505-627 to measure length.
Denver Instrument Company XE Series milligram digital scale Model 300. to measure mass.
Omega HX-11V temperature/humidity probe to measure temperature and relative humidity.

Results to Date:

With the assistance of Miguel Noriega, I have tested the feasibility of several coatings including polyurethane, water seals, epoxies, and a vapor deposited coating.
The coating that has demonstrated the most promise is the 105 West System Epoxy with 209 hardener. The following graphs contrast the performance differences between a 105 Epoxy coated Rohacell strip and an uncoated Rohacell strip.

Explanation of Graphs:

The vertical lines indicate the time at which the environment was altered during the 400 hours of testing. The temperature and relative humidity levels of the various environments are given. In the legend below the graph, the letter-number identifies the strip, and the numbers in parentheses tell which hardener was used, how many coats were applied to the strip, and how long the coating was allowed to precure before it was applied to the Rohacell. The y-axis shows the percent change in length relative to the length at zero time.

105/209 Epoxy Results:

Graph #1

The three replicate strips with the same coating performed in a surprising consistent manner with tightly distributed readings.
Even though the coated strips expanded/contracted in a similar manner to the uncoated strips, their percent change was dampened by a factor of about three. This is accurately reflected with the environment change from 20 C at 100% Relative Humidity to 50 C at 100% Relative Humidity. Here the uncoated strip changed about - 0.6% in length, whereas the coated strips shifted about - 0.2%. Even a single coated strip showed the dampening effect with respect to environmental changes.

Graph #1

Graph #2 (Graph #2 is an extension of Graph #1)

When all the samples were brought back to room ambient conditions, the uncoated strip took a large and permeate - 1.4% shift in length while the coated strips leveled out with only a - 0.35% shift in length.

Conclusions:

The coated Rohacell strips appear to dampen the effects of humidity/temperature changes in many cases. It is most pronounced with temperature changes at humidities greater than 50%.
When the strips are brought back to room ambient conditions, the coated strips show less hysteresis; i.e., the ability to return (close) to their original length.

Additional Graphs:

Just Bare Rohacell

Just Parylene Coated

Just Parylene Coated vs. Bare Rohacell

Lightly Coated Epoxy vs. Bare Rohacell

Parylene-Epoxy Coated vs. Bare Rohacell

Epoxy-Parylene Coated vs. Bare Rohacell

Light Coating Epoxy, then Parylene